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SITE INVESTIGATION

Foundation design over existing tunnels, Jon Morris SC recently carried out an investigation for an educational establishment wishing to construct new facilities on a well-established campus site. An old abandoned railway tunnel was known to traverse the campus at depth, and nearby buildings had suffered damage as a result of a localised collapse of the tunnel in the early 1970s.

In the early stages of negotiation, the client located an archived report describing the collapse and its causes. The report also confi med that the tunnel had been backfi led beneath the collapsed buildings prior to the redevelopment of the area, but provided very little information on the section of tunnel beneath the new site.

It was possible that the tunnel remained unfi lled beneath the proposed development and presented a risk to the foundations of any new buildings. After further discussion with the client, a drawing was located which gave details of a grouting contract to completely fi ll the tunnel, and this was subsequently confi rmed by direct investigation at one of the tunnel portals which had been securely bricked up. This meant the risk of a catastrophic failure due to collapse of the tunnel could be discounted.

This example demonstrates that even in what might be expected to be an ideal situation - a public sector client that had owned site for many years and had a comprehensive archive - vital information on the history of a site can be hard to come by. It is necessary to be persistent and to emphasise to all parties the importance of timely information in the assessment and control of ground risk.

Development of existing buildings, Stuart Wagstaff Redevelopment in London is a relatively constant market. A number of recently completed projects have seen this with relatively new buildings - 1960s and newer - that require ground investigation.

The majority of these projects need a significant amount of preparatory work to construct boreholes for investigative purposes, particularly if basements are present. Increasingly SC is being asked to undertake this work blind, with no available information on the existing structure or local service or tunnel information at the time of appointment.

The company has been expected to source a significant amount of information for certain projects, all of which impacts on the timing of the exploratory phase, to the client's dismay. Much of this information should be sourced by the engineers at an early stage, especially if time constraints are tight. This is increasingly the case in today's market, particularly if ground investigation is a likely requirement.

Once on site, ground investigators are commonly encountering unexpected structural elements of the buildings, which delay and increase the cost of investigation. But often they find that once problems are encountered, drawings of the existing building are produced.

SC believes that engineers and clients need to appreciate the importance of investigation and the relevance of details that might affect it. Where information is not sourced from the outset, this generally leads to increased time and greater cost to the client.

Contamination assessments and planning submissions, Alan Watson SC was recently involved with schemes where local authorities (LA) had not been provided with the latest development proposals, not only after the ground investigation, but even after development had begun.

Contamination assessments carried out after development is underway are subject to additional pressures, not only on time and resources, but also on the assessment process itself. This is particularly the case if optimistic assumptions have been made during the course of the early design.

Initially, it had been impossible for LAs to properly assess risks associated with potential contamination in the absence of a well prepared application. In one case the developer had not shown that about 95% of the development would be hard cover or buildings, reducing the risks from low levels of potential contamination.

At another site, basic ground gas control measures had been incorporated in the design from the outset, but this had not been communicated to the LA. The result was an inevitably cautious approach by the LAs that requested further investigative work prior to fi nal planning approval.

LAs are often unwilling to hold the hand of developers, particularly as sources of advice and guidance are readily available in the commercial sector. They tend to adopt a cautious approach if the applicant has not submitted a fully coherent package. SC was instructed to liaise with the LAs and the issues were resolved once development proposals were made clear, and in one case, without the need for further investigation.

In a third case, an LA consulted the Environment Agency (EA) on the assessment of risks to the groundwater regime within a major aquifer from possible volatile or semi-volatile organic compounds.

This was beneath a former furniture factory in the High Wycombe area west of London. Initially the EA expected an intensive intrusive investigation covering the whole, very large site.

The former site occupiers were still contactable and a detailed layout of the furniture-making activities was obtained. This allowed SC to restrict additional intrusive investigation of particular relevant areas and provide a targeted assessment.

One problem with this kind of situation can be that it is not always obvious to site owners and occupiers what constitutes useful information for the purposes of a ground investigation. A solution could be ensuring that questions are thought out properly and put to the right people, for instance those who have worked on the site when it was operational, or those responsible for record keeping.

Conclusion

These examples involving previous owners have an important feature in common: the former occupiers had remained contactable and it had been possible to recover records from archives. Clearly, this will not always be the case for some other sites such as those that have been derelict for many years or where the company concerned is no longer in business.

Another issue is whether the client for the purposes of the ground investigation is the site owner or prospective vendor or the purchaser or developer. A vendor may not be sympathetic to requests for information if there is no obvious advantage to the sale.

On the other hand, a purchaser will never want to buy a 'pig in a poke' and in this regard may have the upper hand in such discussions. Clearly, these factors may well override the geotechnical or geoenvironmental questions, and availability of data can rest on what the relevant parties are willing to divulge.

Nevertheless, the above examples of the benefits of timely information to assist ground investigations could be employed to encourage engineers to ask the right questions and as a reminder to all vendors, purchasers, developers and their advisors that the provision of seemingly insignificant information can often be beneficial.

Soil Consultants senior geotechnical engineers, Alan Watson, Jon Morris and Stuart Wagstaff and geotechnical engineer Hannah Azzopardi.

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