Piezometric error for fully grouted piezometers installed in clay layers

Vahid Marefat, Robert P. Chapuis, François Duhaime

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Instrumentation and Monitoring

ABSTRACT: In hydrogeology and geotechnical engineering, measuring pore pressure within clay deposits is rarely considered a straightforward task. Modern piezometers with diaphragm transducers are often used for pore pressure monitoring within clay deposits. For this type of transducer, some practitioners claim that the fully grouted installation method has more advantages than the classic method with the transducer surrounded by a sand filter. In this study, analytical and numerical solutions were derived to calculate the piezometric error for steady-state flow conditions and for piezometers installed with the fully grouted method. The solutions show that the magnitude of the error is strongly related to the permeability of the grout and the vertical hydraulic gradient. The results confirm that an error is induced if the grout permeability (Kg) is greater than the permeability of the surrounding soil (K2) by about one order of magnitude. For a very low vertical hydraulic gradient (iv 0.1), grout seals can be up to two order of magnitude more permeable than the surrounding formation with negligible error. For a permeability ratio lower than Kg/K2=10, the piezometric error has no significant variation with borehole depth while for higher permeability ratio, the error increases with depth. RÉSUMÉ En hydrogéologie et en géotechnique, on considère rarement les mesures de pression interstitielle dans les dépôts d'argile comme des valeurs faciles à obtenir. Des piézomètres modernes avec des transducteurs à membrane sont souvent utilisés pour mesurer la pression interstitielle dans les dépôts d'argile. Pour ce type de transducteur, certains praticiens prétendent que la méthode d'installation avec injection complète de coulis présente plusieurs avantages par rapport à la méthode classique où le transducteur est entouré de sable filtrant. Dans cette étude, des solutions analytiques et numériques ont été développées pour calculer l'erreur piézométrique pour des écoulements stationnaires et pour des piézomètres installés avec injection complète de coulis. Les solutions montrent que l'ampleur de l'erreur est fortement liée à la perméabilité du coulis et au gradient hydraulique vertical. Les résultats confirment qu'une erreur est induite si la perméabilité du coulis (Kg) est supérieure à la perméabilité du sol environnant (K2) par approximativement un ordre de grandeur. Pour un gradient hydraulique vertical très faible (iv 0,1), le coulis de scellement peut être jusqu'à deux ordres de grandeur plus perméable que la formation environnante avec une erreur négligeable. Pour un rapport de perméabilité inférieure à Kg/K2=10, l'erreur piézométrique ne varie pas de façon significative avec la profondeur de forage, tandis que pour un rapport de perméabilité plus élevé, l'erreur augmente avec la profondeur. 1 INTRODUCTION Measuring pore pressures within a clay layer is rarely considered a straightforward task in both hydrogeology and geotechnical engineering. Due to the significant lag time of open standpipe piezometers, faster diaphragm-type transducers are often used to monitor pore pressures within clay formations (Penman 1961). The sand-filled intake zone procedure has been adapted and used with diaphragm-type piezometers (e.g., vibrating wire piezometers) by the geotechnical industry for decades (Dunnicliff 1993). Some practitioners have proposed to eliminate the sand filter around the piezometer, and to grout completely the borehole after having positioned the sensor. The notion of installing piezometers directly into the grout without a sand pack around them was first suggested by Vaughan (1969). However, the performance of this installation method is still questioned by some in the hydrogeology and geotechnical communities. A piezometer performance can be defined by its time-lag and measurement accuracy (Mckenna 1995). Time-lag represents the duration taken by a piezometer to reach its equilibrium pressure after a pore pressure change in the area surrounding the sensor (Hvorslev 1951). Deviation of the measured pore pressure from the real pore water pressure of the natural soil before drilling can be called piezometric error. Recently, a number of investigators have advocated and tried to gain accurate and reliable pore pressure measurements for both single and multiple diaphragm-type piezometers installed with the fully grouted method (McKenna 1995; Mikkelsen and Green 2003; Contreras et al. 2008; Simeoni et al. 2011; Simeoni 2012). Proponents of the fully grouted method cite several advantages for this method. These advantages include reduced cost, ease of installation, no risk of failure for the sand pack of deeper wells, the opportunity to install several piezometers within the same borehole or share the same borehole with other geotechnical instruments, and the possibility to adapt the fully grouted method to the installation of horizontal or inclined piezometers. (McKenna 1995). Assessments of the response time for modern diaphragm-type piezometers installed within fully grouted

RÉSUMÉ: metric error for fully grouted piezometers

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Cite this article:
Vahid Marefat; Robert P. Chapuis; François Duhaime (2014) Piezometric error for fully grouted piezometers installed in clay layers in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper212,author = Vahid Marefat; Robert P. Chapuis; François Duhaime,title = Piezometric error for fully grouted piezometers installed in clay layers,year = 2014}