The Use of the Fully-Grouted Method for Piezometer Installation

Abstract

Introduction The fully-grouted method described in this ar t ic le entai ls instal l ing a piezometer tip in a borehole which is backfilled entirely with cement-bentonite grout. Part 1 of this article presents a detailed discussion of the fully-grouted method, including the installation procedure and theoretical background, as well as a seepage-model analysis used to evaluate the impact of the difference in permeabilities between surrounding ground and cement-bentonite grout. Part 2 describes laboratory test results for six cement-bentonite grout mixes and field examples of applications of the fully-grouted method. Both parts of this article are based on the paper, “The Use of the Fully-grouted Method for Piezometer Installation,” presented at FMGM 2007: Seventh International Symposium on Field Measurements in Geomechanics, and are published in GIN with permission from ASCE. A crucial parameter for the success of the fully-grouted method is the permeability of the cement-bentonite grout. Vaughan (1969) postulated that the cement-bentonite grout should have a permeability no greater than two orders of magnitude higher than the surrounding soil in order to obtain representative pore-water pressure readings. Unfortunately, there is limited published data on the permeability of cement-bentonite grout mixes. Figure 1a shows the typical piezometer installation commonly known as a Casagrande or standpipe piezometer. With this installation, the tip of the piezometer (e.g., slotted PVC pipe or porous stone filter) is surrounded with a high permeability material, commonly referred to as sand pack. Above the sand pack is a bentonite seal typically consisting of bentonite chips or pellets. The installation is finished with cement-bentonite grout to the ground surface. This installation relies on a sizable intake volume and a narrow riser-pipe diameter to obtain a pore-water pressure reading in the riser pipe without significant time lag (Hvorslev, 1951). With the development of diaphragm piezometers (e.g., pneumatic and vibrating wire), the method developed for standpipe piezometers was used for diaphragm piezometer installations (Dunnicliff, 1993). This has been a common practice for decades and the resulting installation is shown on Figure 1b. However, because of the GEOTECHNICAL INSTRUMENTATION NEWS