Soil Bearing Capacity under Pipelines 2

[Davidmat]

Are there specific methods (other than the classical shallow foundation formulas) for estimating the bearing capacities of the soils underlying an RCP in a trenched excavation?

I am a geotechnical engineer in the process of designing a foundation system for a 54-inch diameter (RCP) combined sewer interceptor for the I-195 relocation project in Providence, Rhode Island. The installation method is expected to involve excavating a 25 foot deep by a 12 foot wide trench supported by braced steel sheeting with the sheeting left in place. A subsurface investigation has been completed (involving soil borings) has been completed with noncohesive soils encountered.

Sincerely,

David M. Matheson, P.E.
Project Engineer

 

[KAM_imported]

Pipeline installation is generally not a bearing capacity or settlement issue because the soil beneath the pipeline normally "feels" an unloaded condition as the excavated soil weighs more than the pipeline and transmitted fluid which replace it. The issues are creating a stable working mat for the pipe to rest on (usually by placement of a few inches of stone or gravel), proper dewatering techniques, and proper backfill placement and compaction. If the soil is very compressible (peat, for example) partial excavation and replacement with a suitable lightweight structural backfill material may be necessary. The soil-structure interaction problem becomes one of determining whether the conduit can be classified as rigid (does not deform under load) or flexible (will deform under load). There are published methods of designing buried conduit structures in soil (refer to Chapter 23 of Foundation Engineering Handbook).

 

[14243]

Hi David:
If the subsurface soil consists of sand with SPT blow counts more than 10, there should not be a problem in supporting the RC pipes. I don't know why do we need to determine soil bearing capacity?

 

[Ginger]

David

As Kam explains, this is a soil/structure interaction problem and you need to check the net load on the formation following pipe installation to get an idea of settlement or heave.

However, one thing I have noticed is that your trench seems to be very wide when compared with your pipe diameter. This is important because of the phenomenon of "narrow" or "wide" trench condition.

If you draw a scaled cross section of your trench with the pipe in it you will see that there is a lot of fill around the pipe. In time, or under ground level surcharge load, this fill will compact and impose a downward load on the pipe. This downward load can be resisted by shear stress generated at the sheet piles. But your trench is so wide that the pipe will not feel this beneficial uplift from the piles. Now re-draw with a trench approx 80" wide. The narrow trench effect can be seen to reduce the load on the pipe. this also saves on excavation & backfill costs.

Regards Andy Machon