Insitu Assessment of a Cement-bentonite Containment System

Slurry trench cut-off walls are widely used in the UK to contain contaminated groundwater. Specifications for these walls (ICE, 1999) call for a minimum unconfined compressive strength of lOOkPa and a hydraulic conductivity lower than 1 × 109 m/s. Tests for these properties are carried out in a laboratory on samples cast from the poured slurry and allowed to set; currently there is no accepted method for testing the performance of the walls after construction. There are reasons to suspect that laboratory derived values do not reflect the properties in situ. For example there are no guidelines for the appropriate confining stress to apply to the laboratory samples when testing the hydraulic conductivity. An unrepresentative value will significantly skew the results. Material heterogeneity has a major influence on the measured hydraulic conductivity (the scale effect) and this is masked by the small size of the normal laboratory sample. To address these and other issues a series of field measurements of hydraulic conductivity were carried out in an 7 year-old cement-Bentonite containment wall, using a self-boring pressuremeter, a straddle packer system and piezocone. These cover the range of insertion procedures, from minimal to grossly invasive. The experimental programme examined the influence of insertion disturbance and different test procedures, and showed that consistent data can be obtained by a variety of means. In general the wall was found to be much more permeable than the initial laboratory tests had indicated, a difference that at this stage appears to be due to the scale effect.

• INTRODUCTION

• SITE DESCRIPTION

• FIELD TEST RESULTS AND DISCUSSION

• CONCLUSIONS

• ACKNOWLEDGEMENTS

• REFERENCES