Soldier pile and lagging wall in saturated low permeability soils
Soldier pile and lagging wall in saturated low permeability soils
(OP)
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?
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?





RE: Soldier pile and lagging wall in saturated low permeability soils
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.
RE: Soldier pile and lagging wall in saturated low permeability soils
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.
RE: Soldier pile and lagging wall in saturated low permeability soils
RE: Soldier pile and lagging wall in saturated low permeability soils
RE: Soldier pile and lagging wall in saturated low permeability soils
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