Actes du colloque - Volume 2 - page 89

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Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
where: (σ1-σ 3)max = asymptotic value of stress difference
  axial strain
Ei = initial tangent modulus i.e., the slope of   curve
While modeling, the initial Young’s modulus was assumed to
be 5 MPa for dry sand considering the lower soil stiffness in
small scale footings. Following Terzaghi’s (1943) suggestion
that the Young’s modulus reduces by 50% in submerged sand,
the initial Young’s modulus in this sand was taken as half of
that of the dry sand. The asymptotic stress difference relates
closely to the ultimate strength of the soil mass and was taken as
the bearing capacities of footings on dry and submerged sands
obtained from pressure-settlement curves derived from the
model tests. The test on circular footing placed on dense sand
was modeled in this paper. The rise of water table depth was
simulated using appropriate parameters and correction factors at
various water table depths were observed.
Figure 5. Water table correction factor diagram for 100 mm diameter
circular footing obtained from experimental results and numerical
modeling.
Figure 5 shows the comparison of water table correction
factor diagrams obtained from numerical modeling (dotted line)
and experimental results (solid line). The diagrams were similar
in shape, both being curved rather than linear as previously
proposed by some researchers. Also, both the curves indicate
that the effect of water table depth is negligible at a greater
depth, whereas settlement increases rapidly as the water table
gets closer to the footing base. The assumed soil parameters
may contribute to the differences in correction factors obtained
from numerical modeling and laboratory testing.
6 SUMMARY AND CONCLUSIONS
Laboratory model tests were carried out to investigate the effect
of various factors on increase in shallow foundation settlement
when subjected to fluctuation in ground water level. Additional
settlements at various water table depths were observed and
water table correction factor diagram for each case was
obtained.
The results show significant increase in settlement as the soil
immediately below the footing level gets saturated. The results
clearly indicated that the increment is higher in soils having
lower density; however, the increment is significant even in
dense soils. The effect of footing shapes on additional
settlement in saturated sand was not evident from the results.
Comparison of applied pressure-settlement curves in dry and
submerged sands suggest that the additional settlement due to
submergence increases with the stress level. Modeling a circular
footing in FLAC and its comparison with test data confirms that
the correction factor diagram is not linear, and the correction
factor increases at a faster rate in the vicinity of the footing. The
results obtained will help to understand how the fluctuating
water level affects the shallow foundation settlements on
granular soils and will allow designers to apply appropriate
correction factors for water level rise. There is a scope for
further investigations to identify the effect of other important
factors (e.g. depth of embedment, footing width, and soil
gradation) in settlement behaviour of shallow footings with
changing groundwater level. More laboratory testing with
different initial densities might be useful to develop water table
correction factor charts for varying relative densities and shear
strength parameters. Also, advanced soil models can be used to
study the effect of rising water table on shallow foundation
settlement on cohesionless soils.
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