In Hong Kong, there is an increase of using large excavated piles supported by benotnite (barrette piles and bored piles) for the foundations of high-rise building. In most cases, due to the geological conditions, many foundations have been designed as floating piles to resist the superstructure load by shaft resistance. According to the current design practice, the shaft resistance of a large excavated pile embedded in saprolites is either ignored or limited to 10 kPa, if no in-situ load test is carried out....[ Read more ]

In Hong Kong, there is an increase of using large excavated piles supported by benotnite (barrette piles and bored piles) for the foundations of high-rise building. In most cases, due to the geological conditions, many foundations have been designed as floating piles to resist the superstructure load by shaft resistance. According to the current design practice, the shaft resistance of a large excavated pile embedded in saprolites is either ignored or limited to 10 kPa, if no in-situ load test is carried out.

In this thesis, five comprehensive full-scale pile loading tests were carried out by the author in three different sites to study soil-structure interaction behavior between piles and saprolite soils. Construction factors including types of pile (i.e., barrette or bored piles), construction methods, and construction practice were studied. The results were compared with existing pile loading test database in Hong Kong. Moreover, a series of preliminary laboratory triaxial tests were carried out to mimic the swelling behavior of surrounding soil around an excavated trench. The performance of soil mixed with different amounts of bentonite was studied in a large direct shear box to investigate the influence of bentonite on shear strength of saprolite.

It was found that for the large excavated bentonite-supported piles (barrette piles and bored piles), the shaft resistance was highly mobilized at 1.0% to 3.0% of pile diameter (or equivalent pile diameter by area). Besides, it appears that the shaft resistance of piles showed a better correlation with effective overburden stress (β) than that of average SPT-N value (τ_s/N). For barrette piles constructed by grab and chisel or hydrofraise, the empirical correlations of β were similar, varying between 0.35 and 0.5. However, these values were, in general, higher than that of bored piles constructed by Reversed Circulation Drilling, varying between 0.1 and 0.2.

It was evident that the shaft resistance decreased with an increase in duration of trench opening time. For barrette piles constructed by grabs and chisel or hydrofraise, β could decrease from a range of 0.4-0.5 to a range of 0.1 - 0.2 for long duration construction. It is suggested that a trench opening duration of one day may be a useful guide to distinct the two ranges of β values.

Preliminary laboratory triaxial test results showed an exponential increase of radial strain due to a reduction of horizontal effective stress. Swelling behavior is significant for soil adjacent to an excavated trench. Laboratory direct shear test results revealed that the angle of shear resistance of a well-mixed soil was not affected if the percentage of bentonite (by weight) was less than 2%. However, beyond the 2% value, the angle of shear resistance of the mixed soil decreased with an increase in the percentage of bentonite significantly. At 10% by weight of bentonite, the angle of shear resistance was reduced to only 15%.