Use of CPT as a soil mixing verification tool: some practical observations

Mass soil mixing is a versatile ground improvement technology for marginal and brownfield sites. Dry soil mixing is relatively common in the UK for the improvement of ground with very wet and/or organic materials. Wet mixing is less commonplace and involves introduction of a fluid grout with simultaneous rotavating of the soil with a mixing tool. This paper presents the results of CPT investigations into wet mixed soils with a view to use the results as a means to verify the strength and consistency of the mixed materials. A high volume of data has been extracted from two active sites. CPT soundings are found to be purposeful in terms of demonstrating the overall improvement effect and integrity of the mixed soil volume spatially and with depth. Existing CPT testing of mass mixed soil is very limited and this testing has allowed the mixed soil to be compared to existing CPT soil behaviour indexes to give an insight into their usefulness for characterising mixed material and its behaviour.

Some insights into wet soil mixed properties from a database developed through routine project work

Current good practice guidance and codes of practice for soil mixing typically focus on strength as the primary criterion for achievement of project specifications. For example, correlations for stiffness with strength are commonly used in lieu of direct measurements. Long-term consolidation or creep of mixed soil is rarely measured in practice, either in the laboratory or in the field with the exception of a small number of good-quality case studies. This paper presents some insights into mixed soil properties, which are not routinely measured or specified as well as strength, gleaned through the course of normal project work where the overall concept required mixed soil properties other than strength to be measured. All data was taken from wet soil mixing projects which involves the introduction of cementitious grout under medium or high pressure in conjunction with mechanical rotavating of the soil. The projects include both mass mixing and deep soil mix columns using proprietary technologies briefly described in the paper. All projects forming the database were undertaken in the UK.

A Tensile Strength Apparatus with the Facility to Monitor Negative Pore – Water Pressure

This article presents a new testing method for investigating the behavior of clayey geomaterials subjected to a tensile (negative) total stress. The method includes the use of high-capacity tensiometers to measure the pore–water pressure of the test specimen, an aspect which has not been demonstrated in any other direct tensile testing method. This addition allows interpretation of failure data in terms of effective stress rather than total stress, which is the approach that should be pursued in the saturated range. The test specimen shape and loading method have been modified from those commonly seen in existing literature to ensure that the direction of the major principal stress in the failure zone coincides with the direction of the externally applied tensile force, allowing for a more accurate analysis of tensile failure. Results are shown for saturated specimens and compared to results obtained for the same soil in uniaxial compression, using a modified version of the presented uniaxial tensile method, and a triaxial compression test. It is demonstrated that crack initiation occurs by shear failure if the data are interpreted in terms of effective stress rather than total stress and that the failure mechanisms under tension do not differ from compression.