Abstract:
Deep excavations to accommodate basements have become widely attractive in densely
populated cities in recent years. Excavations that extend below groundwater table need to be
watertight and expensive structures in the form of diaphragm walls and secant pile walls are
needed. Above the ground water level, much simpler structures in the form of soldier pile walls
can be used. However, with the infiltration of rainwater the matric suction will be lost and
saturated cohesion values are used in the designs normally.
Using a capillary barrier, infiltration of water into the soil underneath could be reduced and a
much higher apparent cohesive value accounting for the presence of a matric suction can be
used and the design of the soldier pile wall can be optimised. This feasibility of this proposal
was studied with an instrumented laboratory model and experimental results were verified by
a numerical simulation done with SEEP/W software, modelling the process of infiltration.
Results of the laboratory model instrumented with tensiometers and moisture sensors revealed
that matric suctions of the soil below the capillary barrier remained high over 8 days
withstanding a rainfall of 10 mm/h. A failure occurred after 9 days. Subsequently, the study
was extended to a 6 m deep field excavation laterally supported at three levels by a soldier pile
system. Effectiveness of capillary barriers of width 1 m, 2 m and 4 m with 300 inclination was
studied numerically. With the capillary barrier of 2 m width, high matric suctions were
maintained underneath of CB over 4 days of continuous rainfall. The force on the struts were
only 60 kPa during this rainfall. In the absence of a capillary barrier the strut forces would have
increased by 250 %.
The results of the study revealed that in the presence of a capillary barrier the infiltration would
be minimised and design of the soldier pile wall system could be significantly economised. A
capillary barrier of a limited width of 2 m would be sufficient making it feasible in the limited
space available in sites.
