Excavation inevitably induces stress changes and soil movements that might affect the performance of the existing pile foundations. Most of the previous research focused on the lateral response of end bearing piles to excavation induced horizontal soil movements. Settlement and tilting response of floating piles, due excavation induced stress relief and soil settlement, is not well understood.

The research aims to investigate the settlement and tilting behaviour of floating piles and load transfer mechanism adjacent to multi-propped excavation in dry sand and saturated clay. Two research methodologies, centrifuge and numerical modelling, were adopted. In the dry sand, two centrifuge tests were performed considering elevated pile group (PG) and piled raft (PR) adjacent to the ex...[ Read more ]

Excavation inevitably induces stress changes and soil movements that might affect the performance of the existing pile foundations. Most of the previous research focused on the lateral response of end bearing piles to excavation induced horizontal soil movements. Settlement and tilting response of floating piles, due excavation induced stress relief and soil settlement, is not well understood.

The research aims to investigate the settlement and tilting behaviour of floating piles and load transfer mechanism adjacent to multi-propped excavation in dry sand and saturated clay. Two research methodologies, centrifuge and numerical modelling, were adopted. In the dry sand, two centrifuge tests were performed considering elevated pile group (PG) and piled raft (PR) adjacent to the excavation to study the influence of raft contact pressure on PR settlement and tilting compared to PG. In the saturated clay, five centrifuge tests were conducted. The first four tests were conducted on a single pile adjacent to an excavation, to investigate the influence of working load and over-consolidation ratio (OCR) on excavation induced pile settlement. Thereafter, one test was performed to investigate the excavation induced PG settlement and load transfer mechanism. Furthermore, numerical back analysis and parametric study were performed to provide insight into the excavation induced pile-soil interaction.

The tests in dry sand reveals that PR experiences larger excavation induced settlement than PG. Relative pile-soil settlement and raft contact pressure show that a gap is formed between the raft and ground surface that causes the load transfer from the raft to the piles resulting larger PR settlement than PG. In contrast to settlement, similar tilting is observed in PR and PG. This is because of, the difference of load transfer to pile toe for front and back pile is the same in both PG and PR resulting similar magnitude of tilting. Distinctive progressive change in pile head load is found in PG and PR due to excavation. In PG, load is transferred from rear pile to front pile. On the contrary, pile head load is increased for both the front and rear piles in PR. Furthermore, the excavation-induced settlement of the PG is found to decrease by 4 times with increasing density from 30 % to 90 % due to increase of soil stiffness. Moreover, it is found that working load has significant influence on PG settlement while minor effect on PG tilting. The influence of wall stiffness on PG settlement and tilting is found to be insensitive for shallower excavation (H_e/L_p < 0.5), while it influences significantly for excavation deeper than pile depth (H_e/L_p ≥ 1.0). The use of diaphragm wall reduces the maximum settlement and tilting up to 40 % and 90 %, respectively, compared to the sheet wall.

For a single pile in clay, pile settlement is found to be 3 times larger in lightly over-consolidated clay (OCR=1.8) than heavily over-consolidated (OCR=4.6). Due to higher initial void ratio in lightly over-consolidated clay causes larger pile settlement because of lower soil stiffness. Also, the initial working load has significant influence on short-term and long-term pile settlement. The settlement of the single pile increases by 3.0 and 2.6 times when the factor of safety (FS) is reduced from 4.0 to 2.5 at the completion of excavation (H_e/L_p = 0.4) and 2 years after, respectively.

For the PG in sand and clay, it is found that settlement increases with the advancement of excavation while tilting depends on H_e/L_p. This is because of gradient of soil movements depends on H_e/L_p. Moreover, the minimum threshold value of wall stiffness when excavation depth has no influence on PG tilting is 450 and 7000 MN-m² in sand and clay, respectively.