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Minimising ground movements around deep excavations in soft soils

Panchal, J. P. (2018). Minimising ground movements around deep excavations in soft soils. (Unpublished Doctoral thesis, City, University of London)

Abstract

This research concerns the influence of a range of construction methods, acting at or below excavation formation level, on ground movements of the retained surface attributed to a 12m deep excavation in very soft to soft soil. Movements around excavations arise as a consequence of the removal of soil and lateral wall deformations. The work examined the behaviour of excavations that were supported by a high stiffness embedded retaining wall whilst modelling a variety of construction techniques. Four distinct construction methods were modelled which could be regarded as surcharging the formation level or stiffening the ground below excavation formation level. The specific techniques that were explored include underwater excavations, bermed excavations, deep soil mixing and double walled excavations. This study aimed to determine the efficiency of these construction measures on reducing the magnitude and extent of displacements occurring behind the retaining wall.

Experimental data were obtained from twenty-two plane strain centrifuge model tests undertaken at 160g. The geometry of the model comprised a pre-formed excavation where the retaining wall was laterally supported by a continuous prop acting over the majority of the height of the wall and the excavation formation level was surcharged by a pressurised rubber bag. Pressure in the bag at formation level was reduced at a constant rate to simulate the stress change caused by the excavation process. Vertical movements at the retained ground surface were measured using displacement transducers whilst subsurface deformations elsewhere in the model were determined from the analysis of digital images captured by cameras viewing the front of the model through a Perspex window. The magnitude and extent of movements were quantified and the general patterns of ground deformation were identified for the construction methods implemented.

A series of reference tests were conducted to provide a baseline against which modified excavation tests were compared. The stiff wall and continuous prop supporting the retaining wall ensured that the reference tests quantified the magnitude of displacements at the retained surface arising simply as a result of heave at the formation level. The main test series investigated a range of construction methods that aimed to surcharge or stiffen the formation level. Additional tests were also undertaken to evaluate the influence of wall embedment on the performance of the excavation system. Direct comparisons were also drawn between tests in an attempt to establish the significance of wall crest fixity on soil movements.

The use of all of the special construction techniques investigated were shown to reduce the magnitude of vertical displacement behind the retaining wall and at the formation level; in addition to reducing horizontal displacements at the toe of the wall. Increasing the retaining wall embedment depth in the main test series generally reduced the magnitude of vertical settlement by a factor of two, however the effect was less pronounced in the reference tests. Improving the fixity of the crest of the wall delayed excavation collapse and, where additional support mechanisms were not employed, pinning the crest of the wall was shown to reduce maximum settlement in the reference test by a factor of three.

Of the four supporting construction methods the underwater excavation was found to be the most effective owing to the reduced change in vertical stress during the simulated excavation. Various deep soil mixing geometries were modelled and similar excavation behaviour was observed, however deep soil mixing ground treatment extending to the toe of the retaining wall and across 2/3 of the excavation demonstrated a slight reduction in settlement. Similar behaviour was observed for double walled excavations. Combining underwater excavations with a double wall was shown to further reduce maximum settlements however little additional benefit was observed when performing an underwater excavation with a deep soil mixed soil layer at excavation formation level.

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/21044
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