*Article* **Behaviour of the Steel Welded Grid during a Simplified Pullout Test in Fine Sand**

**Marcin Cwirko ´ 1 and Małgorzata Jastrz ˛ebska 2,\***

**Featured Application: A potential application of the study is the use of the technology described in the construction of bridges, in particular for the construction of retaining structures within abutments.**

**Abstract:** This study considered the possibility of using steel gabion baskets made of welded mesh for a soil-strengthening function. Examples of such applications are known for meshes made of wires with diameters from 6.3 to 12.7 mm and mesh openings from 125 to 225 mm. In the case of a welded grid, the pulling resistance of fine-grained soil consists of two factors: frictional resistance and bearing resistance. Therefore, for the purposes of this issue, a simplified laboratory pullout test was carried out with four types of welded steel grid (common in Europe) embedded in the fine sand. The geometry of the grid (opening size: 76.2 × 76.2 mm), the type of steel (low carbon steel, tensile strength from 500 to 700 MPa), the diameter of the wire (2.7–4.5 mm) and its cover (ZN + PVC or ZnAl) were taken into account during the analysis. It was unequivocally stated that as the stiffness of the steel grid itself increases, its strength increases during the pullout test, which is not so obvious in the case of popular steel woven meshes. In addition, it has been shown that steel welded meshes with wire diameters less than 6 mm are suitable for soil reinforcement in structures with gabion facing, and the determined apparent friction coefficient (*μk* = 0.39–1.47) takes values similar to the friction coefficient given in references for welded meshes of larger diameters. This is a positive premise for starting further research on the use of wires of smaller diameters for welded mesh production used as soil reinforcement.

**Keywords:** pullout test; retaining structures; gabions; bearing resistance; welded steel grid; woven steel grid; reinforcement of embankments

#### **1. Introduction**

Retaining structures are used in the case of construction of buildings where there is a difference in ground level. A special place for the use of such structures are infrastructure projects, such as the construction of roads, railroads, bridges, or airports. Examples of existing implementations in Poland are depicted in Figure 1. Another innovative solution is the use of gabion baskets in the design of a highly sustainable agricultural building (with a cellar), located in a farm in the south of the Tuscany region, Italy. Gabions and natural stones were used for the realization of the foundations, the ground retaining walls, and all other bearing walls, including the external wall, which remains entirely above ground [1].

Retaining structures made of gabion baskets, i.e., cuboidal steel mesh baskets filled with ballast material, constitute a method of direct reinforcement for the mechanical protection of slopes [2]. The history of their use in construction dates to the time of the pharaohs (c. 5000 B.C.), when reed and rock elements were used to build dams and to prevent erosion and strengthen the banks of the Nile [3]. Similar structures were erected in

**Citation:** Cwirko, M.; Jastrz ˛ ´ ebska, M. Behaviour of the Steel Welded Grid during a Simplified Pullout Test in Fine Sand. *Appl. Sci.* **2021**, *11*, 9147. https://doi.org/10.3390/app11199147

Academic Editor: Ana María Camacho López

Received: 7 August 2021 Accepted: 9 September 2021 Published: 1 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

China around 1000 B.C. along the Yellow River [4]. It is believed that the first modern wire mesh gabions were made in the Bologna area of Italy around 1879 [5].

**Figure 1.** Examples of the use of gabion baskets in Poland (own pictures): (**a**) gravity retaining wall made of gabion baskets; (**b**) the slope of a motorway embankment reinforced with gabions; (**c**) gravity retaining wall supporting a motorway embankment; (**d**) an example of a gabion wall erected in harmony with the environment.

> The use of gabions filled with stones is environmentally friendly (Figure 1d) [6]. The main reason for the limited impact on the environment is the reduction in the materials used compared to traditional reinforced concrete structures, especially steel and concrete, the manufacture of which requires the use of cement, which is a highly energy-consuming raw material to produce. It is true that the steel of the gabion baskets is also an energetically expensive material, both in terms of production and the recycling process. However, it has the feature of being able to be totally recycled an unlimited number of times. Even the zinc used for steel galvanization is fully recyclable [1]. Both natural and recycled materials or alternative materials—e.g., clothing and plastic—can be used as ballast material for gabion baskets [7,8]. The development of anticorrosion protection techniques has made steel wire structures durable. Their durability is estimated at over 100 years [9–11].

> The fact that about 30–40% of the volume of gabions are air voids has a positive influence on the durability of the gabion structures, thanks to which they are characterized by excellent water permeability (hydraulic conductivity *k* > 2.5 × 10−3 m/s), which facili

tates the drainage of water from behind the retaining structure and prevents its level from rising [10–16].

Phenomena related to the influence of water on retaining structures are often neglected or treated with insufficient care when analysing the surroundings of bridge structures, which leads to numerous minor failures that are difficult to eliminate due to the scale of earthworks. The importance of proper drainage system implementation is often underestimated, both at the design and execution stages [17]. A geotechnical engineer should always be consulted for design solutions in such situations, and it should be noted that gabion retaining structures, apart from their structural function, also drain water from the soil medium.

An additional advantage is the fact that the gabion structures are deformable, thanks to which they do not break, but adapt to changing external conditions [18], which allows the use of gabions in areas of mining damage (e.g., retaining structure by the motorway in Ruda Sl ˛ ´ aska, in Silesia, a vast area of mining activity in southern Poland [19]). Gabions also have the ability to absorb noise [20], which allows them to be used for sound-absorbing partitions (e.g., noise barriers on the railroad in The Hague, Netherlands). Furthermore, the gabion revetments have the capacity for vegetation restoration (it provides a good habitat for floral, faunal, and microbial growth) and resist periodic flood disturbance [21,22]. Gabion structures are easy to assemble and do not require extensive construction site facilities, and their landscape and architectural aesthetics allow them to be adapted to the surrounding natural terrain (e.g., retaining structures on Victoria Road in Cape Town, South Africa).

Gabion structures made for the purpose of soil reinforcement technology deserve special attention. The whole structure is made of woven mesh, more precisely doublewoven mesh (Figure 2a) [23]. Gabion retaining structures made using welded meshes (Figure 2b) appear mainly as gravity structures (e.g., Figure 1a,c) or as structures made of soil reinforced with geosynthetics (e.g., Figure 1b) [24]. The use of welded steel meshes as backfill reinforcement seems to be the next step in the development of this technology. A diagram of such a solution is presented in Figure 3.

