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.
Research Training Materials Video Research Some insights into wet soil mixed properties from a database developed through routine project work Free | PDF | Support Materials Read more Presentation to the Temporary Works Forum December 4th 2019 €5.00 | eBook | Support Materials Read more A Tensile Strength Apparatus with the Facility to Monitor Negative…
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.
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.
Presentation to the Temporary Works Forum on the use of propping frames in routine deep excavations.
Mechanisms of failure in saturated and unsaturated clayey geomaterials subjected to (total) tensile stress
The paper presents an experimental investigation into the mechanisms of tensile failure in clayey geomaterials under saturated and unsaturated conditions. An experimental apparatus was developed to test specimens in uniaxial tension with the facility to monitor suction (pore-water tension) using high-capacity tensiometers. This allowed interpretation of failure data in terms of effective stress and average skeleton stress for saturated and unsaturated specimens, respectively. Experimental data from normally consolidated samples showed that failure under uniaxial tension occurs in shear and tensile cracks form as a combination of mode I and mode II fracture. In the saturated range, when samples were prepared with de-aired water, tensile failure occurred at deviatoric stresses corresponding to the critical state line derived from triaxial and uniaxial compression tests. When using non-de-aired water and at suction levels approaching the air-entry value, failure occurred at deviatoric stresses lower than the ones corresponding to the critical state line derived from compression tests. It has been suggested that water cavitation may be one of the mechanisms that control premature rupture of saturated clay when subjected to a (total) tensile stress state. Finally, tensile failure data from unsaturated samples showed that there is continuity between saturated and unsaturated states.
Mass soil mixing and deep soil mix columns are a versatile ground improvement technology for marginal and brownfield sites. Dry soil mixing is relatively common in the UK for improvement of ground with very wet and/or organic material. Wet mixing is less commonplace and involves introduction of a fluid grout with simultaneous disaggregation of the soil with a rotating mixing tool. This paper presents the results of strength verification testing carried out across multiple projects in the UK covering differing soil types with varying project specification criteria. Some conclusions are drawn with regards to the factors affecting strength progression and in understanding the mechanics of the mixing process. Mixing time per unit volume of mixed material is identified as an important parameter for mass mixing. In addition, discrete element modelling has shown promise in understanding the mechanics of deep column mixing.
Statistical inferences from a database of lime and cement modified soil in earthworks for residential construction in the UK
Use of lime for soil moisture conditioning is frequently used for the treatment of soils significantly wet of optimum in order to achieve density-driven specifications for cohesive materials. In a similar fashion, cement is frequently used for the treatment of soil of lower clay content to improve strength overall. This paper presents an assessment for a comparably large database of routine compaction testing across varying soil types in the UK where lime and/or cement has been used in ground improvement for the preparation of structural upfill specifically in residential construction projects. Statistical inferences are drawn from the data and presented for guidance for refinement of design approaches in such projects. It is concluded that the techniques produce a highly reliable structural upfill in terms of relative compaction and percentage air voids.
Geotechnical characterisation of fine-grained alluvial & proluvial soils for a motorway project in Kosovo
Description and classification of fine-grained soils for large earthworks projects is extremely important to the success of the project, particularly for embankment construction. Establishing the correct profile of undrained shear strength and appropriate drained (effective stress) strength parameters is also critical in consideration of aspects of the project such as founding soils for embankments and stability analysis for deep cuttings. This paper discusses the investigation and characterisation of fine-grained alluvial and proluvial soils in the context of a large motorway scheme in Kosovo. The methodology of geological identification and geotechnical investigation of these soils is described and engineering parameters, including soil classifications, are presented.
A simplified parametric study of particle trace in soil mixing simulations using discrete element modelling
The focus of the study is be the fundamental parameters that affect the mixing process and, specifically, the disaggregation of the native soil through examination of the mechanisms of a body of granular particles by use of Discrete Element Modelling – a means to model arrangements and interactions of granular particles explicitly as individual particles. This is distinct from traditional soil mechanics approaches which have, in general, tended to treat soils as a continuum, accounting for what David Muir-Wood referred to as “particle-continuum duality”. The trend of increase in commercially available computational power has given rise to a corresponding rise in Discrete Element Modelling and associated commercially available software which until recently, had been beyond nominally available computing power. This has been a particular focus in the powder technology, food & agricultural industries and chemical engineering fields.