Response characteristics of cylindrical piezometers

Abstract

The finite difference solutions presented for the fundamental process of pore pressure equalization in a soil piezometer system are the first of their kind to be published for cyhndrical piezometers. The response-time relationships established for cylindrical piezometers (of radius a and length b) are of the same general form as those for spherical piezomelers, the exact shape of a particular relationship being a function of the ratio a/b, the soil compressibility m and the flexibility of the piezometer measuring system V. The soil-pierometer system characteristics can be best represented by a control parameter λ = 47πbm/V. The value of this parameter characterizes the shape of the response-time curve, for which there is a unique time ratio t90/t50 and time factor ratio T90/T50. On this basis, the cylindrical equalization chart is constructed, which provides a direct means for the analysis of equalization tests in cylindrical piezometers. Interpretation of laboratory tests on piezometers embedded in clay triaxial specimens shows that equalization tests can provide reliable in situ measurement of soil permeability and good estimations of coefficients of consolidation. Les solutions de différences finies présentiées relativement au processus fondamental de l'égalisation de la pression interstitielle dans un système sol-piézomètre sont les premières de ce type à être publiées en ce qui concerne les piézométres cylindriques. Les relations réponse-temps établises pour les piézomètres cylindriques (de rayon a et de longueur b) sont de la même forme générale que celles qui correspondent aux piézroméres sphériques, la forme exacte d'une relation donnéé étant fonction du rapport a/b, de la compressibilité du sol m et de la souplesse du système de mesure du piézomètre V. La meilleure façon de représenter les caractéristiques du système sol-piézomètre est de faire appel à un paramètre de contrôle λ = 4π2bm/V. La valeur de ce paramètre caractérise la forme de la courbe réponse-temps pour ladquelle il existe un rapport unique de temps t90/t50 et un rapport de facteur de temps T90/T50. L'abaque d'égalisation cylindrique est établi à partir de ces données; cetabaque permet d'analyser directement les essais d'égalisation dans le cas de piézomètres cylindriques. L'interprétation des résultats des essais en laboratoire de piézomètres fichés dans des échantillons triaxiaux d'argle montre que les essais d'égalisation sent capables de fournir des mesures in situ fables de la perméabilité du sol ainsi que de bonnes évaluations de coefficients de consolidation.

Hydraulic conductivity in upland blanket peat: measurement and variability

“…Comparing the hydraulic conductivity values calculated using both theories shows that the mean value of k* (the hydraulic conductivity calculated using the response time charts derived numerically by Brand and Premchitt (1982)) was much lower than k 90 . Baird and Gaffney (1994) reported that both rigid and compressible soil theories gave values of hydraulic conductivity for each piezometer installation at their fenland site within a factor or two of each other.…”

“…Full treatment of the calculations and derivations used are provided by Brand and Premchitt (1982) and Baird and Gaffney (1994) and so only a summary is provided below. Hvorslev's (1951) used the basic differential equation that describes saturated flow through a falling head permeameter to produce a solution to the pore pressure equalisation process between a piezometer system and an incompressible soil.…”

“…Brand and Premchitt (1982) used a numerical solution to equation 4 to show that the soil -piezometer system was well represented by a control parameter:…”

“…The shape of the head recovery is characterised by λ for which there is a unique ratio between t 90 (time taken for the head to recover to 90 % of initial head difference between piezometer and soil) and t 50 (Premchitt and Brand, 1981;Brand and Premchitt, 1982). Using λ as a control parameter, Brand and Premchitt (1982) derived equalisation monographs based on t 50 and t 90 that can be used to calculate the hydraulic conductivity and the coefficient of consolidation. These have been used in the following analysis.…”

“…However, S2 C20 shows pronounced deviation from the curve with the data points (open triangles) forming a more gently sloping curve than that described by Hvorslev (1951). Table 1 presents mean values from the experiments for each site and depth using both equation 1 and values calculated from the nomograph of Brand and Premchitt (1982). Values of hydraulic conductivity tend to be higher at S1 than the other two sites.…”