Slope Excavations in "Cohesionless" Soils

[theCorkster]

I'm working on a project involving temporary excavations to provide working benches for installation of permanent shoring walls. The boring lots indicate the material is fill comprised of Ml, GM, SM, GP materials. Stability analysis of existing topography indicates the slope is not stable as is without minimal cohesion (100 psf) in the ML, GM and SM materials. Since this material is fill and was at one time compacted, the presence of some cohesion due to dilation during shearing would be expected. Review of NAVFAC and USBR Table 8 for compacted soils does provide some guidance on the soil cohesion issue.

Currently, I'm not aware of any standard or process to provide specific data other than performing SPT tests,in place soil density tests or geophysical methods to provide guidance on the soil consistency (loose, medium dense, dense or very dense). And of course, all of this data would then have to be correlated to known empirical data.

SPT data could be used at comparable elevations to the boring logs SPT test elevations to give some guidance as to whether the soil classification and consistency is similar or not. While this provides guidance on friction angle it does not give any quantifiable indication of cohesion.

Measuring in place moisture/density with NDT gauge would require laboratory vibratory table or impact compaction tests to assess relative compaction; this information, in conjunction with classification would provide guidance on friction angle but no cohesion.

Finally, surface shear wave velocities could be measured with the slope gradient, and the wave velocity correlated to ranges of SPT.

Regardless, each of these approaches involves considerable costs and time, which may be unavoidable.

I'd appreciate any experiences or alternate approaches you may be familiar with.

 

[SlideRuleEra]

1) I'm working on a project involving temporary excavations to provide working benches for installation of permanent shoring walls.

2) Stability analysis of existing topography indicates the slope is not stable as is without minimal cohesion (100 psf)...

3) Regardless, each of these approaches involves considerable costs and time, which may be unavoidable.

4) I'd appreciate any experiences or alternate approaches you may be familiar with.

1) For a project in the USA, construction (temporary excavation) is controlled by OSHA requirements... which do not necessarily need complex test results for justification, because:

2) This is OSHA Type "C" soil (the worst).

3) If your fee is being paid by the Contractor, see how much (of his own) money he wants to spend on soil testing versus just assuming worst case and conservatively designing/constructing temporary excavation.

If your fee is being paid by the Owner, don't do any testing, have the contract documents include that soil is OSHA Type "C", make what soil information you currently have available, and require the Contractor submit his temporary excavation plans for your review and acceptance (before work begins).
If you do the temporary design and require Contractors to use it, the Owner may pay dearly (not only for your testing and design but by limiting the Contractor's "means & methods" options).

4) I've dealt with both sides of issues like this... as a bridge construction contractor (who happened to be a PE at the same time) making the Owner pay dearly for specifying construction "means & methods"...

and, later, as an in-house Owner's engineer performing review of Contractor's technical submittals (and accepting several that were truly innovative and cost saving for both Contractor and Owner).