Thursday, December 13, 2012

Ground Investigation.

This chapter is a summary of experience in dealing with a large variety of ground conditions on which to build a wide range of structures. It may help young engineers who tend to deal with soil properties, geotechnical engineering and superstructure design only – but sometimes give too little attention to ground investigation on which such engineering topics are dependent. This chapter may also be a helpful recapitulation for the experienced engineer.

The ground or sub-strata material needs to be considered as part of the structure, for, like the superstructure, it will be subject to stress, strain and deformation and also possibly to deterioration. If that part of the ‘structure’ is defective or fails then experience shows that it can be the most expensive structural failure to remedy. Furthermore, no matter how well or expertly the superstructure is designed, if the foundation fails it is possible that the superstructure will also fail. Foundation failure is one of the largest causes  of cost claims.

For example, claims for subsidence damage to foundations and buildings following the 1995–96 period
of dry weather in Britain rose to £475 million. It has been stated that piling contractors do not have adequate site investigation details for over half the projects for which they are invited to tender. Before a foundation can be designed it is necessary to know what load the ground can support, how it will react under the load, both in the shortterm and over the structure’s life, and also the effect of this new loading on adjoining structures. Without this information, safe and economic design is difficult and may not be possible. Further, the design should be practical and buildable so the designer should be aware of the contractor’s likely construction methods and possible problems. (Site construction progress can be slow until the foundations are complete, i.e. the building is ‘out of the ground’.)

Before a foundation can be constructed the contractor needs to tender for the project, plan methods of excavation, temporary works and ground treatment, and be forewarned of possible problems, etc., to enable skilful, safe and rapid construction of the foundations. Standard forms of agreement between the design engineer and his client usually state that the designer should exercise ‘reasonable skill and ability’ and meet the standard of a ‘reasonably competent practitioner’. The designer is not expected to be an expert in construction or a specialist in ground treatment. The designer should be wary of non-standard forms with clauses which increase the duty of care to ‘fitness for purpose’. The fact that a cause of failure could not have reasonably been foreseen is no defence with such a clause, nor would professional indemnity insurers accept any obligation.

The ground information is obtained by means of a site investigation. Site investigation, like X-rays and other tests on a sick patient, is not an exact science. The investigation of the ground – as laid down by geological processes, some-times modified by previous construction, mining, etc., and possibly subject to future change – requires detailed planning, careful collection of information, testing and analysis, to be as reliable as possible. Most importantly, it requires the application of engineering knowledge, judgement and experience.

The results need to be reported clearly, precisely and without ambiguity – but it should be appreciated that the result of the most thorough investigation is an  estimate  and not necessarily an accurate forecast. (It has been stated, cynically, that ‘there is only one way to determine the  exact soil conditions and that is to dig it all out, examine it and replace it’ – the designer would then be faced with the problem of building on a fill!)

Even the most thorough, detailed and careful survey and investigation can sometimes lead to false conclusions.

Isolated pockets of peat, meandering channels of loose,  saturated sands, fissures, filled-in shafts and wells, etc., can remain undetected. It is always advisable, therefore, to include in the project estimate a contingency item to cover the possible additional expense of dealing with unforeseen foundation construction difficulties. The engineer should remember that no two samples of soil will have identical properties and that most of the soil tested and reported  on will now be in the testing laboratory and not left on site.

The soil encountered on site is likely to differ (in varying degrees) to that previously tested and due allowance
should be made for this at all stages of the design and  construction process.

Since investigation, analysis and reporting (i.e. interpretation of the results of the investigation) should be based on readily available knowledge and established soil investigation procedures – it may be difficult to plead ignorance in a later dispute over a failure. The designer must obtain the available and relevant data from reliable sources and must interpret that data, not necessarily with over-sophisticated mathematics but with sound judgement and skill.

The costs of a site investigation are low in relation to the overall cost of the project. Engineers can find it difficult to get their client to agree to spending upfront monies resulting in nothing more than a report. The fact that the information contained within this report can be crucial in saving significant sums of money in the design and construction stages of the project is often overlooked. Engineers need toeducate their clients on the costly results of  cheap, inade-
quate soil surveys.

Though a percentage of the capital cost is useful as a preliminary estimate it must be appreciated that it is not a true guide for every site and sufficient funding should be allocated to site investigation to ensure both economic foundation design and construction. Sites and the structures built on them are so varied that it is not possible to  fix firm cost percentages without details of the site and  proposed structure. For example, a low-rise housing estate to be built on well-known and tested London clay overlying Thames ballast is likely to incur less investigation cost  than that for a multi-storey, heavily loaded structure on a suspect, highly variable glacial deposit. Contractors tender- ing for excavation without adequate (or with suspect) site information may gamble, ‘load’ the tender or claim high rates for ‘extras’ and variations.

Clients who object to the cost of a survey or foundation design should be informed of the risks of such cost-cutting.

Clients rarely accept the responsibility of the risk or refuse the additional finance for survey and design.

Delays in construction due to inadequate investigation can easily cost more than any money ‘saved’ by cheap surveys.

Extra-over costs in amending foundation design or construction methods to cope with undetected problems can substantially exceed the total cost of an inadequately funded investigation. Time spent in advising the client to provide adequate funding is therefore worthwhile and often essential. In addition, it is advisable that the client should be made aware that the survey cost begins as an estimate which may need revising as the investigation  proceeds.

Many clients are in a hurry for early handover of the completed project (with the increasing need for early return  on capital investment) and can find time spent on site investigation to be an irksome and unnecessary delay to construction start. The engineer should resist any temptation to skimp the survey and have regard for the client’s long-term interests.

When analysing tenders it is important to have a clear understanding of the kind of sampling and test regime which will actually be required. In the following example (Table 3.1 (a)), Firm 1 appears cheaper initially because their set-up on site and rates of boring and sampling are lower. But when an analysis is undertaken to include the anticipated laboratory testing requirements (Table 3.1 (b)), Firm 2 is the more cost-effective, due to their lower testing rates.

Table 3.1 (a) Site investigation tenders as received.  
(b) Analysis of site investigation tenders

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