Cement grouting during installation of ground anchors in non-cohesive soils

Abstract

Pressure grouting during installation of grouted ground anchors is known to increase anchor capacity in non-cohesive soils, but little information is available on correlations between applied grouting pressures, duration of grouting, ground conditions and increase of anchor pull-out capacity. The presented PhD study is concerned with processes taking place during installation of grouted ground anchors in non-cohesive soils, where filtration of the cement grout is assumed. It was aimed to determine the influence of pressure grouting on the stresses on the anchor body and the properties of the adjacent soil. The knowledge of those is considered prerequisite in order to determine the anchors pull-out capacity. In the first part of the PhD thesis, a series of laboratory experiments is presented, which was carried out to understand the filtration process of cement grouts and to determine the properties of the filter cake material. Using a filtration press the rate of filter cake build-up was investigated, taking into account the influence of grouting pressure and initial water/cement ratio of the grout. The test results were used to evaluate different analytical approaches to simulate the filtration process: a two-phase filtration model and classical consolidation theory. Both models were found appropriate, and calculation parameters were determined. In addition to the filtration tests, the mechanical properties of the fresh, uncured, filter-cake material were investigated. Applying soil mechanical investigation methods, strength and stiffness properties could be determined. In the second part of the PhD thesis in-situ tests during anchor installation in sands are presented. On three test sites the grout pressure was measured inside the borehole during and after anchor installation. Measurements confirmed a grout filtration inside the borehole and indicated the increase of radial stresses on the anchor body. Additional flat-dilatometer soundings (DMT) and cone penetration tests (CPT) showed the influence of the grouting process on the radial stresses in the adjacent soil. In the third part of the thesis a numerical model is proposed to simulate the filtration process of cement grout in a fully coupled flow-displacement finite element analysis. Based on the two-phase filtration model a filter criterion was implemented, which defines the phase change from liquid to solid grout based on the discharge of water. The phase change was realised by changing the material properties of the grout elements. With the presented model the grouting during anchor installation was simulated and the influence of different parameters could be determined. The transfer of grouting pressures from the liquid grout to the soil through seepage forces in the filtercake is simulated and the residual stresses after grouting determined. The findings can now be used as starting point to simulate the load transfer mechanisms of grouted ground anchors in numerical analysis, taking into account installation effects.