Abstract:
Bored and cast in situ bored piles are used to support the heavy super structure loads,
while transferring them the hard rock layers. Because of that, in order to design
single piles or group piles, it is very important to know the carrying capacity
characteristics of the pile. To determine the settlement characteristics of the pile head
with the load, static pile load tests are playing a significant role. It is really important
to know about the skin friction distribution along the pile shaft and the deviation of
the applied load in to the friction through the shaft. Such information can be obtained
by using the instrumented pile load tests, where strain gauges are installed along the
pile length. However the instrumented pile load test is expensive and not always
carried out in all pile construction sites, in Sri Lanka. Further, since it is affected by
many factors and the processes, the outcome from the instrumented pile load tests is
not easy to interpret.
To find out the carrying capacity of the piles, empirical formulae and factors
available can be used. However, use of software packages based on finite element
analysis to find out the carrying capacity of the pile may provide an excellent
opportunity to obtain results easily and quickly, if the accuracy of the results can be
established. Because of that, this study was aimed to find out the carrying capacity of
cast in situ bored single piles using commonly used finite element software PLAXIS
2D and compare the results with the instrumented pile load test results obtained in
the field. Further, the differences and the difficulties of the interpretation of results
with their potential reasons were discussed within the study.
Results for two borehole tests and instrumented pile load tests were obtained and
compared the real world data with the numerical simulation of such test with same
conditions. Input parameters for the Finite element software used were Young‘s
modulus of the soil and rock, poission‘s ratio and the shear strength parameters of the
soil. Young‘s modulus for the soil layers were calculated from the energy correction
method and for rock layers it was calculated using the Hong Kong geo guide lines
(Geo,2006) and rock mass rating values. Shear strength parameters for the soil layers
v
were calculated using the borehole data and the method proposed by Bowels. For
rock layers it was used the Hoek-Brown formulae, proposed by Hoek and Brown.
The best match results with the field data for weathered rocks were given when used
twice the Young‘s modulus for rock layers. For the bored piles socketed in to fresh
rock, the best match results with instrumented pile load test results were given when
used half the value of the Young‘s modulus of rock which was found using the Hong
Kong geo guide lines (Geo,2006).