**Figure 2.** Comparison of the mesh opening: (**a**) woven (double-woven) mesh; (**b**) welded mesh.

**Figure 3.** Diagram of a gabion retaining structure for soil reinforcement technology.

In addition, lower demand for steel mesh, the considerably lower consumption of ballast material for filling the gabions, the reduction of the necessary transportation, as well as the lower labour demand constitute other advantages of such gabion structures. Structures using soil reinforcement technology are ultimately 25% to 60% cheaper to build than massive structures [24–28]. Ciomcia [25] carried out a meticulous analysis of costs and construction time on the example of the construction of a typical bridge abutment, showing that the construction of the bridge abutment using soil reinforcement technology with steel elements is 35% to 37% cheaper and takes from 38% to as much as 63% less time, compared to the construction of the abutment in the traditional monolithic form. Jayasree [26] analysed the costs of erecting retaining structures with a height of 1 to 10 m, showing that the construction of gabions, both massive and with soil reinforcement, is about 30% cheaper than the construction of RCC (reinforced cement concrete) retaining walls. For higher structures from 5.5 to 10.0 m, the savings range from 48% to 52% for massive structures and from 70% to 78% for reinforced soil gabion structures).

Gabion baskets made of woven or welded mesh are always selected on the basis of their deformability, which is a function of wire thickness, mesh size, type, and the grain size of the ballast material, as well as the construction of the gabion basket itself. The same parameters determine the selection of an appropriate gabion mattress solution for the regulation of riverbeds depending on the speed of flowing water [11].

In turn, the additional use of steel meshes as soil reinforcement requires the determination of their deformability after building in the soil medium, the force pulling them out of the soil, and the friction coefficient between the reinforcement and the soil, which also directly affects the length of the anchorage. Woven mesh, when stretched in the air (without building into the ground), tends to narrow, which is a result of the geometry of the mesh openings. This phenomenon drastically increases the tensile elongation of the woven mesh. It does not occur when stretching welded mesh with rectangular openings, because

the force is transmitted along the wires and their cross-sectional area directly affects the mesh elongation during its stretching. It turns out that after building in the soil medium, both types of mesh (woven and welded) are characterized by similar values of relative deformation. Pullout tests are dedicated to the determination of tensile forces, and consist of building a reinforcement mesh in a box with soil, applying a vertical load, simulating the weight of the entire geotechnical structure, and then slowly ejecting the reinforcement through the slot in the box, while simultaneously measuring both force and displacement. In this type of research, woven meshes exhibit a different mechanism of cooperation with the ground from welded meshes [29].

All these premises were the starting point for this research paper, which focuses on the observation of the behaviour of welded steel mesh during a simplified pullout test in fine sand. Pullout tests are performed to determine the coefficient of friction between the reinforcement and the soil, the value of which is essential for the correct and safe design of reinforced soil structures [30].

In construction site practice, woven and welded meshes are commonly used in the construction of gabion retaining structures. In the case of retaining structures with soil reinforcement, a double-woven mesh is typically used if the baskets are also made of it, or geosynthetics, regardless of the material of the gabion basket. Gabion welded mesh for soil reinforcement was not used. Although welded mesh has been successfully used to reinforce soil in light retaining structures since the 1980s, it is made of wires with a much larger diameter than those used for gabion basket production.

The essential issue in the design, performance, and durability of the use of a structure with soil reinforcement is the connection of the wall facing with the soil reinforcement. This connection is often the weakest point in terms of strength, effectively limiting the use of the total load-bearing capacity of the soil reinforcement, and is the site of potential failure during use. Integrating the facing with the reinforcement (e.g., extending the base of the gabion to such a length that it is also the reinforcement of the soil) makes it possible to eliminate the problematic connection, as is currently the case in structures where woven meshes are used to make gabions and reinforce soil.

The use of welded steel meshes as backfill reinforcement seems to be, of course, the next step in the development of this technology. This is important because welded meshes can achieve higher tensile strength than braided meshes due to the technological possibility of using thicker wire for their production.

#### **2. Materials and Methods**

Soil reinforcement with steel welded meshes identical or similar to those used for gabion structures is a field for discussion among researchers dealing with these issues. There are many indications that the use of this type of reinforcement is possible and justified [29]. In this study, it was decided to explore the subject of the cooperation of the soil reinforcement system in a simplified laboratory pullout test. For this purpose, four types of welded steel mesh and one type of coarse-grained soil were used, and the mesh was built in that soil. The question to be investigated was how the different types of reinforcement in a particular soil type affect the force required to pull the reinforcement out of the soil during the test.

#### *2.1. Fine Sand*

During laboratory tests, dry fine sand (FSa) with the following parameters was used: grey-yellow colour, calcium carbonate content CaCO3 < 1%, angle of internal friction *ø* = 24.0◦, cohesion *c* = 0.9 kPa, coefficient of uniformity *C*u = 2.57, coefficient of curvature *C*c = 0.71, optimum water content (OWC) *w*opt = 8.6%, maximum dry density *ρ*ds = 1.76 g/cm3. A detailed description of studies identifying the fine sand under consideration was presented by Sułkowska [31].

#### *2.2. Welded Steel Grid*

Welded meshes are manufactured of steel wire with a tensile strength of 500–700 MPa in accordance with the PN-EN 10223-8 standard [32]. Wires with a diameter of 2.2 mm to 5.0 mm are used, and the edge wire may be the same or thicker than the filling wire. The opening is rectangular and its most common sizes are 50 × 50 mm, 50 × 100 mm, 75 × 75 mm or 100 × 100 mm, as well as 76.2 × 76.2 mm (3 × 3 inches).

A commonly used anticorrosion protection of welded meshes is used to cover the wire, with one of three types of coating according to the PN-EN 10244-2 [33] and 10245- 2 [34] standards:

Laboratory tests were conducted on four types of welded meshes, which differed with respect to wire diameter as well as the type of anticorrosion coating used during production. They had been chosen as they are widely used for the production of gabion baskets available on the European market. It is worth mentioning here that soil reinforcement with gratings made of thick steel rods (6.3–12.8 mm in diameter) is used in practice in the North American market [35]. However, soil reinforcement with both gratings made of steel rods and welded steel meshes made of thinner wire (such as those used in the production of gabions) is an innovative method on a European scale.

In the series of tests, sheets with dimensions of 762 × 1676 mm, consisting of 11 tensile longitudinal wires and 23 transverse wires, are used, and in the case of the largest diameter wire mesh, also sheets with dimensions of 305 × 1676 mm, consisting of 5 and 23 wires, respectively. The smaller number of wires was the result of the limitation of the maximum force that could be applied during the test. The choice of meshes with type 2 and type 3 coatings was based on the manufacturer's guaranteed long-term protection against corrosion under the conditions prevailing in the soil or in contact with the soil. This period is 80–100 years, and even more than 120 years respectively. The types of meshes are presented in Table 1.

