The dilemma of designing retaining structures and earthworks in clay

Richard Herraman

My job with the then Department of Planning, Transport and Infrastructure (DPTI) now the Department of Infrastructure and Transport (DIT) required me to design retaining structures and earthworks in clay soil. When I started with DPTI design methods for retaining structures assumed the soil had little or no cohesion whereas I observed that some high near vertical excavations in stiff unsaturated clay were stable for a long time. This could only arise from cohesion in the clay caused by tension in the water in the pores of the clay, referred to as matric suction. I realised that, if this cohesion due to suction (referred to as apparent cohesion) could be quantified reliably, more lightweight (i.e. cheaper) retaining structures could be used. I also realised that, as retaining structures and earthworks are generally above the watertable, it would be necessary to use unsaturated soil mechanics for analysis and design. This presentation is about using unsaturated soil mechanics to get strength parameters for clay soils for use in designing retaining structures and earthworks and the dilemma of determining a suction value for design.

About the speaker

Richard Herraman Currently Higher Degree by Research student Department of Civil, Environmental and Mining Engineering University of Adelaide Formerly Principal Geotechnical Engineer Department of Planning, Transport and Infrastructure, SA

I managed the Geotechnical Engineering Group of DPTI for 20 years. During this time my group provided a range of geotechnical engineering services for many successful transportation infrastructure projects. During this time I became aware of shortcomings in the planning of geotechnical investigations for excavations and retaining structures in unsaturated clay, the analysis of data from the investigations and its use for design. I worked with John Woodburn (consulting engineer), my colleagues in DPTI, the Australian Geomechanics Society and the University of Adelaide to improve practice in this area of engineering.

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