Soldier pile and lagging wall in saturated low permeability soils

[pgyr]

My firm is designing a sewer line and lift station for which I did the geotechnical investigation. The lift station will require an excavation approximately 24 feet deep, 60 feet by 40 feet. The project is outside of the area of my professional experience, i.e. dewatering and shoring in low permeability soils. My initial assessment is this: 1) dewatering prior to excavation utilizing deep wells or well points is impractical/uneconomical due to low permeability soils and may not be required, 2) sheet piles will not be drivable into the hard underlying strata, 3) a soldier pile and wood lagging wall constructed around the perimeter of the excavation below the water table seems practical and constructible.

Three borings were advanced to 30 feet at the lift station location. The upper 20 feet of the soil profile consists primarily of lean clay, silty clay, and fine sand layers with groundwater at about 8 feet. Clay and sand strata are deposited in distinct layers with layer thickness generally less than 6 inches. SPT values in the upper 20 feet range from 12 to 17. From 20 to 30 foot depth are strata of weakly cemented claystone and very dense fine sands/sandstone with SPT values of 50 plus. The sands at depth are saturated. Hydraulic conductivity of ST samples from 10-20 ft depth range from 10-6-10-8 cm/sec. CD triaxial tests are underway to obtain shoring loading design data.

Not having any real world experience with this type of installation, I’m wondering what I am overlooking. Is the pile and lagging wall constructible and cost effective? The biggest concern I have would be excessive caving preventing lagging installation as the dig progresses downward. Initial calcs show inflows into the excavation (seeping through the lagging) to be easily handled by trench and sump type dewatering measures. I just get the feeling that I might be overlooking a fatal flaw or a significantly better alternative?

 

[GeoPaveTraffic]

First, since you do not have experience with this type of project, you need to get someone in your firm who does to review your assumptions and calculations. This form is not a substitute for an appropriate in-house review. That said a few comments about your approach.

First, I don't think the permeability and triax tests were neccessary, they will not hurt, but will not really help. I'm assuming that the permeability tests were conducted vertically on samples taken from the site; of much more use would have been in-place tests. It is likely that the horizontal permeability into the excavation will be much higher than indicated by the testing, assuming that the sand layers are somewhat interconnected. However, it probably will not matter, but you need to talk with someone who had dug a deep hole in the same stratigraphic units to be sure. As for the triax, the earth pressures should be based on emperical data for shoring excavations such as (gama * H) - (4 * c) etc.

As for how I would shore it... I think that driving sheet piles to refusal and then using two layers of interal bracing would likely be the most cost effective. However, soldier piles and lagging with either tie backs one layer of internal bracing would likely work just fine. Part of it depends on what is nearby and how much movement you can stand at the top of the excavation.

One question, if you firm is designing the project, are you also building it? Around here the design of temporary shoring is part of the contractors responsibility not the project designer.

 

[pgyr]

Geopave,
Thanks for the post. I too appreciate the need for peer review and it will happen. Water inflow calc's were performed conservatively, with assumed hydraulic conductivity an order of magnitude higher than the test results. Our firm is not building the lift station, however, our contract has us providing a basic design for excavation and dewatering for the lift station. Thanks for the shoring and materials testing tips. At this point, I am trying to get a handle on what the cost of the excavation will be to determine if the project can be built within budget constraints. Contractor submittals of a shoring plan stamped by a PE will be required prior to construction.

 

[GeoPaveTraffic]

Ok. I still recommend talking to someone that has dug a deep hole in the area since the horizontal conductivity of the sand "could" be much higher than you have assumed.

 

[DRC1]

This is a tricky little problem. First, although Geopave has a point about testing, I think it is great to have any data other than SPT's. The sand layers may aloow significant water or posibly make the clay unstable. On the other hand it may be no big deal. Local experience may be the best guide. Trying to drive sheets or piles through 50 blow material is a slow process. Not knowing much about the local conditions and not having run any numbers, I think I would look at driving sheeting down to th claystone, making the footprint large enough to leave about a 5 ft bench. Then, assuming the claystone was competent enough, excavate the claystone to grade. I would probably install two wells with submerible pumpms to try to intercept the water from the sand layers and posibly dewater the claystone. The sheeting would require two levels of bracing.

 

[pgyr]

Sewer line construction in the same alluvial basin was performed with a trench box and in-trench dewatering, depth 15' plus. The contractors description of the soil profile/water inflow matches the boring logs pretty closely. They had difficulty controlling the extent of caving as they advanced the trench box-no surprise. I anticipate the same difficults during the construction of the incoming sewer line....dewatering via well-points may be required to trench for the sewer line.
In any case, we will excavate a test pit or two at the lift station site to gain visible insight on groundwater inflow and caving conditions.
I think DRC1's methodology is reasonable, oversizing the pumps conservatively.
Thanks for the suggestions