No one likes paying for something they don’t need, and yet this is happening all the time in geotechnical design. A lack of detailed information about ground conditions means that most geotechnical designers take a cautious approach, often delivering designs that are more conservative than they need to be.

All design codes include factors of safety that are there to ensure a structure can cope with the loading conditions that might occur. However, in ground engineering there are far more unknowns than in structural engineering, says Aecom ground engineering director Niall Corney. 

“Geotechnical designers have to make a lot of judgments and assumptions because they don’t always have enough information," he says. "They of course want to minimise risk so often have to use worst-case design input characteristics. That process adds up even before the factors of safety are applied, thereby compounding the inherent conservatism.” 

Corney estimates that between 20% and 30% of costs, materials and embedded carbon could be saved on the ground engineering elements of many building substructures – foundations, basements, retaining walls, and so on – if more was known about the ground conditions to assist in designing the most efficient solution. “You may not even need foundations at all in some embankment situations,” he points out.

The way projects are procured is the overriding reason for conservatism in design, with individual design disciplines working in isolation and little opportunity for specialist contractors to use their expertise. 

“If you look at your overall building, the way the work is procured is that the building is chopped up into packages – for example, the concrete frame and the geotechnical elements are separated out,” explains David Hard, chief engineer at Bachy Soletanche. “We need to take a much more holistic approach to find the most sustainable solution overall, which might mean more cost or materials somewhere in the process, but globally it comes out as being the better option.”

He adds: “By the time we see a proposal, all of the design has been done. We can come up with alternatives but there are usually a lot of restrictions based on what the client or designer has already fixed and quite often there is not enough time to develop an alternative.”

Even within geotechnical design, it is possible that, owing to the nature of the contract, piles, load transfer structures, retaining walls and abutments might all be designed by different parties, each of them trying to minimise their share of risk and therefore being overly conservative in their design.

A more collaborative contractual environment in which the client, designers and construction team work together is likely to result in better solutions in which risks are managed appropriately.

There is risk in every geotechnical or construction project, whether it comes from unforeseen ground conditions or an unexpected global pandemic. A client (also called the ‘owner’ or ‘employer’ in contract documents) must decide who they think is best placed to carry the different potential risks, with many opting to pass as much of it as possible down the chain to the designer and the contractor. 

Faced with the possibility of being held liable if something goes wrong, most designers will take a cautious approach, says Kelvin Higgins, senior partner at Geotechnical Consulting Group. 

“If you know you’re going to be held liable for anything that goes wrong, it changes your attitude to how you produce your design," he says. “There may be greater risks in the geotechnical environment because you are dealing with ground conditions that can be variable, or there may be other factors that come into play that weren’t apparent at the start.”

The International Tunnelling Association (ITA)’s Strategy for Site Investigation of Tunnelling Projects says that clients may believe they are saving money by offloading the risk to designers or contractors, but in fact the opposite may be true: “The frequently encountered practice among owners worldwide of attempting to transfer the total geotechnical risk to contractors, especially in DB [design and build] contracts, does not facilitate the proper management of risks and does not liberate the owner of his final responsibilities. 

"This transfer of geotechnical risk – especially when accompanied by a reduced initial effort in ground investigations – may eventually be paid for by the owner in terms of either conservative design and/or increased risk of contractual claims, revised design, and schedule overruns.”

Contractual risk, and arguments between clients and contractors over what constitutes 'unforeseen ground conditions', can be mitigated by the inclusion of a geotechnical baseline report (GBR) in the contract documents. 

The purpose of a GBR is to establish the allocation of risk between the client and contractor in relation to the ground, which it does by including contractual statements that define the geotechnical conditions the contractor can expect to find during construction. If the actual conditions are equal to or better than the baseline, the target cost is unaltered; if conditions are worse than the baseline and the contractor can demonstrate a loss then a compensation event is triggered.

GBRs are becoming increasingly popular in tunnelling, with some insurers including them as a condition of insurance. Contract documents for the Crossrail project incorporated GBRs not just for the tunnelling but for all civil contracts that included significant ‘in-ground’ works, such as depots and stations.

John Davis, senior partner at Geotechnical Consulting Group, carried out an assessment of Crossrail’s experience of using GBRs and found that they were successful in three ways:

1. They were relatively straightforward to write
2. They were relatively straightforward to apply
3. The cost outcomes struck a reasonable balance between cost certainty and risk transfer

In addition, comments by senior figures from Crossrail’s major civils contractors suggest that the GBRs helped them to understand the assumptions that the client presumed would be taken into account during the pricing of the tenders.

