Grouted ground anchors and the soil mechanics aspects of cement grouting

John D McKinley, Cambridge University
Geotechnical Engineering Group


Civil Engineers have used grouted ground anchors to transmit tensile loads into the soil because of the technique’s economic and practical advantages. Their construction will affect the local ground conditions such as stress state and void ratio, and must, therefore, influence the anchor’s subsequent performance. This study concentrates on the construction phase of low pressure grouted ground anchors in fine to medium sands, for which the anchor diameter increase is small and permeation of grout through the soil is negligible. It examines the basic mechanisms of cement grout injection, and divides into two parts.

The first part characterises the behaviour of fresh water-cement mixtures in terms of existing soil mechanics concepts, using the results of filtration, consolidation and sedimentation tests on three commercial Portland cements. This characterisation is both appropriate and useful. It appears that the filtration of cement grout takes place by the laying down of a stiff compacted layer due to the removal of excess water, where this layer has soil-like properties. The calculated values for the fluid limit, permeability and compressibility are consistent across the range of tests, and would be typical of a reconstituted silt.

The second part considers the injection of cement grout into a borehole, modelled as a plane strain, axisymmetric consolidation process with multiple “soil” regions and material accretion against the borehole wall. The governing equations are derived and form the basis of the finite difference computer program Racoon, whose development and testing are described. For conditions typical of grouted ground anchor construction in the United Kingdom, Racoon predicts that it takes less than ten seconds to remove the grout’s excess water, and that the residual effective stress on the anchor body is approximately 40% of the injection pressure. The determinants of the filtration time and the residual stress are discussed.

Keywords: grouted ground anchors, cement grouts, fine to medium sand, Portland cement chemistry, pressure injection, suspensions, filtration, sedimentation, consolidation, creep, geotechnical centrifuge, fluid limit, permeability, compressibility, axisymmetric coupled transient flow, numerical analysis, finite differences, filtration times, residual stresses, construction effects.