Abstract:
Compensation grouting (also known as frac grouting) helps compensating and preventing settlements on structures e.g. during tunnelling. During the injection, grout-filled fracs are formed, causing the structure above to lift. Major cost factors are the required construction time and the grout quantities. In cohesive soils almost the entire grout volume is reflected in the resulting frac structure. In coarse-grained soils, however, the mechanisms of filtration and penetration of the grout into the pore space heavily reduce the lifting effect. Furthermore, these mechanisms affect the resulting spatial formation (shape, orientation, length and thickness) of the frac structures. Hardly anything is known about the grout bodies actually produced. Moreover, there is no simple method to check the suitability of the chosen grout mixture and to quantify the influence of these mechanisms. An extensive research project at the Center of Geotechnics of the TU Munich examines the resulting grout bodies to improve the economic efficiency of the process. An absolute special feature are the large-scale tests of compensation grouting carried out in a test pit (built-in soil volume of approx. 40 m³ ). To investigate the spatial formation and development of the frac structure, the grout bodies were exposed, documented and analyzed. Real-time (liquid levelling system) and area-wide deformation measurements (hydrostatic line measurement) helped to record the uplift of the surface. In addition, the development of a laboratory test device to test the suitability of grout mixtures in coarse-grained soils is presented. Ready-mixed products used in construction practice and own grout mixtures were tested on different coarse-grained soils. The test results allow conclusions about the filtration and penetration behavior of the grouts during compensation grouting in coarse-grained soils. The experimental investigations in the research project make an important contribution to optimizing the practice of compensation grouting by determining the suitable grout mixture in advance and predicting the lifting effect more realistically.