Foundation Performance on Re-Engineered Ground

[Pitchstone]

I am looking to track down references and/or case studies which explore the performance of shallow foundations, ideally strips although rafts would also be useful, for typical residential low rise structures founded on relatively minor thicknesses of engineered earthworks (say 1.5 to 4m) {issues relating to underlying strata notwithstanding}.

The reason for this is I wish to form a robust argument as to the validity of using a suitably graded material placed under a defined earthworks specification with post placement validation testing to demonstrate that such a material is a suitable bearing stratum for the construction proposed.

The case in point is that I am looking at a number of sites where it is necessary to re-profile and re-engineer the existing site soils to a uniform depth to construct a development platform. The typical construction solution would either be to adopt deep foundations to found on natural strata at depth by the use of deep trenches or piles or to adopt raft foundations. (N.B.deep foundations are not suitable due to contamination issues at these sites). Although the raft solution is not particularly financially onerous it does represents an uplift in build out costs per unit when viewed against standard foundation strips, there is a significant cost saving if such abnormals could be avoided.

UK regulatory views and standards are non supportive of designs of foundations which bear directly onto a made ground bearing stratum. Re-engineered soils are, technically, made ground, and as such materials can have drastically different engineering properties which vary sufficiently over the construction to lead to both differential and total settlement issues. However, my rationale is that an homogenised, re-engineered soil will have adequate properties which are consistent at the scale relevant to the construction to be a founding stratum for traditional strips.

Your thoughts

 

[BigH]

Pitchstone: I'm away from my "references" at the moment - on home leave. However, I would check on the use of bridge abutment piers on embankments. This is becoming quite standard practice. I believe, in a lot of US states and Canada - they form the last 25 m or so of the embankment using well graded crushed stone; then they place the "floating" abutment on top - often using >300 kPa for allowable bearing pressures. Of coure, a lot also depends on the settlement characteristics on which the embankment is placed. If very soft clay, then likely need to use pile supported abutment. I remember a few states having studies of this. Check out:

http://www.fhwa.dot.gov/bridge/geopub.HTM

 

[iandig]

If you can source the following book, it should provide the informaiton you require:
BR 424: Building on Fill: Geotechnical Aspects; Second Edition. Written by J A Charles and K S Watts, ISBN 1 86081 509 X
This document contains a number of case studies dealing with building on fill, including a 'model earthworks specification' which is used on many projects in the UK to assess and confirm the suitability of engineered fill for the placment of foundations within the fill.
The book is published by the BRE [Building Research Establishment] and the second edition was issued in 2001 so it is still relatively new and up to date.
From reading your thread, this is the book you need.