“In geotechnical engineering, design efficiency depends on how much you know about the ground,” says Aecom’s Corney. “The more information you have about the ground conditions, the fewer assumptions you have to make, and the less conservative your designs are.” 

Unfortunately, many clients view ground investigation (GI) as a cost rather than an investment and are reluctant to spend money so early in the lifecycle of a project. “It is a financial investment and it can be a lot of money to put in at the start when it is uncertain if the job will progress or not,” explains Arcadis’s Gillarduzzi.

Still, skimping on ground investigations at the start of a project is a false economy, according to Corney. “The client will pay one way or another. They might think they’re saving £5,000 by reducing the scope of the GI but if the designer is then, as a consequence, having to make assumptions that build in conservatism, the money they’re looking at is buttons,” he says. “You might easily save £20,000 by changing the pile diameters or spacing, or by using ground improvement rather than piling.”

The ITA’s Strategy says: “Any apparent economy in terms of cost and/or technical involvement by the owner could result in an overly conservative (or even too optimistic) design, bigger residual risks and/or unidentified geological/geotechnical risks.”

In her 2017 Glossop Medal Lecture – entitled "Variability and ground hazards: how does the ground get to be ‘unexpected'?" – Geotechnical Consulting Group senior consultant Jackie Skipper identified research that quantified how much a lack of understanding of ground conditions could cost clients in the long run. Some of this research is shown in the graphic below.

The UK’s Office of Road and Rail (ORR) highlighted this in a report on the A21 Tonbridge-Pembury dualling scheme in 2017, which said: “The sensitive wooded site meant ground investigations were not as comprehensive as they could have been. As a result, the extent of contaminated land at the site was not fully understood, and the groundwater levels and site geology more complex than expected. Woodland translocation and moving protected species such as dormice could only occur at certain times of the year. Statutory utility diversions, programmed well ahead of the start of the project, were delayed. This all had a knock-on effect on the completion date for the scheme.”

The report’s author, David Hunt, head of economics and policy at the Highways Directorate, says: “Our biggest takeaway was the importance of pre-construction surveying and investigation, particularly in such challenging areas.”

Bachy Soletanche’s David Hard adds: “It is possible that carrying out more site investigations or pile testing won’t necessarily change the end solution a lot, but it will remove an element of risk so that when it comes to pricing the tender, the contractor will have more certainty.”

The ITA Strategy says: “On completion of a large or major project, a budget for the site investigations of about 3% (potentially increasing up to 8%-10% depending on the complexity and depth of the underground work, the need for exploratory galleries/shafts, or the use – such as nuclear or hazardous waste) of the project construction cost should be considered as normal.” Most clients spend considerably less than this, some as little as 0.2%.

It adds: “The owner [client] must be aware that investigation should be planned on the basis of needed information and not on the basis of cost, and that sufficient time and budget have to be devoted. Economies on the investigation phases could apparently save time on the design and/or tender schedule but would generally not… achieve the best and most economic project.”

Still, the solution is not merely to spend more money on ground investigation – it is about doing the right investigations at the right time. 

As far back as 1968, geotechnical engineer Rudolph Glossop said in the 8th Rankine Lecture: “If you do not know what you should be looking for in a site investigation, you are not likely to find much of value. What you look for should be suggested by the natural environment, and by the nature of the constructional problem to be solved. Thus, a detailed programme of investigation cannot be decided on day one and adhered to, and the engineer who in the long run is responsible for the solution of the constructional problem should not expect to order a site investigation and then dismiss the matter from his mind until a report is placed upon his desk.

“The number and location of boreholes, trenches and pits, and the number and nature of the tests to be made, whether in-situ or in the laboratory (in other words, the questions to be answered), should be decided as work proceeds at meetings between the engineer and those responsible for the investigation. In the case of a large structure, such as a dam, modifications may be necessary during the progress of the main work as yet more knowledge of the site is gained from excavations.”

In it's Strategy for Site Investigations of Tunnelling Projects, the ITA has mapped the typical components of site investigations against the design stages of a project to indicate where they will have the most impact. The flowchart below demonstrates the interlink between the multiple stages of site investigations and three design phases.

The strategy also outlines typical stages of design and the components, at various phases, of site investigations. It also emphasises that investigations should continue during construction if required.