Bearing capacity of soils under seawalls

[jeabramian]

Hello everybody,

As a port engineer, I frequently have to deal with gravity walls. Usually, the height of these massive concrete structures is 15, 20 m or more. They have water on one side and the water table and different soil layers on the other (including pavement). I checked many soil reports, and they are dark, as they do not give many details of the criteria they use to compute the bearing capacity of the soil. Here are my doubts:

1) Should they use the drained or undrained parameters of the foundation soil? I understand that undrained parameters are used if you have sudden loads that would increase the water pore pressure. But can an accidental load that only works for a few hours mobilize the water pressures so quickly to 18 m of depth?
2) Do small loads compared to the weight of the gravity wall or soil above the foundation layer work the same way? I'd tend to think there would be some dissipation effect in these cases. Am I wrong?
3) The soil-bearing capacity formula includes a q factor that takes into account the effective stress at the foundation. Should that q include the accidental loads?
4) Is is true that in the ULS case, the FOS for the bearing resistance could be close to 1? An for the SLS over 2.5?
5) I recently checked one 100-year-old gravity wall and its soil-bearing capacity. Next to these walls usually circulate reachstackers, equipment that imposes about 100 t/sqm on the pavement. The pressures are much higher than those indicated by the soil capacity. This has been going on for decades, and the wall didn't move an inch. How can I explain that?

Sorry for all these questions, but I am very curious and anxious to understand how these work.

Thank you!

Jorge

 

[geotechguy1]

1) Should they use the drained or undrained parameters of the foundation soil? I understand that undrained parameters are used if you have sudden loads that would increase the water pore pressure. But can an accidental load that only works for a few hours mobilize the water pressures so quickly to 18 m of depth?

You should use parameters that are relevant to whether the shearing process is drained or undrained (caveat emptor words "shearing process" - a soil can be 'drained' in the sense of the overall mass being at a steady state or not having an excess PWP but still have undrained conditions on the shear surface)

2)

I couldn't quite understand what you mean here, can you provide a sketch?

3)

The q factor is accounting for overburden pressure due to burial depth basically. I wouldn't count an 'accidental load' if it is acting in your favour though?

4)

What do you mean by ULS and SLS here? Is this about a seismic case?

5)

Things are quite conservative, often.


To me it sounds like you have hired a 'copy paste geotech report' geotech who doesn't actually understand anything but you need someone who understands soil mechanics, finite element modeling and cyclic loads and advanced investigation / testing techniques for your case.

 

[Smoulder]

Agree with Geotechguy1. These big walls are meant to have the geotech engr do the geotech design. Boilerplate report given to structural/port engr is not good practice. Both engrs should know enough about the other so info can be exchanged though. If geotech can't comprehend ULS then you're asking for trouble or v conservative design.

1) Usually undrained for clays because of time to dissipate the pressure. Drained case often governs.

2) Also not sure what you're asking, but maybe see #1.

3) Look at full shear planes. Usualky no load on berth side so accident load doesn't help there.

4) Don't refer to FOS when talking ULS. It's a workjng load term. Unity Check or something similar. The safety margin is built in at various points throughout the calculation, not one factor at the end. A ULS design with adequate capacity will have FOS >1.5 approx for static loads if you did working load calcs for comparison.

5) Pavement design is different to foundation design. For foundation, reach stacker axle is 100 tonne which is small compared with 15m of wall load.

 

[jeabramian]

Thank you for your answer. Here are some additional questions and clarifications.

Description

-Usually, the top layer is a 10-inch concrete pavement, and the layer below is clayey or silty about 30 ft thick.
The foundation layer is either sand or a hardened mix of fine sands, clays, and silts (N=40 blows or so), depending on the foundation's depth, which could run between 15 and 25 m (this is typical in the ports of Buenos Aires, La Plata, and Dock Sud).
-The water level ranges between 2 m to 4 m from the top of the pier.
The loads are usually mobile cranes on rails or rubber tyres, depending on the port, container trucks, reach stackers, and stacks of containers. Heavy equipment that weighs 100 t or more each.
These ports are about 100 years old, meaning the soils should be very consolidated. They have endured heavy loads along their study.
In one soil study (I did not hire them; I've just got them from different port authorities) performed by a Canadian-specialized geotechnical firm, the clayey layer was considered to be drained because it is at the top and there are some drain holes in the wall.
-In a different study, the specialist considered the top layer undrained for another gravity wall.

About the questions I asked

1) Given all these facts, I wonder if the top layer should be considered drained or undrained. From what I read, clayey layers should be considered drained. Sorry, but I did not understand your explanation of the shearing process. And what about the foundation layer if it is a mix of compacted, hard clays and silts? Being fine grains, should choose undrained conditions even when they are very hard and consolidated?

2) I don't have a sketch, but what I mean is: given a small load -let's say a truck- that is acting for a short period of time -let's say two hours-, would that load have enough time to develop water pressures 18 m below the surface?

3) About the q factor, several studies have considered the effective pressure to compute the foundation's bearing capacity. For instance, one made by an Argentine soil specialist using Geo5 considered the crane's weight, which makes the foundation capacity much higher. Using different cranes (different weights), the bearing capacities are larger or smaller. Now, I have computed the capacity without the burden of the crane and then the bearing capacity would be reduced so much that the wall would be unstable. But the wall is standing still for decades! Of course, I am using the submerged density for soils under the water table.

4) No earthquakes here. Here the concept of ULS is related to a maximum condition without factoring the loads, in this case, a very heavy storm with maximum winds. SLS considers magnifying the service loads by a factor (normal winds).

Summarizing, I have doubts what kind of conditions should be used - undrained or drained - for the different layers, if q has to include the overburden pressures or not to compute the bearing capacity, and if a temporary load could develop the stresses so quickly that could affect the deeper layers (foundation).

I appreciate your comments and thanks again.

Jorge

 

[Smoulder]

I am not a geotech so hopefully one will chime in if I'm wrong.

For lateral pressure:
Analyse both undrained and drained cases for sustained load. Take the worst case. Then add undrained case for incremental short term load.

For bearing capacity:
Do similar but you add the two utilisations. (sustained bearing pressure/drained bearing capacity) + (short term increment load/undrained capacity).

For the q factor, see below figure. Overburden confining the full failure zone increases capacity. You never have overburden on the berth side so taking this benefit is questionable.

https://www.researchgate.net/profil...n-soil-underlying-a-shallow-strip-footing.png