**Table 1.** Basic geometrical parameters of the welded meshes used in the test.

#### **3. Test Procedure**

The basic pullout type test is carried out in accordance with the EN 13738 standard [36] in a large-scale pullout apparatus with a box with internal dimensions of 1600 mm × 600 mm × 360 mm (length × width × height). The test consists of pulling the reinforcement uniformly out of the soil at a speed not exceeding 10 mm/min, at a given vertical tension (up to 200 kPa). The displacement of the reinforcement points inside the box is monitored by linear variable differential transformers (LVDT) with the corresponding value of the pullout force. Both values are recorded continuously in the computer system [37]. Originally, the pullout test was dedicated to geosynthetics, but eventually it also came to be used to study the interaction between soil and reinforcement in the form of welded or woven steel mesh. As a result, many researchers conduct tests that are called simplified ones, in a suitably modified apparatus and according to a procedure adapted to the situation (e.g., [38–41]). In this study, the authors carried out a simplified pullout test. This test is consistent with the concept of the standard method; however, the simplification is related to the way force and displacement were measured. The pullout force was set and not measured, and the

displacements were measured at the extreme nodes of the reinforcement and mounting brackets, which were not embedded in soil. Finally, the test was conducted in accordance with the following procedure:

**Figure 4.** Mesh installed on the level of the slot height in the box.

During the test, a total of four tests were carried out according to the above procedure, one for each type of welded steel mesh.

#### *3.1. Test Stand*

For this research, a stand was constructed (Figure 5). The test stand was originally intended for the tests of geosynthetics (geogrids) carried out by Kawalec [42]. Then it was adapted for pullout tests by the authors of this study. It consisted of typical pullout test equipment such as two hydraulic actuators, a force gauge, four electronic dial displacement sensors with a range of 0–100 mm, and a steel load frame (length 3260 mm × width 2160 mm), welding mesh mounting brackets, and a steel soil box with internal dimensions of 900 mm × 900 mm × 630 mm (length × width × height) and sturdy retaining trestles. The dimensions of the soil box were selected on the basis of the observations of Kim [43] and Prashanth et al. [39], who used boxes ranging from 400 to 2000 mm in length, 200 to 1100 mm in width and 100 to 1100 mm in height. The whole structure was fixed to a concrete floor, ensuring the sliding of the frame elements in the horizontal plane. The geometry of the frame, especially the position of the axis of the hydraulic actuator and the axis of the force that stretches the welded mesh in relation to the articulated joint of the frame, causes the force that stretches the mesh to be exactly twice as large as the force applied by the hydraulic actuator. Displacement sensors were applied to the clamping jaws in such a way that the average displacement for each jaw showed displacement both in the plane of the mesh itself and half the width of the mesh. This method of mounting the sensors facilitated their installation and did not require the installation of sensors inside the box with the soil.

The mounting brackets were constructed in such a way that there were two bolts on each side of each of the tensile wires of the welded mesh to twist the independent parts of the bracket. The tensile force from the bracket to the steel mesh was transmitted through a 30 × 5 mm steel flat bar, which ensured an even distribution of force across the whole width of the mesh.

The aforementioned retaining trestles made of steel channel bars allowed one of the arms of the loading frame to be anchored to the floor. Thanks to this solution, it was possible to expand only one side of the frame and thus to pull the steel mesh out of the soil in the central part of the test stand. Horizontal load was applied gradually every 3.1 kN and after each load was applied and the reading stabilizes on the jaw displacement sensors, the indication of these sensors was noted.

The vertical load was applied once to the value corresponding to the stress of approx. 60 kPa in the level of the reinforcement. The force was applied using a hydraulic actuator, which from the top rested against a very thick and durable reinforced concrete ceiling, and from below it transferred the force to the ground by means of a strongly ribbed steel cover, which ensured the even distribution of force over the entire surface of the box in the box. After the indication of the vertical force, measuring force was stabilized, the reinforcement was pulled out of the soil.

A view of the test stand is shown in Figure 5. To get as much information as possible from this test, it was also decided to install digital displacement dial sensors directly to the grid, but on the opposite side to the mounting jaws.

The test stand during the work is shown in Figure 6.

#### *3.2. Test Conditions*

The test was conducted in a dry and closed room at room temperature. The soil water content was controlled during its construction. If necessary, water was added in sufficient quantity to obtain the optimum water content. The soil was compacted manually using an electric plate compactor. An important factor to be taken into account when analysing the issue of meshes embedded in soil is the state of normal stress in the soil at the level of the tested reinforcement. The intention of the authors was to illustrate the stress induced in an embankment of approximately 3–4 m height. Using hydraulic actuator 2, a vertical stress

at the level of the reinforcing steel mesh of about 60 kPa was obtained, which meets the assumptions adopted by the authors of this study.

(**a**)

(**b**)

**Figure 5.** Schematic of the test stand for pulling meshes embedded in the soil: (**a**) top view; (**b**) longitudinal section.

**Figure 6.** The test stand for pulling meshes embedded in the soil.

#### **4. Results and Discussion**

During the test, the test with the Zn + PVC 3.20 mm mesh resulted in the destruction of the mesh in the form of its tearing outside the box with the soil. In other cases, ejection of the reinforcement from the fine sand was achieved. For easier interpretation, the results obtained from the test were converted to unit width reinforcement, and these are presented in Figure 7.

**Figure 7.** Force–displacement diagram from a simplified pullout test for fine sand.

On the basis of the test results, it can be unequivocally stated that in the case of the Zn + PVC 3.20 mm mesh, the key parameter is its low strength, as it was the destruction of the mesh that caused the termination of the test (Figure 8). In the case of the remaining three meshes, the condition for termination of the test was that the load bearing capacity of the soil reinforcement system was reached (i.e., the mesh ejection from the soil).

**Figure 8.** Damage of Zn + PVC 3.20 mm mesh after the pullout test—large displacements of the wires that were broken during the test are visible (the photo is greyed out for better visibility of the mesh against the floor).

The approximate values of the unit force necessary to damage the soil reinforcement system during a pullout test were also determined, and the results are presented in Table 2.

It has also been shown that as the stiffness of the reinforcement increases, the pullout resistance of this reinforcement increases as well, which is consistent with the observations of Yuan and Chua [44].

The values obtained for fine sand are similar to those presented in [44–47]. The authors of [44–47] also carried out similar tests in sands (or soil containing a lot of sand fraction in the case of [46]), with similar values of normal *σ*n (the overall range is 2–140 kPa), but for different types of reinforcement, proving the validity of the conducted test. A summary of the test conditions is presented in Table 3.

Furthermore, the decrease in the displacements along the length of the reinforcement embedded in soil during pullout tests of whole meshes embedded in soil medium was analysed. In the tests performed in fine sand, it was found that the displacements recorded on the sensors behind the box (BACK) were smaller than those recorded on the sensors at the force application side (FRONT) by about 30–40%, as presented in Figure 9.

**Table 3.** Summary of selected test conditions.

**Figure 9.** Recorded displacements at the front and back of the box for: (**a**) 4.30 mm Zn + PVC mesh; (**b**) 3.20 mm Zn + PVC mesh; (**c**) 4.50 mm ZnAl mesh; (**d**) 3.00 mm ZnAl mesh.

