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Lpile soil properties in backfill

Lpile soil properties in backfill

(OP)
I'm working on a project for steel helical piles being installed into backfill which is yet to be completed. I could potentially recommend lengthening the proposed piles and installing the helices into undisturbed soil. Even then, there are lateral loads specified and the properties of the soil in the backfill are relevant. Any ideas as to what specifications I should be recommending for the backfill and what soil properties to use in the Lpile analysis? The backfill will be low plasticity clay. In the clay's natural state it is soft (cu ranging from 25kPa to 75kPa). Any advice would be appreciated.

RE: Lpile soil properties in backfill

(OP)
For example, if I were to specify the CL fill material to be compacted to 95% of standard proctor density, what soil properties (shear strength and unit weight) might one expect?

RE: Lpile soil properties in backfill

Your project Geotechnical Engineer needs to provide you with this information.

Mike Lambert

RE: Lpile soil properties in backfill

(OP)
I appreciate the advice Mike, and I completely agree. I think I'll be forced into making some assumptions as the piles are being fabricated shortly and the back fill is not yet completed (or started). Any recommendations on conservative assumptions that could be verified with a proctor test?

RE: Lpile soil properties in backfill

Not without knowing the local practice and materials that will be used.

Mike Lambert

RE: Lpile soil properties in backfill

Based on experience with silty clay fill in the Midwestern US, compaction to minimum 95% standard Proctor can be done within Optimum -3% to optimum + 3%,possibly wider. The consistency at the drier end will be very stiff to hard, and at the upper end, just stiff. Exposed to water for a while, it may reduce to "firm to stiff". The unconfined strength could be as low as 1500 psf.

If you can't vary the design in the field, I suggest compacting samples under reduced energy to 95% standard, soaking them, and running unconfined strength.

If you assume the soil will not become saturated, you had better be a good prophet. Cultural practices can change during the life of the structure.

RE: Lpile soil properties in backfill

What are expected pile loading? For what structures? If just for light poles, go for lower bound Cu to check the minimum embedment depth, relax the compaction requirements.

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