Oxford University
Civil Engineering
Department of Engineering Science
Piezocone performance in layered soils
A joint research programme between University of Sheffield and University of Oxford sponsored by EPSRC
Investigators: Dr Charles Hird (Sheffield) and Prof. Gilliane Sills (Oxford)
Background
During the last 20 years there has been an enormous development of in-situ testing and profiling devices for soil deposits, e.g. the self-boring pressuremeter, the dilatometer and the piezocone. These devices are now playing an increasing role in site investigation for engineering works.
The piezocone combines measurements of penetration resistance and pore water pressure in the vicinity of a cone being pushed steadily into the ground. Used as a profiling tool, it can provide information about the sequence (types and thicknesses) of the soils present. This is especially useful in alluvial and estuarine deposits containing interbedded, layered or laminated soils (clays, silts and sands) where the layered fabric can have a very strong effect on the overall permeability or consolidation rate. The rate of consolidation normally governs the development of settlement and the gain of strength beneath embankments and other structures built on soft ground. Therefore, it is of fundamental importance for the safe and economical design of such structures.
Surprisingly, although field experience of detecting relatively thin sand layers in clay deposits has been reported, systematic studies of this capability have been lacking. However, one such study has recently been conducted at Sheffield University using a 1cm2 piezocone with a filter at the tip. Further research is now under way, under a wider range of controlled conditions, to establish the performance limits of different sizes and configurations (in terms of the filter position) of piezocone. There are advantages to measuring the pore pressure at more than one position. It is generally recognised that one filter should be placed at the shoulder, so that the measured cone resistance can be corrected for the effect of water pressure on the back on the cone (the area ratio effect). A second filter placed on the cone face or at the tip can provide additional information about soil type and is probably more effective in detecting thin layers.
In clay deposits the piezocone can also provide an estimate of the coefficient of consolidation, through interpretation of pore pressure dissipation tests, and a reasonable amount of experience of this has been gained, both in the field and in calibration chambers. This use of the piezocone is attractive, given the difficulty of measuring the coefficient of consolidation reliably in the laboratory. However, the theories used in interpreting the measurements assume that the soil is homogeneous and in layered soils interpretation is likely to be more difficult.
Programme
The overall aim of the proposed research is to improve the current knowledge of piezocone performance in layered soils (alternating layers of clay and silt or sand). Specific objectives are:
· to assess the performance of a piezocone equipped with multiple filters in detecting relatively thin permeable layers in a clay deposit.
· to assess the performance of a piezocone in measuring the coefficient of consolidation in clay layers bounded by more permeable layers.
· to provide guidance on the interpretation of piezocone data for the overall rate of consolidation in layered soils.
It is proposed to conduct a series of piezocone drives into layered soils in a calibration chamber, in the manner already pioneered at Sheffield for the 1cm2 instrument. In order to accommodate a larger piezocone, the cells would be larger than those used previously.
Each experiment will be carried out in a 400mm diameter test cell containing layers of preconsolidated kaolin and silty sand of 400-600mm depth. The existing techniques for soil layer construction will be used so that regular layers can be formed, whose characteristics are well known.
Two different piezocones will be driven into the layered soil, while appropriate boundary stresses are maintained:
· a 1cm2 piezocone made by Fugro Limited, as used in the previous research on layered soils, with alternative filter positions (cone tip or cone shoulder).
· a 5cm2 piezocone developed for Oxford University by Fugro Limited with 4 filter positions, simultaneously used (cone face, cone shoulder, and two locations on the shaft).
Pore pressure dissipation tests will be carried out at a variety of carefully controlled elevations relative to the sand layers. The main test series will consist of up to 15 experiments, with various soil layer configurations.