The relationship described above clearly indicates that the type of mesh affects the anchorage length in the passive zone. On the basis of the studies of Teerawattanasuk et al. [48], it is known that the displacements along the length of the reinforcement decrease in a curvilinear manner with increasing distance from the face of the tested structure. As the authors of this study only have the value of this displacement at two points, assuming its rectilinear slope, the anchorage length (for the maximum pullout forces applied in the tests) for the fine sand tested is approximately 2.3 to 3.0 m. These lengths are certainly underestimated, as the displacement value does not decrease in a linear way. Unfortunately, due to the lack of a third measurement point along the length of the reinforcement, the exact course of the curve describing this phenomenon cannot be determined.

It is worth noting that, on the basis of the research carried out, it is possible to easily determine the value of the apparent friction coefficient *μk* using Relationship (1):

$$
\mu\_k = \ T\_{\max} / (\mathbf{2} \cdot \mathbf{S} \cdot \sigma\_v),
\tag{1}
$$

where:

*Tmax*—a maximum pulling force [kN] (according to Table 2),

*S*—a mesh area [m2] (0.34 m2 for ZnAl, 4.50 mm and 0.75 m2 for others),

*σv—*a vertical stress at the level of the reinforcing steel mesh [kPa] (60 kPa in test).

The summary of the friction coefficients between the reinforcement and the soil *μk* calculated according to the above formula is presented in Table 4. The obtained values of the friction coefficient 0.39–1.47 are similar to those given by Lajevardi et al. [45], who reported values of the coefficient *μk* equal to 0.31–0.72. It is worth noting that in this study, for a steel mesh with a wire diameter of 4.5 mm, the friction coefficient (*μk* = 1.47) is much greater than that of Lajevardi et al. [45] for a steel grid with a wire diameter of 12 mm (*μk* = 0.72). This can be explained by the fact that the friction coefficient is determined not only by the diameter of the mesh wire (the larger it is, the greater the coefficient of friction is), but also the mesh opening (the smaller it is, the greater the friction coefficient is, because more wires are located on the same surface across the force action). In this study, we used 76.2 × 76.2 mm mesh opening, while Lajevardi et al. [45] examined grids with the mesh opening from 125 × 140 mm to 125 × 505 mm. On the other hand, Bergado et al. [46] showed coefficients of friction equal to *μk* = 1.04–1.78 for welded meshes with a wire diameter of 6.3–12.7 mm and a mesh opening from 152 × 152 mm to 152 × 225 mm. At the same time, it was confirmed that the coating type of the mesh wire affects the friction coefficient. The zinc-coated wires of the same diameter (or similar) to the polyvinyl chloride (PVC)-coated ones are characterized by a higher coefficient of friction, which is consistent with the results of the hexagonal wire mesh studies realised by Teerawattanasuk et al. [48]. By comparison, Bergado and Teerawattanasuk [47] demonstrated for woven meshes (with a similar geometry as in Lajevardi et al. [45] and Bergado et al. [40]) a coefficient of friction equal to *μk* = 0.2–0.30. This clearly shows that the welded meshes are more suitable for soil reinforcement than the commonly used woven meshes.

**Table 4.** Coefficient of friction between reinforcement and soil *μk* based on the presented tests.

#### **5. Conclusions**

This study deals with the interaction of welded steel mesh, used for the production of gabions, with the soil depending on (1) the soil type (in this case fine sand FSa) and (2) the mesh parameters. The tested meshes (four types) were differentiated with respect to the diameter of the steel wire (2.7–4.5 mm) as well as the type of anticorrosion coating (Zn + PVC or ZnAl). Each of them had the same opening size: 76.2 × 76.2 mm. For the analysis, a proprietary stand was constructed (based on the geogrid test stand) to carry out a simplified pullout test.

Undoubtedly, it can be concluded that it is possible to make a retaining structure using soil reinforcement technology using steel welded meshes with a much smaller diameter (3.0, 3.8 and 4.5 mm) than 6–12 mm, used for the production of gabion baskets. This is the basic premise for the application of sustainable geotechnical principles in practice.

To confirm the above statement, a trial design replacement of the retaining structure with an extension of the abutment was performed using the parameters obtained during the tests described in this study. Originally, the retaining wall was designed using soil reinforced with flat steel bars with a facing of precast concrete elements. The wall has a height of 9.50 m from the top of the foundation to the highest point of the backfill. The replacement design of this retaining wall assumes that it will be made of gabions of a welded mesh made of Zn + PVC 4.30 mm wire. The facing made of gabion baskets is vertical. Facing basket reinforcement 1.0 m thick and 9.0 m long were adopted at a spacing of 0.75 m between the three lowest reinforcement layers and 1.0 m between the higher layers of soil reinforcement. A detailed description of the design process, the solutions adopted, and the geometry of the structure is presented in [29,49]. It is hereby shown that gabion retaining structures with welded meshes made using reinforced soil technology are possible for use in infrastructure construction applications.

Furthermore, on the basis of the conducted tests and the analysis of the obtained results, it can stated that the low deformability (hence greater stiffness) of the tensile welded meshes embedded in soil allows them to be used for soil reinforcement in all types of reinforced soil structures, including the abutments of engineering facilities (the maximum deformation in the elastic work phase was only equal to 0.35%). In the case of woven meshes, this type of use is not so obvious.

Another valuable observation is the dependence whereby the mesh with higher stiffness (e.g., made of a thicker wire, with smaller opening or stiff welded node), during the pullout test, obtained higher resistance to pulling from the soil compared to a less stiff mesh. This is a favourable phenomenon from the point of view of the use of retaining structures. The reinforcement effect can be explained by the favourable phenomenon of mesh wedging between soil grains. Taking into account that fine sand was considered in this study, it may be assumed that the effect will be greater in soils with a larger grain size. The authors plan to continue the initiated series of studies using other soils.

In addition, the obtained results are a good basis for further model tests both in the laboratory and in situ on welded meshes in function of reinforcing soil. The main objective is to propose all coefficients necessary for the design of the engineering structures mentioned above in the future. In this study, the first of such parameters was determined the apparent friction coefficient *μk*. The next ones require more advanced research.

**Author Contributions:** Conceptualization, methodology, M.J. and M.C.; investigation, validation, ´ research tests, M.C.; formal analysis, M.J. and M. ´ C.; writing—original draft preparation, M.J. and ´ M.C.; visualization, writing—review and editing, M.J.; funding acquisition, M.J. All authors have ´ read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding. The APC was partially funded by the professor's grant No 03/070/RGP19/0125 awarded by the Rector of the Silesian University of Technology (Gliwice, Poland).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study is available in the source materials included in the References.

