This Paper describes a liquid electric analogue model and a finite difference model used to establish reliable values for the shape factors of cylindrical piezometers with length/diameter ratios of up to 15. For piezometers with length/diameter ratios greater than 4, it was found that the shape factor is given with negligible error by the relationship: F = 7d + 1·651. An examination of the effect on the shape factor of piezometer proximity to the watertable showed this to be negligible for practical purposes, except where the piezometer is within a few piezometer lengths of the water-table. Shape factors were also established for cylindrical piezometers embedded in cylindrical soil specimens subjected to all five possible boundary conditions. The measured shape factors were higher or lower than the infinite values depending upon the boundary conditions and the proximity of the boundaries to the piezometer. These data give some guidance to theperformance of pore pressure probes used in laboratory specimens, and they are useful in the analysis of steady state flow conditions and for the assessment of response studies of piezometer systems. Cet Article décrit un modèle analogique électrique fluide et un modèle à différence finie servant à établir des valeurs fiables pour les coefficients de forme de piézométres cyhndriques dont les rapports longueur/diamètre peuvent atteindre 15. Pour les piézomètres dont le rapport longueur/daimétre est supérieur à 4, on a trouvé que le coefficient de forme était donné, avec une erreur négligeable, par la relation F = 7d + 1·65. L'influence sur le coefficient de forme de la proximité de la nappc phréatique est pratiquement négligeable sauf lorsque le piézomètre était à quelques longueurs de piézomètre de la nappe phréatique. Les coefficients de forme ont également été établis pour des piézomètres cylindriques enfoncés dans des échantillons de sols cylindriques soumis aux cinq conditions aux limites possibles. Les coefficients de forme mesurés étaient supérieurs ou inférieurs aux valeurs infinies dependant des conditions aux hmites, et de la proximité des limites par rapport au piézomètre. Ces données permettent de se faire une certaine idée de la performance de sondes de pression interstitielle utilisées dans des échantillons de laboratoire et sont utiles pour l'analyse des conditions d'icoulement stationnaire ainsi que pour I'évaluation des réponses des piézomètres.

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.

Introduction A recent paper by Pierson (1983) provided sound evidence of the contribution of soil pipes to landslides. Whereas the existence of pipes has been documented in a good number of publications, their importance in the context of slope stability has not generally been appreciated. Attention is now focusing on pipes in Hong Kong as a major contributory factor to frequent landsliding. Pipes in Hong Kong The hilly terrain of Hong Kong is prone to landslides brought about by the heavy seasonal rainfall ( Lumb 1975 ; Brand 1984 , 1985 ). The granites and volcanic rocks are weathered to considerable depths, the surface residual soil often being overlain by a mantle of colluvium. Until recently, it was not realized that the existence of pipes was an important factor in the overall stability of some of Hong Kong's hillsides, and that they have undoubtedly been the main cause of some of the failures that have occurred in Hong Kong cut slopes ( Premchitt et al ., 1985 ). The pipes in the residual soils and colluvium of Hong Kong cover the range of types described in the Paper and discussed previously by Crouch (1976) and Jones (1981) . They vary considerably in diameter, in length and in depth in the soil profile. In some cases, they are suspected of comprising anastomosing networks which cover large areas ( Nash & Dale 1983 ). Pipes in Hong Kong are most commonly exposed in shallow excavations in A and B soil horizons, as described by Crouch (1976) . These are most commonly attributed to

A severe rainfall event occurs once every two years in Hong Kong, causing widespread landslides which result in human casualties and much damage. It has recently been established that these landslides are caused largely by short-period, high-intensity rainfall, with nearly all failures occurring within a few hours of peak intensity. The failures are therefore attributable mainly to rapid transient variations in groundwater conditions. These findings are in contradiction to previously accepted theories on failure mechanisms, and it is therefore necessary for these to be reconsidered.A new theoretical model is now being examined in Hong Kong to explain the rapid landslide response following high-intensity rainfall. This is based on well-defined hillslope hydrological theories. Of particular significance in this context are thought to be subsurface erosion pipes and tunnels which have been found to exist in some of Hong Kong's hiilslopes, especially those carpeted with colluvium.

No abstract