a plurality of at least two plate or panel - type members are formed with edge portions which hook together to define a key - forming passage therebetween , with tongue - and - groove type interlocks directed laterally from the elements defining both sides of the passage . the passage is filled with initially flowable , solidified key - forming material injected therein under pressure sufficient to cause the injected material to flow along and fill all , or one or more selected portions of the passage . the elements of the panels defining such passage are so formed that the pressurized , injected material tends to expand the passage , and thereby urge adjacent edges of the two panels toward abutting relation with each other , whereby further expansion of the passage is prevented . upon solidifying , the injected material comprises a key , which interlocks the panels and seals the joint therebetween .