**Acknowledgments:** The authors would like to express their gratitude to the company BETAFENCE Sp. z o.o. which delivered the steel welded grids for this study free of charge, and the company CzekAmet PL Sp. z o.o. Sp. k. delivered clamping jaws for this study free of charge.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


*2.7. Biochemical Determinations: Salivary Amylase Activity and IL-1*β *Concentration*

The salivary amylase activity (EC <IP_ADDRESS>) was assessed spectrophotometrically using an alkaline solution of 3,5-dinitrosalicylic acid (DNS). The absorbance of samples was measured at 540 nm, accompanying the increased concentration of reducing sugars released during the hydrolysis of starch catalyzed by salivary amylase. Salivary amylase activity was determined in duplicate samples and expressed in μmol/mg protein [23].

Salivary interleukin 1β (IL-1β) concentration was determined by ELISA using a commercially available kit from EIAab Science Inc. Wuhan (Wuhan, China) in accordance with the manufacturer's instructions provided in the package.

#### *2.8. Biochemical Determinations: Redox Assay*

All analyses were performed in duplicate samples. Absorbance and fluorescence were measured with Infinite M200 PRO Multimode Tecan microplate reader. The results were standardized to 1 mg of total protein. The content of protein was evaluated by the bicinchoninic method (BCA) using a ready-made reagent kit (Thermo Scientific Pierce BCA Protein Assay Kit, Rockford, IL, USA) and a bovine serum albumin standard (BSA).


**1. Introduction**

Hip fractures (HF) are a frequent problem in elderly patients, and are related to osteopenia and osteoporosis. Around 1.6 million patients suffer from HF per year [1], and by 2050, the global incidence is believed to become 4.5 million [2]. Reduced bone density, female sex (female/male ratio greater than 2/1 in those over 50 years of age), low weight and reduced physical activity are main risk factors for HF [1,2]. The mortality rate among patients who suffer a hip fracture is 5–10% one month after the fracture and 20–30% in the first year [1,2].

Hip fractures can be classified as intracapsular or extracapsular, and the latter are subdivided into basicervical, intertrochanteric/pertrochanteric and subtrochanteric. Up to half of such fractures are intertrochanteric, usually occurring in elderly patients as a result of low-energy trauma [3]. The main treatment of these trochanteric fractures is surgery, which can be extramedullary or intramedullary. Previously, extramedullary treatment with sliding hip screw (SHS) was the most indicated, but some studies showed that nailing gave better fracture fixation results for uncommon trochanteric fractures, especially subtrochanteric

**Citation:** Méndez-Ojeda, M.M.; Herrera-Rodríguez, A.; Álvarez-Benito, N.; González-Pacheco, H.; García-Bello, M.A.; Álvarez-de la Cruz, J.; Pais-Brito, J.L. Treatment of Trochanteric Hip Fractures with Cephalomedullary Nails: Single Head Screw vs. Dual Integrated Compression Screw Systems. *J. Clin. Med.* **2023**, *12*, 3411. https://doi.org/ 10.3390/jcm12103411

Academic Editor: Moshe Salai

Received: 18 March 2023 Revised: 29 April 2023 Accepted: 1 May 2023 Published: 11 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fractures, so the use of nailing has dramatically increased even though there is no evidence that it is superior to the SHS in a simple intertrochanteric pattern [3].

Some of the most-used intramedullary nailing alternatives in our region are the Gamma3 nail, PFNA (Proximal Femoral Nail Antirotation) and TRIGEN InterTAN. The latter differs from the others in that it offers the possibility of two cephalo-cervical screws that provide linear compression and additional resistance to the rotation of the femoral head, while the others use a single screw [4,5]. Several studies and meta-analyses have compared the use of these methods of treatment without reaching a definitive conclusion on which fixation method is most appropriate to reduce complications and improve prognosis [6–17]. Independent risk factors for early mortality already reported in the literature are: male sex, dependence on others for the basic activities of daily living, American Society of Anesthesiologists (ASA) score > 2, older age and medical complications occurring while an inpatient [18]. Fracture stability also plays a key role in the prognosis for these patients, not only influencing early device failure requiring reoperation within 12 months, but also increasing the rate of mortality after trochanteric hip fractures by up to 1.6 times [19]. Studies such as Chehade et al. also describe an increase in early osteosynthesis failure associated with the use of double lag screw systems [19]. Attending to only unstable hip fractures, such as subtrochanteric fractures, Panteli et al. identified six risk factors associated with reoperation: age < 75 years old, pre-injury femoral neck shaft angle, choice of nail, varus reduction angle, fracture-related infection and non-union. The addition of a proximal anti-rotation screw did not confer any benefit [20] in terms of reoperation or survival rates. Figures 1 and 2 show typical cases of pertrochanteric hip fracture treated with an endomedullary nail.

**Figure 1.** Pertrocantheric hip fracture treated with a short Gamma3 nail.

**Figure 2.** Pertrocantheric hip fracture treated with a short InterTAN nail (letter D for right lower limb).

For a time, in our service, we had the impression that systems with two cephalocervical screws had a higher rate of reoperations. As such, the aim of this study is to retrospectively review major post-surgical complications as they relate to the type of nail used, comparing single head screw nails to dual integrated compression screw nails, with nails being implanted at a third-level hospital. The hypothesis of our study is that cephalomedullary nailing with double head screw systems present a greater number of post-surgical complications compared to single screw systems.


**4. Typical optoelectronic scanners signals**

Different shapes of signals are generated during an optical scanning process, depending on the kind of light source and the sensor of the scanner. Some precision semiconductor optical sensors like CCD or PSD produce output currents related to the "centre of mass" of light incident on the surface of the device See [18]. All light registered by the CCD or PSD origi‐ nates an ideal signal shape as shown in figure 17(Image credit: Measurecentral.com).

A typical position measuring process includes an emitter source of light, as a laser diode or an incoherent light lamp and the position sensitive detector like CCD or PSD as a receiving device, which collects a portion of the back-reflected light from the target. The position of the spot on the PSD is related to the target position and the distance from the source, see[19].However, the real photon distribution on the sensor depends on the characteristic di‐ mensions related to the diffraction pattern of the light in the space. Common examples of signals generated by the light registered on a CCD camera appear in a study about superresolution by spectrally selective imaging, shown in figure 18 (Image credit: A.M. van Oijen and J.Köhler).

