In the 1930s, the firm designed the first bolted concrete tunnel linings, for the London Passenger Transport Board. The new tunnel linings were used on the Ilford extension of the Central line between Redbridge and Newbury Park. MHA designed a road tunnel at Dartford and supervised the construction of its pilot tunnel beneath the Thames, but preparations for construction of the full-size tunnel were stopped in 1931 due to economic difficulties. G L Groves became a partner in 1933, but it was decided to leave the name of the company unchanged.