pipeline and soil design paramters

[gwkwong]

I am working on a pipeline project which my Client is asking me for the soil design parameters (density, cohesion, friction angle) for his pipeline stress analysis. We did the soil investigation for this line which i can estimate the insitu soil parameter. However, I know that the contractor will excavate and probably end dump the soil back into the trench with minimal or no compaction.

I can estimate the insitu soil parameters based on the literature. I even consider to provide my Client a bulk factor to reduce the soil density due to excavation. However, I didn't come across any factors for cohesion of backfill soil. The insitu soil cohsion is about 12 to 60 kPa. Is it reasonable to reduce the cohesion and how?

 

[msucog]

i would expect cohesion to be "broken" for the backfill even though some will be present

 

[BigH]

Can you be a bit more specific on what the soil types you are expecting to encounter? Is the cohesion you reporting the undrained shear strength of a clayey soil? - or the shear stress intercept of a Mohr's circle or stress path plot? You have indicated that the contractor will simply excavate and "probably" through the soil back into the trench. If this is the case, I'd go on record that this is unacceptable. The specifications should have clear performance requirements (compaction level, layer thicknesses, acceptable backfill material, etc.) - if so and the contractor simple "throws" the soil back in - then someone should hold the contractor to task - for non-compliance. The contractor should not get away with doing what he has stipulated he would do when he signed the contract. I would surmise that the designers stress analysis would be somewhat based on certain parameters and the specifications should be such that the parameters are achieved. Bulking factor - yes - but only for purposes of excavation and disposal (or spoil) - obviously you will need to waste a good percentage of the fill - if you are to maintain grade. I suggest that you work with the designer and develop your parameters (that he wants) to meet his needs - and at the same time to ensure proper project requirements.

 

[gwkwong]

The soil is very soft to stiff, medium plastic clay (till), N values ranges from 2 to 12. As I know, a lot of pipelines were laid down without any properly compaction in Alberta. They were all done by contractor "observation" or best practice. This is a small 200 mm diameter pipe and will bury about 1.5 m below grade. The designer is looking for the undrain shear strength of the backfill material. The guidelines for design is based on ASCE American Lifelines Alliance "Guidelines for the Design of Buried Steel pipe July 2001". Practically, the soft material is very hard to compact because it is wet. I agree that some compaction shall be done to backfill the trench. But in reality, they don't.

 

[BigH]

Hard to believe that clayey till only has N values of 12 max - and for an N value of 2, doubt this is really till unless it has been wholly remoulded. With these N values - and rough correlations with undrained shear strength, you have a huge range of about 15 to 75 kPa. However, the undrained strength in practice depends on how it is put back as you know. Clay soil, being "lumpy" is hard to compact unless you have the right equipment (kneading action is the best). The reality you speak of - if the specs say compact and they don't, they shouldn't be paid. Simple as that (but hard to put into place, I agree). You might check out clay fills in the 1960 Boulder ASCE special publication for properties of clayey fills. It's an oldie but goodie.