A Method and Electronic Device to Detect the Optoelectronic Scanning Signal Energy Centre http://dx.doi.org/10.5772/51993 405

**Figure 17.** Ideal photon distribution on CCD and PSD sensors.

more accurate and faster than CCD because the PSD is a continuous sensor, while CCD is a matrix of dots switched on and off and its resolution depends on how many dots are located

PSD has an infinite resolution because it is a continuous sensor, therefore the digital resolu‐ tion of a PSD depends not on the PSD itself. Alignment sensors using CCDs have to be pro‐ grammed to do multiple measurements at every step to improve accuracy and to lower noise because linear CCDs have a low resolution. To have the same accuracy of a PSD, CCD should perform no less than 32 measurements and hence calculate the average measure‐ ment. However, CCD is generally preferred to PSD because PSD needs an expensive circuit

Different shapes of signals are generated during an optical scanning process, depending on the kind of light source and the sensor of the scanner. Some precision semiconductor optical sensors like CCD or PSD produce output currents related to the "centre of mass" of light incident on the surface of the device See [18]. All light registered by the CCD or PSD origi‐

A typical position measuring process includes an emitter source of light, as a laser diode or an incoherent light lamp and the position sensitive detector like CCD or PSD as a receiving device, which collects a portion of the back-reflected light from the target. The position of the spot on the PSD is related to the target position and the distance from the source, see[19].However, the real photon distribution on the sensor depends on the characteristic di‐ mensions related to the diffraction pattern of the light in the space. Common examples of signals generated by the light registered on a CCD camera appear in a study about superresolution by spectrally selective imaging, shown in figure 18 (Image credit: A.M. van Oijen

nates an ideal signal shape as shown in figure 17(Image credit: Measurecentral.com).

on the sensor. Typically a linear CCD has 1024 or 2048 dots.

design including Analogue-to-Digital conversion [17].

**4. Typical optoelectronic scanners signals**

**Figure 16.** PSD Operating principle.

404 Optoelectronics - Advanced Materials and Devices

and J.Köhler).

**Figure 18.** Real photon distributions as a function of the detector for different diffraction patterns.

Based on A.M. van Oijen and J.Köhler study, we can observe that the spatial distribution function of light has an Airy-function-like shape, see [20]. It is well known that CCD, CMOS and SPD use the light quantity distribution of the entire beam spot entering the light receiv‐ ing element to determine the beam spot centre or centroid and identifies this as the target position. However, they are not the only sensors that generate a similar Gaussian-like shape, there are still a lot of sensors to be further investigated. For example, a simple photodiode can also originate a similar Gaussian-like shape, when it is used as a sensor on a scanner with a rotating mirror, [21]. Figure 19 below illustrates a hypothetical spot model, and at‐ tempts to explain how the signal is created by the photodiode on a scanner with a rotating mirror.

tre of the signal. In the following section, we propose a new method and its respective cir‐

A Method and Electronic Device to Detect the Optoelectronic Scanning Signal Energy Centre

http://dx.doi.org/10.5772/51993

407

**Figure 20.** Scanning sensor Gaussian like shape, measurements at different angular positions of the light source.

In the previous sections, we described different sensors and scanners that produce output currents related to the "the centre of mass" of the light incident in the surface of the device. In this section we will compare some techniques to find the energy centre of the signal and

Peak Signal Algorithms are simple statistic algorithms for non-normally distributed data series [23] to find the peak signal through threshold criteria statically calculated [23]. The al‐ gorithms which identify peaks in a given normally distributed time-series were selected to be applied in a power distribution data, whose peaks indicate high demands, and the high‐ est corresponds to the energy centre. Each different algorithm is based on specific formaliza‐ tion of the notion of a peak according to the characteristics of the optical signal. These algorithms are classified as simple since the signal does not require to be pre-processed to smooth it, neither to be fit to a known function. However, the used algorithm detects all peaks whether strong or not, and to reduce the effects of noise it is required that the signal-

**5. Signal processing methods to locate signal energy centre**

**5.1. Time-series simple statistics algorithms for peak detection**

to-noise ratio (SNR) should be over a certain threshold [23]:

eventually discuss their advantages.

cuit to find the centre of the signal by an optical scanner.

**Figure 19.** Principle of electrical signal formation during rotational scanning.

In this case, the signal created, as a similar Gaussian-like shape, goes up(Fig. 19, a) and falls down (Fig. 19, e), and a fluctuating activity takes place around its maximum area in figs. 19 (b-d). As we mentioned before, in a real practice the signal becomes noisy, see [22]. The ex‐ periment recently developed by Rivas M. and Flores W., with the scanner shown in Figure 6, for angular position measuring and using an incoherent light source and a simple photo‐ diode, validated the model shown in Figure 20. During experimentation, it has been ob‐ served that the optoelectronic scanning sensor (photodiode) output is a Gaussian-like shape signal with some noise and deformation. This is due to some internal and external error sources like the motor eccentricity at low speed scanning, noise and deformation that could interfere with the wavelength of the light sources. Other phenomena could also affect, though, such as reflection, diffraction, absorption and refraction, producing a as seen in Fig‐ ure 20.

As we can see, the photodiode signal originates a similar function to a CCD, consequently, it is possible to enhance the accuracy measurements in optical scanners with a rotating mirror, using a method for improving centroid accuracy by taking measurement in the energy cen‐ tre of the signal. In the following section, we propose a new method and its respective cir‐ cuit to find the centre of the signal by an optical scanner.

**Figure 20.** Scanning sensor Gaussian like shape, measurements at different angular positions of the light source.


3. Theory and working principles

In the use of optical fibres for temperature sensing, measurements can be conducted using both intrinsic and extrinsic techniques. The most common technique for assessing temperature changes using optical fibres is the use of interferometry. Examples being fibre Bragg gratings (FBGs), extrinsic Fabry-Perot interferometers (EFPIs), Mach-Zehnder interferometers (MZIs) and Sagnac interferometers (SIs). As the fibre requires a light source to conduct the measurement, various types have been explored such as; broadband light source [26] monochrome laser [21] and swept laser [27]. Measurements can be carried out by observing either the transmitted spectrum, or the reflected spectrum. When recording the reflected spectrum losses occur in the back reflections through the fibre and at the coupler between the light source and spectrometer. That said however, this method allows

for the fibre to be used as a point measurement device in applications where transmission is impractical, i.e. biomedical [28], automotive [29], and pharmaceutical [30] sensing. As the modus operandi of any particular optical fibre sensor is modulation of the light source, this can be carried out via intensity, frequency, or phase modulation. The latter of which will be the main focus of the review as this is the method typically employed by LiF-OFTS.

## 3.1 Fibre Bragg gratings

2.1 Construction

Applications of Optical Fibers for Sensing

2.2 Measurement stability

environment.

98

2.3 Distributed sensing

temperature sensing performance [25].

3. Theory and working principles

Commercially available silica fibres are widely used in the development of optical fibre temperature sensors owing to their small cross-sectional area and consequentially their ability to be implemented in restricted areas. However, the relatively low cost of silica fibre compared to more exotic fibres such as those manufactured from fluoride or synthetic sapphire may be a factor to consider. As is reported, optical fibre sensors are particularly suited to environments where an electronic sensor may not have sufficient protection from EMI, and where line of sight to the measurement point is obstructed [21]. Furthermore, optical fibre sensors do not represent a potential source of ignition in an explosive environment; can be biocompatible; and can be made to work over very long distances. To date, optical fibre temperature sensors have been implemented in applications ranging from in-vivo biomedical sensing [22], to structural and geothermal engineering [23].

