pumped into the trench 48 to prevent the walls of the trench from collapsing preferably the bentonite is a high swelling wyoming type sodium based bentonite consisting mainly of montmorillonite and the slurry contains a concentration of from about 3 to 6 bentonite in water the concentration of bentonite will vary depending upon the type of ground to be excavated as is understood in the art the slurry is formed by mixing bentonite and water at high shear as is well known when excavation has been completed i e when the depth of the trench is substantially equal to the desired height of the vertical wall the trench bottom is cleaned the trench preferably has a depth substantially equal to the height of the wall to be formed then concrete footing 51 is placed in the bottom of the trench preferably using the tremie method as shown in fig4 d to a depth at which a small portion e g 1 to 3 ft of the bottom edge of the pre cast panel will be embedded in the concrete when the panel is in place in the trench before the concrete has set pre cast panels 16 are then inserted into the trench.