Laterally Loaded Pile in Cohesive Soil 1

[MasterrrrBlasterrrr]

Hi


I am a structural engineer asking a soil mechanics question.
Not after an in-depth answer to the following.



Why is the ultimate capacity of cohesive soil to restrain a laterally loaded pile related to the Undrained shear strength of the soil?
EG, Broms Theory uses 9*Cu.


My basic understanding is that undrained behaviour relates to load that is applied in the short term, and water cannot drain from the pores between the soil particles.
What I do not understand is that - for loads that are applied in the long term, where drainage can occur - why is it applicable to use Cu?
Or - for short term loads that are applied to cohesive soils that have mainly air in the voids, not water: Why is it applicable to use Cu as opposed to C'?




Thanks

 

[GeoPaveTraffic]

You indicated that you were not looking for an indepth answer, so I'll try.

Generally, the assumption with cohesive soils is that the short term (i.e. undrained) soil properties control and that the long term properties of the soil are better.

Note that this is not allways the case, particuarly with heavily overconsolidated clays.

Mike Lambert

 

[MasterrrrBlasterrrr]

Hi GeoPaveTraffic


Thanks for the reply.

The following discussion is intended for the benefit of my general understanding.
In a real project I would rely on the advice of the project geotechnical engineer


I would like to raise a specific design example to talk further on this subject.
The example is a cantilever soldier pile shoring wall.
Lets say - pile diameter of 450mm at 2.0m centres, cantilevering H = 3.0m, supporting stiff clay with Cu = 50kPa.


The soil loads acting on the shoring wall are long term, obviously - and lead me to question what resistance is provided by the DRAINED soil condition?

I know from experience that for the cantilever height "H", I will need about 2*H length of pile socket if the piles socket into reasonable soil.
Or, if the piles socket into rock (at the bottom of the retained soil), I will need about 1*H socket length (or greater) depending on the rock properties.

I would like to talk about the example where the piles socket into clay (say Cu=50kPa).
From Brom's Theory for ultimate capacity, I get a lateral restraint of 9*50 = 450kPa (although zero in the top portion of 1.5*450mm).
I understand that this is the ultimate capacity of the soil, and using a "broad brush", would reduce this by a factor of 3.0 for use in structural design as the "allowable" load.
Strictly speaking, there is no "allowable" lateral load - just a load at which the deflections are acceptable.
For arguments sake, I am saying that a Factor of Safety of 3 is reasonable.

This still gives me a pressure resisting the loads of 150 kPa laterally on the piles, or 67 kN/m for a pile diameter = 450mm.


The following part is where I am confused:


If the clay is "dry", and there is no pore water pressure to assist in taking the load - I dont see how it can be relevant to use the undrained soil strength, and
The Drained soil strength seems to be MUCH less??, as below:

Drained shear strength = σ' tan(φ') + c'

C' is often a low number, 5 kPa or so.
σ' tan(φ') - varies with depth, but would take many metres of depth to get anywhere near 150kPa (as above)

I don't understand how to apply these soil parameters into the structural design of the pile. The values are too low.
I then revert back to using the passive resistance of the soil with an appropriate factor of safety, ie Broms Theory for cohesionless soil.


Thanks

 

[GeoPaveTraffic]

First let me say that I'm not familure with Broms Theory. Second, if I were designing a case where soldier piles were being installed for a permanent installation in clay I would check the embedment using both drained and undrained properties.

One way that I would check the long term resistence is passive pressure acting on the embeded portion of the soldier pile. Depending on your soil, Kp could be 2.5 or so. Apply a factor of safety and you can determine your required embedment. Some people will apply the load over more width of soil than just the pile, up to 2 daimeters, I generally do the same.

Mike Lambert

 

[MasterrrrBlasterrrr]

Mike


See attached for an extract on Brom's Theory for laterally loaded piles in cohesive soil.


Tom

 

[274490]

Hi,MasterrrrBlasterrrr, Bro's theory is applicable for pile under the lateral loads. This is not the theory for analysing the retaining wall structure. Passive and active earth pressure theories are more appropriate for retaining wall analysis.

Please refer to some soil mechanics books such as Bowles foundation design and analysis for more details.