Underground Singapore 2011
A pressuremeter’s perspective on soil stiffness K.H. Goh, W.M. Cham & D. Wen Land Transport Authority, Singapore
ABSTRACT: The stiffness of soil is an important parameter that affects the deformations of ground and adjacent structures arising from construction activities such as tunnelling and deep excavations. Whilst constitutive models and analytical methods have been derived to predict soil deformation from its stiffness, engineers face a difficult task of identifying soil stiffness from routine site investigations. This paper discusses the use of pressuremeter testing in site investigations to estimate the soil modulus for design. The paper begins by summing up the use of pre-bored pressuremeters in in-situ testing. In particular, the interpretation of pressuremeter curve to obtain the reload modulus will be discussed. Instead of describing the pressuremeter test results using a single modulus value, it is proposed to interpret the pressuremeter modulus in relation to the corresponding strain levels for each test. By correlating the pressuremeter modulus to the strain levels, it is possible to identify guidance for elastic modulus in geotechnical design and analysis. This is illustrated using actual pressuremeter tests completed in Bukit Timah Granite soils and other soils in Singapore. All the pressuremeter tests results presented in this paper are OYO type pressuremeters, where the cavity expansions are monitored directly using displacement transducers.
1 INTRODUCTION The stiffness of soil is an important parameter that affects the deformations of ground and adjacent structures arising from construction activities such as tunnelling and deep excavations. Whilst constitutive models and analytical methods have been derived to predict soil deformation from its stiffness, engineers face a difficult task of identifying soil stiffness from routine site investigations. A tool to determine the soil modulus in-situ is the pressuremeter. The pressuremeter consists of a long cylindrical device, placed in a borehole and radially expanded into the surrounding ground. It is designed to apply a uniform radial pressure to the sides of the borehole through a flexible membrane, and the pressure is increased to create an expansion of the borehole cavity. During the pressuremeter test, measurements of the applied pressure and the corresponding expansion of the cavity are taken so that they may be interpreted into ground properties. The cavity expansion is measured either by measuring the volume of fluid change in the cylindrical device or by measuring the change in cavity radius using displacement transducers. Furthermore, the type of pressuremeter is also differentiated in terms of how the pressuremeter is installed. Figure 1 (as extracted from BRE, 2003) shows the three main types of pressuremeter testing that are available – (a) the pre-bored pressuremeter which is installed in pre-formed boreholes, (b) the self-boring pressuremeter which is able to form its own hole with minimal disturbance on the ground, and (c) the full displacement pressuremeters which is inserted into the ground without soil removal and the ground is displaced by the passage of the pressuremeter. Mair and Wood (1987) and Briaud (1992) discussed the different types of pressuremeter test methods and interpretation. The pressuremeter tests are usually conducted to procedures described in ASTM D4719 (2000) and BS 5930 (1999).
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