Trenching for pipelines 1

[ONENGINEER]

Pipelines are going to be placed in a trench of 4 m wide. The invert of pipes would be 5 m below the surface ground elevation. Ground is sandy silt (SPT=11), clayey silt (SPT=17), clayey sand (SPT=4) and below 7 m depth silty sand (SPT=15). Groundwater table is high.

We are using pre-fab cage with steel plates.

Could someone please advise 1-2 references/design manual to design the above trench - including heave, embedment length. Thank you very much.

 

[Ron]

Lateral earth pressure for the sides with surcharging by equipment above.
Potential quick conditions from groundwater. Be careful of sand layer at bottom. Assume you will dewater well below this level.

 

[BigH]

CalTrans Trenching and Shoring Manual - do a search - easily found

 

[ONENGINEER]

I went through the CalTran which referred to USSteel manual, which itself refers to W C Teng, 1962. I have still challenges on lateral distribution of pressures behind the cofferdam for the stratified soils. The above reference dopes not converge with Foundation Engineering Handbook's recommendations by T D Dismuke, 1991.

Wonder if there is a more recent reference which may take all the above into account. Thank you.

 

[BigH]

Are your stratified soils so different? As you are building a shoring box, it will be strutted so a "braced" configuration would be appropriate - I think I'd simplify your "stratification" into a "worse" condition and a "best" condition - see how different they would be for your box design and then go from there. Be robust with your box design. You have never said at what depth the SPT 4 clayey sand is located - is it at the invert depth? Still, look at simplification to get a green or red light.

 

[ONENGINEER]

I have 1 m med dense sand over 2.8 m firm silty clay over 1.2 m loose sand. The sand continues under the base of excavation but become med. dense. For the 5 m to the excavation level, may a ask how you would suggest the worse and best conditions.