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Compensation grouting to control deep excavation ground movements

Halai, H. (2018). Compensation grouting to control deep excavation ground movements. (Unpublished Doctoral thesis, City, University of London)

Abstract

The research conducted concerns the application of compensation grouting, particularly compaction grouting, as a method to reduce and control the ground movements generated in the soil by a deep retained excavation in firm to stiff clays. Compaction grouting involves the injection of grout into the soil to create a spherical or cylindrical grout bulb. This research investigates the effectiveness and limitations of the method to provide a preliminary understanding of the influence of the grouting volume, timing and position on the vertical settlements at the retained surface and horizontal displacements of the wall relative to the behaviour observed from the corresponding scenario in which only excavation occurs.

Experimental data were obtained from a series of 10 successful centrifuge model tests undertaken at 100 g. The plane strain models consisted of a pre-formed 12 m deep (at prototype scale) retained excavation temporarily supported by the pressure of a dense fluid acting against the wall and formation surface and a relatively flexible retaining wall propped at the top. The dense fluid was removed and the subsequent soil movements at the retained surface, wall and the formation level were measured, using a combination of transducers and analysis of digital images taken of targets embedded in the front face of the model and wall. Three reference tests were conducted to establish the magnitude and pattern of soil and wall displacements generated by excavation alone. Idealised compaction grouting was modelled simultaneously with excavation in the remaining tests with the injection of ‘grout’ (water) into sealed latex tubes (supported by a perforated Nylon tube), inserted in the soil behind the excavation. The start of injection relative to the timing of the excavation varied amongst the tests. Grouting was continued until positive compensation of the local surface settlement was noted or significant horizontal displacements of the wall were observed.

The tests showed that there appears to be no distinct relationship between the grout volume and the displacements at the retained surface above the injection and at a wall depth of 0.75 times the excavation depth, H due to injection. However, it was seen that for injections below a depth of 0.25H different critical volumes existed beyond which a positive compensation of the retained surface deteriorated or negative compensation increased at a greater rate. This was also reflected in the wall behaviour.

The different injection initiation times showed that greater positive compensation effects could be achieved with injections up to a depth of 0.5H when conducted during the excavation, rather than in the period after. Timing had no influence on injections below this depth. The influence of injection timing was found to be secondary to the injection position.

A linear relationship between the depth of injection and either positively or negatively compensated settlements was noted from the tests. Positive compensation of the ground surface is possible for injections conducted above a depth of 0.5H. However, below this depth a significant negative compensation effect on surface settlements and horizontal wall movements was noted regardless of timing or volume. Greater positive compensation effects and reduced negative effects on the wall were noted with increasing distance from the wall. Regions behind the wall have been identified where grouting provides positive compensation of the surface with minimal influence on the wall and where only negative effects are observed at both the surface and wall.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering
School of Mathematics, Computer Science & Engineering > Engineering > Civil Engineering
URI: http://openaccess.city.ac.uk/id/eprint/20078
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