Excavations and the Next Door Neighbour

I have recently seen an increasing number of geotechnical reports that imply that using Ko=1-sinφ’ (the coefficient of earth pressure at rest) to design basement retaining walls is appropriate and will, to a large extent, prevent movement of neighbouring properties. Can this be justified?

Ko, as we all know, is the ratio of the horizontal to the vertical stress in the ground. Although it is generally accepted that Ko = 1 -sinφ’ applies to normally consolidated soils (Bishop 1958), it is a function of stress history. Since most Australian soils are the product of complex weathering and desiccation processes, they are generally over-consolidated and it is probably wrong, more often than not, to assume Ko=1-sinφ’.

Alternatives are available and guidance can be found in papers such as Mayne and Kulhawy (1982), from which the expression Ko =(1 – sinφ’).OCRsinφ’ might be chosen as a more rational alternative. This could present problems if, as is so often the case, testing has been omitted to save money and neither φ’, nor OCR, are known. In this case it might be better to guess a plausible, but conservative, value of Ko based on experience and a knowledge of the local geology. In most cases a “conservative” value of Ko is a high one, but the specific application needs to be considered and a high, or low, value chosen as appropriate. Ko=1 may not be unreasonable, at least as a starting point, in many situations.

It is simple enough to design a retaining wall to withstand Ko stresses, but this does not mean they will eventuate, or that the resulting wall will prevent ground movement. If the objective is to try and minimise movement then analysis of the entire construction sequence is required, using realistic values of the basic parameters. It may be worth keeping the following thoughts in mind:

1. Ko stresses will only be maintained if zero ground movement occurs. Even with diaphragm walls and very stiff, top down, construction this is hard to achieve in practice, so actual stresses are nearly always less than the Ko ones.
2. Relatively small movements of a wall during excavation and anchor installation are enough to cause stresses to drop to the active (Ka) value. This can be demonstrated using WALLAP, FREW, or a finite element/difference program to model the construction sequence.
3. Walls are often allowed to cantilever as much as they can before the first row of anchors is installed. This causes movement that is usually close to the maximum that the wall will experience on completion and the stresses drop to the active value.
4. Stressing anchors to high levels, once a wall has been allowed to deflect, increases stresses in the wall, but does little to redress ground movements that have already occurred.
5. In practice, ground movements are more likely to be limited by undrained conditions during excavation, than by anything that the engineer can do to stiffen the restraint. Whether this can be relied on in a particular instance to limit ground movements needs careful evaluation.
6. Where a basement is built in open cut, ground movements will have occurred during excavation and cannot be reversed. In this case earth pressures on the wall are more likely to be governed by the level of compaction given to the backfill, as suggested by Ingold (1979), than by the original Ko condition.

To me this illustrates that design, of even a commonplace construction element such as a basement wall, requires very careful consideration and my concern is that there is a modern tendency to ignore the complexities. This may be due to a lack of time, a lack of money, or even to a lack of interest in the subject, as Tim Sullivan’s recent discussion of post- graduate study might suggest (Australian Geomechanics, December 2003). Whatever the cause, geotechnical engineers are paid to give advice that is reasonable and appropriate in relation to every one of their projects. This requires site specific data and site specific thought. Relying on “off the shelf” reports with little or no data can have dire consequences and the excuse that it was only a “cheap” investigation holds no water when things go wrong. I urge any of you who provide geotechnical design advice to keep your thinking cap on at all times – to lose it is to court disaster!