Soil stress beneath strip footings
Soil stress beneath strip footings
(OP)
Hi,
I have the following problem designing the foundation of a structure composed by an heavy loaded part connected to a lighter part.
The heavy loaded part of the structure is based on a mat foundation. The other part is based on strip footings. The strip footings are connected to the mat foundation.
In the FEM model I insert the k value (subgrade modulus for the foundation), in order to model the behavior of the soil.
The k value depends on the width of the foundation, thus I insert different value for strip footings and for mat foundation (k value for strip footings in this case is around 5 times bigger than k value for mat foundation). The analysis shows big settlements under the mat foundation, and smaller settlements under the strip footings.
In corrispondance of the connection between strip footings and mat foundation, the settlements are similar, thus the soil stress beneath strip footings is much bigger than the soil stress beneath the mat foundation (this because I obtain the soil stress multiplying the settlements by the k value, that is different according to the width of the section).
This result seems not normal, leading to soil stress beneath the strip footings more than double those obtained with hand calculation.
Keep on going on this way, the only solution seems to define only one mat foundation for the whole structure.
What is you opinion/suggestion about this?
Thanks in advance.
I have the following problem designing the foundation of a structure composed by an heavy loaded part connected to a lighter part.
The heavy loaded part of the structure is based on a mat foundation. The other part is based on strip footings. The strip footings are connected to the mat foundation.
In the FEM model I insert the k value (subgrade modulus for the foundation), in order to model the behavior of the soil.
The k value depends on the width of the foundation, thus I insert different value for strip footings and for mat foundation (k value for strip footings in this case is around 5 times bigger than k value for mat foundation). The analysis shows big settlements under the mat foundation, and smaller settlements under the strip footings.
In corrispondance of the connection between strip footings and mat foundation, the settlements are similar, thus the soil stress beneath strip footings is much bigger than the soil stress beneath the mat foundation (this because I obtain the soil stress multiplying the settlements by the k value, that is different according to the width of the section).
This result seems not normal, leading to soil stress beneath the strip footings more than double those obtained with hand calculation.
Keep on going on this way, the only solution seems to define only one mat foundation for the whole structure.
What is you opinion/suggestion about this?
Thanks in advance.





RE: Soil stress beneath strip footings
RE: Soil stress beneath strip footings
RE: Soil stress beneath strip footings
Hope this helps.
f-d
¡papá gordo ain't no madre flaca!
RE: Soil stress beneath strip footings
-top sands (from 0 level to -2.5m)
-quaternary sands medium dense to dense with deformation modulus Es=70-90MPa (from -2.5m to -12m)
-clayey quaternary sands with deformation modulus Es=60-80MPa (from -12m to -14m)
-sandy loam (weak soil properties) with deformation modulus Es=15-30MPa (from -14m to -25m)
-footbase sands (dense sand) with deformation modulus of Es=80-100MPa (from -25m to -30m)
The level of underground water is -10m
The bottom level of foundation is -2.5m / -3.5m.
The mat foundation has a dimension of 14x35m2
The strip footings are 1.5m width and 16m long.
Mat foundation and strip footings tranfer an average stress on soil of 200KN/m2.
Regarding the sandy loam layer, draw down settlements due to the low Es-values of the encountered clays are considered, and the expected settlements in this scenario are around 3 to 6 cm. For this reason we are performing a plant initiation with constant and distributed life loading in order to reduce the settlements to 2 cm.
RE: Soil stress beneath strip footings
RE: Soil stress beneath strip footings
Once defined k, I multiply it by the influence area in order to define the spring stiffness for the FEM model.
The mat foundation moves down and drags down the strip footings. Can this effect cause the failure of the soil beneath the strip footings but not beneath the slab, even if the 2 foundations are very near and connected?
Anyway, I will advise with the geothecnical engineer.
RE: Soil stress beneath strip footings
RE: Soil stress beneath strip footings
Regarding k I'm using the Vesic theory indicated in Bowles "Foundation analysis and design". It is indicated that k=Es/B/(1-m^2) where m is the Poisson's ratio, B the width of foundation and Es the deformation modulus.
RE: Soil stress beneath strip footings
Which values did you figure out for Es and m?
Shallowest layer? Weighted mean of layers? Else?
Did you use m-values as suggested by Bowles'tables or some more specific indications (0.15 to 0.2 for sands, 0.5 for clayey soils below GW)?
Vesic's formula does not account for influence factors due to the geometry of the setup. When using subgrade modulus to assess overall serviceability and not structural serviceability of the foundation structure problems may arise especially in mat foundations on layered soils...
Anyway, your final result (mat yielding remarkably and dragging down strip foudnations) would also agree with the presence of the weaker layer within the mat's influence area)
RE: Soil stress beneath strip footings
"Vesic's formula does not account for influence factors due to the geometry of the setup", do you mean that I should consider the Steinbrenner factor?