16 the volume enclosed may be immense with the wall encircling such surface features as buildings 21 and trees 20 and the other major surface features the top of the wall 16 is not seen at the gound level 17 since a space 18 is left between the top 19 of wall 16 and ground level 17 to allow for normal water filtration and ordinary conditions to exist for adjacent trees 20 and buildings 21 this is the usual condition but under other circumstances it may be desirable for the wall to extend to the surface or above it as explained below in forming wall 16 the voids in the native soil are filled by injecting grout into the voids thus creating an impervious membrane or wall the thickness of which may vary from a few inches to several feet depending upon the economics of balancing labor time and materials or the physical composition of the soil or matter into which the grout is to be injected the grout may be in the form of a clay any one of a number of commercially available grouts or a combination of two or several of these the material forming tank wall 16 may be pressure grouted and jelled into place in addition to commercial grouts such as tar asphalt 34 am 9 34 obtainable from union carbide corp clays lime degradable pulps concrete flexiblized with rubber such as neoprene natural rubber and the like one may use synthetic polymeric materials as for example the.