In civil engineering applications, such as bridges, use of chirped fibre Bragg gratings (FBGs) provide excellent through life performance as fibre degradation is in excess of 25 years [24] with data transmission losses being minimal [23]. Furthermore, stability of measurements are generally quite good due to performance being driven predominantly by the wavelength stability of light source used. This being an easy factor to account for, as said light source may be kept in a controlled

In addition to the EMI resistance and dimensional advantages of using optical fibre sensors, their ability to provide multipoint sensing with minimal use of fibre is a desirable characteristic. One such method is the use of chirped FBGs whereby multiple FBGs are inscribed on a single fibre, with the grating period modified at each location thus providing high spatial resolution. Wave division multiplexing (WDM) is another commonly used method in multi-point sensing with optical fibres, however this method narrows the usable bandwidth of light proportionally to the number of fibres used. Historically, Raman scattering has been used as an efficient means of multi-point temperature measuring; however, Brillouin sensors reported have shown exceptional strain measurement capability for an equivalent

In the use of optical fibres for temperature sensing, measurements can be conducted using both intrinsic and extrinsic techniques. The most common technique for assessing temperature changes using optical fibres is the use of interferometry. Examples being fibre Bragg gratings (FBGs), extrinsic Fabry-Perot interferometers (EFPIs), Mach-Zehnder interferometers (MZIs) and Sagnac interferometers (SIs). As the fibre requires a light source to conduct the measurement, various types have been explored such as; broadband light source [26] monochrome laser [21] and swept laser [27]. Measurements can be carried out by observing either the transmitted spectrum, or the reflected spectrum. When recording the reflected spectrum losses occur in the back reflections through the fibre and at the coupler between the light source and spectrometer. That said however, this method allows

Fibre Bragg gratings are created by periodically modifying the refractive index of an optical fibre core. At each change of refractive index the reflected light constructively interferes producing a high intensity narrowband signal. This effect is described by Eq. (2) where λB is the Bragg wavelength, neff is the effective refractive index, and Λ is the pitch between each of the modified refractive indices. Figure 1 provides a schematic of an FBG inscribed on a fibre core. As is evident from Eq. (2) care must be taken to eliminate, or account for, mechanical straining of the fibre as this will artificially modify the grating period. Once mechanical strain has been determined the change in Bragg wavelength with temperature is given by Eq. (3). A review on the packaging of FBGs is provided by Hong et al. [31].

$$
\lambda\_B = 2n\_{\rm eff} \cdot \Lambda \tag{2}
$$

$$\frac{d\lambda\_B}{dT} = 2\left(n\_{\ell\overline{\ell}} \cdot \frac{d\Lambda}{dT} + \Lambda \cdot \frac{dn\_{\ell\overline{\ell}}}{dT}\right) \tag{3}$$

Owing to the simple nature of their construction FBGs have been utilised to great success as a means of sensing temperature, however they are not without inherent issues such as damage due to exposure to excessive temperatures [22], and grating orientation to the heat source [32]. Zhang et al. [22] reported the assessment of cylinder head temperature and mixture flow within maritime diesel engines, where significant steps were taken to protect the fibre coating from excessive cylinder head temperatures (873.15 K). Gassino et al. [32] examined the use of FBGs in the presence of strong temperature gradients (�10 K/cm) during thermal ablation of tumours. Several key design factors were discussed from which it was determined the largest sources of error were caused by the temperature gradient along the length of the FBG, and FBG orientation with respect to the temperature gradient.

Figure 1. Schematic of FBG, highlighting grating pitch (Λ) in fibre core.


**Appearance-Based Retrieval for Tracked Objects in Surveillance Videos**

Thi-Lan Le1, Monique Thonnat2 and Alain Boucher3 *1MICA Center, HUST - CNRS/UMI 2954 - Grenoble INP, Hanoi, 2PULSAR, INRIA Sophia Antipolis, 3IFI, MSI Team; IRD, UMI 209 UMMISCO; Vietnam National University, 1,3Vietnam 2France*


*Article* **Lightweight Equipment Using Multiple Torches for Fast Speed Asphalt Roofing**

#### **Alberto Barragán-García 1,2, Miguel Fernández-Muñoz 1 and Efrén Díez-Jiménez 1,\***

Received: 7 April 2020; Accepted: 20 April 2020; Published: 2 May 2020

**Abstract:** In this work, we describe the design and test of a new piece of equipment, developed in order to enhance speed, gas consumption and safety during the manual asphalt roofing process. The novelty of the equipment is based on the use of a set of five parallel gas burners located in front of the roll to maximize heat transfer. The equipment is light and can be used by any worker on any type of roof. It also includes a thermal insulation cover to significantly reduce gas consumption and, thus, to reduce CO2, SO2, and other non-eco-friendly emissions. In this paper, we present the mechanical and thermal design and analysis of the equipment, Computer Fluid Dynamics (CFD) simulations for heat transfer calculation, a description of the manufacturing and assembly, a preliminary thermal test, and an operational test. The results demonstrate an installation speed of 1.75 m2/min, for 3 kg/m2 rolls, which translates to around 700–735 m2 per person per day, more than twice the usual manual roofing rate. Nevertheless, some issues need to be resolved, such as the nonuniform heat distribution and the low heat transfer at the end of the roll installation.

**Keywords:** fuel burners; asphalt roofing; heating equipment


**4.9 Cultural and religious considerations**

Christianity, Islam and African traditional religion are the major faiths in SSA. Interplay of faith, religion and cultural attitudes and their relationship with views on organ donation is complex. Response to illness as God's will negates organ

donation or reception. Belief in resurrection and reincarnation precludes organ donation since the 'new body' may have some missing parts. Desecration of the body of the deceased is reported as a factor prohibiting family members from donating body parts of their deceased relatives.


*Edited by Wilbert S. Aronow*

The latest diagnostic and therapeutic modalities in the management of coronary artery disease by coronary artery bypass graft surgery and by percutaneous coronary intervention with stenting and in the interventional management of other atherosclerotic vascular disease have led to a reduction in cardiovascular mortality and morbidity. This book entitled Artery Bypass provides an excellent update on these advances which every physician seeing patients with atherosclerotic vascular disease should be familiar with. This book includes 27 chapters written by experts in their topics.

