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

摘要

Pressure grouting during installation of grouted ground anchors is known to increase anchorcapacity in non-cohesive soils, but little information is available on correlations betweenapplied grouting pressures, duration of grouting, ground conditions and increase of anchorpull-out capacity.The presented PhD study is concerned with processes taking place during installationof grouted ground anchors in non-cohesive soils, where filtration of the cement grout isassumed. It was aimed to determine the influence of pressure grouting on the stresses on theanchor body and the properties of the adjacent soil. The knowledge of those is consideredprerequisite in order to determine the anchors pull-out capacity.In the first part of the PhD thesis, a series of laboratory experiments is presented, whichwas carried out to understand the filtration process of cement grouts and to determine theproperties of the filter cake material. Using a filtration press the rate of filter cake build-upwas investigated, taking into account the influence of grouting pressure and initial water/cementratio of the grout. The test results were used to evaluate different analytical approachesto simulate the filtration process: a two-phase filtration model and classical consolidationtheory. Both models were found appropriate, and calculation parameters were determined.In addition to the filtration tests, the mechanical properties of the fresh, uncured, filter-cakematerial were investigated. Applying soil mechanical investigation methods, strength andstiffness properties could be determined.In the second part of the PhD thesis in-situ tests during anchor installation in sands arepresented. On three test sites the grout pressure was measured inside the borehole duringand after anchor installation. Measurements confirmed a grout filtration inside the boreholeand indicated the increase of radial stresses on the anchor body. Additional flat-dilatometersoundings (DMT) and cone penetration tests (CPT) showed the influence of the groutingprocess on the radial stresses in the adjacent soil.In the third part of the thesis a numerical model is proposed to simulate the filtration processof cement grout in a fully coupled flow-displacement finite element analysis. Based onthe two-phase filtration model a filter criterion was implemented, which defines the phasechange from liquid to solid grout based on the discharge of water. The phase change wasrealised by changing the material properties of the grout elements. With the presented modelthe grouting during anchor installation was simulated and the influence of different parameterscould be determined. The transfer of grouting pressures from the liquid grout to the soilthrough seepage forces in the filtercake is simulated and the residual stresses after groutingdetermined. The findings can now be used as starting point to simulate the load transfer mechanismsof grouted ground anchors in numerical analysis, taking into account installation effects.