Piezometers

a. Подпись: MOVEMENT OF BACKFILLING OPERATION LEADING EDGE OF ТРСМГи СЇГ Д/ДТІПМ U.S. Army Corps of Engineers
Installation. Piezometers are installed to deter­mine the elevation of the groundwater table (gravity or artesian) for designing and evaluating the perfor­mance of a dewatering system. For most dewatering applications, commercial wellpoints or small screens are satisfactory as piezometers. The selection of well – point or screen, slot size, need for fdter, and method of installation is the same for piezometers as for dewater­ing wellpoints, Holes for the installation of piezomet­ers can be advanced using continuous flight auger with a hollow stem plugged at the bottom with a removable

plug, augering with more or less simultaneous installa­tion of a casing, or using rotary wash-boring methods. The hole for a piezometer should be kept filled with water or approved drilling fluid at all times. Bentoni­tic drilling mud should not be used; however, an organ­ic type of drilling fluid, such as Johnson’s Revert or equivalent, may be used if necessary to keep the drilled hole open. Any auger used in advancing the hole should be withdrawn slowly from the hole so as to minimize any suction effect caused by its removal. When assembling piezometers, all fittings should be tight and sealed with joint compound so that water levels measured are those actually existing at the loca­tion of the wellpoint screen. Where the water table in different pervious formations is to be measured, the riser pipe from the piezometer tip must be sealed from the top of the screen to the ground surface to preserve the isolation of one stratum from another and to ob­tain the true water level in the stratum in which the piezometer is set. Such piezometers may be sealed by grouting the hole around the riser with a nonshrinking grout of bentonite, cement, and fly ash or other suita­ble admixture. Proportions of 1 sack of cement and 1 gallon of bentonite to 10 gallons of water have been found to be a suitable grout mix for this purpose. Fly ash can be used to replace part of the cement to reduce heat of hydration, but it does reduce the strength of the grout. The tops of piezometer riser pipes should be threaded and fitted with a vented cap to keep dirt and debris from entering the piezometer and to permit the water level in the piezometer to adjust to any changes in the natural water table.