ISBN 978-953-51-1025-5

Artery Bypass

Photo by MickeyCZ / iStock


**Wenming Jiang**

Wenming Jiang, Ph.D., professor and doctoral supervisor for the State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China, was selected as the most beautiful scientific and technological worker in the national Foundry Industry in 2021 and the scientific and Technological Innovation (double innovation) talent in Jiangsu Province. He served as the director of the National Magnesium Alloy Society, the director of the National Casting Society, and the Executive Deputy Secretary general of the National lost mold Casting Committee. He has presided over more than 20 scientific research projects, such as the National Natural Science Foundation, the National Key Research and Development Plan, and the National Key Research Plan for Basic Strengthening. He has published more than 150 academic papers, applied for/authorized 35 invention patents, and participated in the compilation of five books/textbooks. He is an Editorial Board Member of journals such as *Materials*, *Metals*, *China Foundry*, *Casting*, and *China Foundry Equipment and Technology.* He has mainly carried out research on magnesium, aluminum alloys, and their precision casting forming theory and technology; 3D printing rapid casting technology; and green casting theory and technology research.


**Author details**

Farhad-Ali1,2\* and Nadeem Ahmad-Sheikh3

\*Address all correspondence to<EMAIL_ADDRESS>
1-Computational Analysis Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam-

2-Faculty of Mathematics and Statistics, Ton Duc Thang University, Ho Chi Minh City, Vietnam-

3-Department of Mathematics, City University of Science and Information Technology, Peshawar, Khyber Pakhtunkhwa, Pakistan-


**3.1. Preparation for model input data**

The image was classified into seven land use/land cover classes, e.g., conifer forest, scrub forest, agriculture land, rangeland, bare soil, built-up land, and water bodies. The image classification was undertaken using the maximum likelihood rule, which provides reliable classification results [26]. Rainfall-runoff model SWAT was calibrated for the target watershed. The statistical measures, such as coefficient of determination (*R2* ) and Nash-Suttchiffe Simulation Efficiency (*Ews*), were used to evaluate model prediction. The *R2* value is an indicator of strength of relationship between the observed and simulated values, while the latter coefficient indicates how well the plot of observed versus simulated value fits the 1:1 line.


*2.4. ompR and marA Effect on Porin Response under Antibiotic Stress*

Next, we investigated the contribution of transcriptional factors in the regulation of porin genes.

The best-described input into controlling porin levels involves OmpR, which regulates *ompC* and *ompF* directly through OmpR–promoter DNA association [41]. The OmpR/EnvZ two-component system may play a major role in antibiotic resistance by modulating porin expression [42]. It was shown that OmpR deficiency in *Y. pseudotuberculosis* and *Y. enterocolitica* reduces bacterial sensitivity to the β-lactams, tetracycline, and polymyxin B, caused by the lack of OmpF and OmpC porins [43,44].

qRT-PCR analysis revealed differential expression of *ompR* in *Y. pseudotuberculosis* depending on the duration of antibiotic stress (Figure 3). Short exposure to antibiotics excluding carbenicillin and tetracycline at 27 ◦C slightly reduced (1.7- to 2.3-fold) its transcription associated with the downregulation of porin genes. On the contrary, prolonged antibiotic stress increased the level of *ompR* transcripts up to 5.3-fold; however, this did not affect the expression of *ompF* and *ompC*. A similar observation was described in the work of Viveiros et al., where the response of the *ompC* and *ompF* genes, despite the increased expression of *ompR*, remained comparable to their levels in *E. coli* cells not exposed to tetracycline [24]. Moreover, it was shown that OmpR positively controls the expression of the AcrAB-TolC efflux pump involved in the adaptive response of the *Y. enterocolitica* strain to different chemical stressors [45]. It is tempting to speculate that a high level of *ompR* transcription in *Y. pseudotuberculosis* may be required to induce this efflux pump system as another general defense mechanism against antibiotics.

Multiple studies have shown that general porin expression is post-transcriptionally regulated by global regulator MarA, involved in multidrug resistance response during antibiotic stress [46,47]. *marA* locus can directly or indirectly increase the level of *micF*, a non-coding RNA stress response gene, causing a post-transcriptional downregulation of *ompF* mRNA and reducing the OmpF synthesis [47]. Two MarA-like proteins have been reported in *Yersinia pestis*: MarA47 and MarA48 [48,49].

To test their involvement in the regulation of *Y. pseudotuberculosis* porin expression under short- and long-term exposure to antibiotics, we measured the levels of *marA47* and *marA48* mRNA using qRT-PCR (Figure 3). We found different activities of *marA* transcripts depending on the time of antibiotic exposure. A short treatment of cells with the antibiotics caused upregulation of at least one *marA* gene, except for chloramphenicol, which did not affect the *marA* expression. In the case of long-term exposure to the tested antibiotics, the level of *marA* expression remained comparable to those of the control samples.

The obtained results led us to conclude that regulation of *Y. pseudotuberculosis* general porins mediated by MarA was observed only for early antibiotic stress caused by sublethal concentrations of tetracycline, kanamycin, and carbenicillin. During long-term antibiotic presence, *marA* transcription remained unaltered and therefore MarA did not play a significant role in porin regulation at the post-transcriptional level.

**Figure 3.** Relative expression levels of *Y. pseudotuberculosis ompR*, *marA47*, and *marA48* under antibiotic stress detected by qRT-PCR. (**A**) 1 h antibiotic exposure, (**B**) 16 h antibiotic exposure. All results are expressed as mean ± standard deviation from three independent experiments. An asterisk indicates *p*-value < 0.05 vs. respective untreated control. Significance was calculated using one-way ANOVA. Km—kanamycin, Tet—tetracycline, Cb—carbenicillin, and Cm—chloramphenicol.


**1. Introduction**

In late 2019, a novel coronavirus, caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) emerged from Wuhan, China [1]. This virus leads to the coronavirus disease 2019 (COVID-19), currently causing a pandemic with worldwide severe economic and healthcare consequences [2]. Among the most dominant clinical characteristics are fever, cough and fatigue, whereas gastrointestinal symptoms are rather uncommon. Critical disease conditions may be caused by severe and sustained systemic inflammatory responses (hyperinflammation or "cytokine storm") and a cytopathic effect, leading to acute respiratory distress syndrome (ARDS), septic shock, metabolic acidosis that is hard to correct, coagulation dysfunction and multiple organ failure. The exact viral host factors that influence the pathogenesis are still being investigated. The SARS-CoV-2 infection binds to human host cell receptors, using human angiotensin-converting enzyme 2 (hACE2), although there are more factors influencing susceptibility to infection and disease progression. Elderly people (>65 years of age) with underlying diseases such as hypertension, chronic obstructive pulmonary disease (COPD), diabetes and cardiovascular disease seem more susceptible to an infection and prone to serious outcomes of this viral disease [3]. Since the spread of SARS-CoV-2 around the world, there have only been a few case reports describing COVID-19 in patients after solid organ transplantation (SOT).

We present a case of COVID-19 in a lung transplant (LTX) recipient presenting with fever and gastrointestinal symptoms in our emergency department.