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The risks and rewards of site investigation

In the November 2018 print issue of GeoDrilling International, Philip Hines, chair of the Federation of Piling Specialist, called for better site investigation. Now Signe Hansen gets an insight into how to avoid the pitfalls of poor site investigation.
The risks and rewards of site investigation

Poor site investigation can mean a lot of things. It may be the manifestation of poor survey design, under-sampling and lack of flexibility to respond to actual site conditions; it may mean poor practice in terms of acquisition or analysis resulting in unreliable results; it may mean results that are delivered too late to influence the design.

Likewise, the consequences of poor site investigation can range from the undesirable, such as increased cost and delay in construction, to the catastrophic, such as structural collapse. In the often-quoted words of Professor Stuart Littlejohn "You'll end up paying for a decent site investigation - whether you have one or not". Despite this, as Hines pointed out in his column in GDI, many piling specialists are still provided with inadequate site investigation data.

Simon Brightwell regional communications manager at Fugro, explains: "Most of us in the industry frequently hear that there is lack of time or lack of budget for anything more than a quick and basic investigation. At Fugro, we are also told that a more detailed approach is not needed because ‘we know enough about the ground already'. This symbolises a lack of appreciation of the importance of ground related risk and the disproportionate effect that unexpected (but often foreseeable) conditions can have on outturn cost and programme."

This is, however, in stark contrast to the reality experienced by industry experts. Hence, when Fugro recently conducted a survey, asking 40 engineers to identify the biggest causes of construction overrun, issues related to the ground and to groundwater were listed as the number one and number two concerns - ahead of design, construction or political factors.

Misunderstanding
Despite the concern within the industry, a lack of understanding of the cost versus benefit when it comes to site investigation continues to drive developers to seek the cheapest bidder. Ash Rogers, design manager at Aarsleff Ground Engineering, explains: "Any potential savings a client may strive to find in the site investigation can be very quickly reversed due to unforeseen ground risks that can cause additional delays and costs to the project overall. In nearly all projects, proper site investigation contributes a very small proportion to the overall project costs and looking for savings here is a completely false economy."

Andrew Waghorn, operations director at piling and geotechnical services provider J. Murphy and Sons, agrees: "Companies are reluctant to spend an additional £1,500 for a decent borehole at the start of a project, without realising that this money will pay for less than half a day's standing for a piling rig if an unexpected problem arises."

Additional factors contributing to poor site investigation are poor advice from consultants and/or associates, poor planning on the foundation type, and lacking flexibility in the scope of the investigation.

Added cost and delays
On top of delays, there are multiple ways a reduction in the site investigation budget can create additional costs later in a project. Firstly, there is the risk that expenses saved will be doubled later as the ground complexity and the need for further investigations become apparent to the construction team. "Often site investigation boreholes are undertaken and a lot of the cost is in the mobilisation but if more pre-planning was undertaken a more useful scope/borehole depth could have been specified. Often we get projects where there are 15m boreholes provided when we need 20m to 25m for example to design the piles for the loads required, and going an extra 5m at the time would cost a lot less than returning to the site to do more investigation," says Waghorn.

Another cost-increasing factor is the over-engineering of foundations which designers faced with high levels of uncertainty are forced to resort to compensate for the lack of a robust ground model. Similarly, contractors will manage higher levels of risk associated with uncertainty by factoring in a bigger ‘comfort blanket' to cover for the lack of information.

"Without proper site investigation the, piling contractor is faced with the choice of allowing very conservative design parameters and making conservative choices on techniques for installation to cater for risks that may well not exist. At concept stage this may skew affordability analysis if the anticipated budget price of the project is artificially inflated due to poor site investigation," adds Waghorn.

On site, the best case in this scenario is that poor site investigation costs the client money that need not be spent due to the conservative choices made. The worst case is that the budget is too low and does not cover actual pile construction costs when the ‘real' conditions are discovered on site or that piles do not perform as anticipated following installation.

Geotechnical failure
While uncommon, geotechnical failure as a result of unexpected, but often foreseeable, ground conditions is a possible consequence of a poor site investigation. "It is very important to consider the inherent major risks of site investigation boreholes not identifying major site risks such as scour features, dissolution features, infilled valleys, greater depths of made ground etc all of which could have a hugely detrimental influence on pile performance which the piling contractor may not be responsible for if they are relying on provided site investigation data which does not depict these risks," Waghorn stresses.

Professor John M. Reynolds, managing director of engineering geophysics consultancy Reynolds International, has, indeed, encountered clients who have suffered major financial setbacks due to an unwillingness to invest in proper site investigation. "To give but one example, a housing developer in Cumbria wanted to do the minimum site investigation so put a borehole down in each corner of his site and found no underground mine workings although it was common knowledge that the site was probably underlain by them. Following a minimal geophysical campaign that demonstrated mine workings to be present, the house builder decided not to undertake any further ground investigation but chose to drill-and-grout as necessary," Reynolds explains and continues: "As the underground mine workings were both more extensive than he thought and also extended way beyond the boundaries of his site, he ended up flooding the underground mine workings in grout both beneath his site and beyond at huge additional expense. The cost almost bankrupted the housebuilder."

Environmental gains
However, as Hines points out, the question regarding site investigation is not just one of avoiding risk but of finding possible ways to increase the reward. And, it is not just about money, time and safety; there are also obvious environmental benefits of investing in adequate site investigation. "At its simplest, an efficient pile design facilitated by proper site investigation needs less pile in the ground. This means that less concrete and reinforcement is used and, if a replacement pile is used, less spoil is generated for disposal. These are obvious wins for the environment," Waghorn points out.

Fugro too, uses value engineering based on geotechnical insight to drive down not just cost but also environmental impact. "Fugro performs advanced bi-directional pile testing using its Loadtest O-cell system to determine the capacity of piles safely, accurately and reliably; the optimisation of foundation design has frequently led to >10 per cent reduction in depth of large foundations including piles and barrettes, reducing excavation impact and cutting the need for steel and concrete," says Brightwell.

Besides, the strategic design of a site investigation can help target the most appropriate geophysical methods to be used as well as the modes of deployment that impact the ground surface the least. In some cases, and depending upon the techniques to be used, aerial platforms such as drones can be used to carry sensors at low height across ground that otherwise might be damaged by or inaccessible to ground-based vehicles. Reynolds explains: "Geophysical methods can be used to test for the integrity of geomembranes in landfills, for example, to check on possible leaks of leachate so that appropriate remediation measures can be designed. They can also be used over closed landfills to help locate buried bunds that separate individual landfill cells, thereby helping to reduce the risk of a borehole puncturing a lining and causing an environmental problem through leakage."

One of the environmental challenges of piling is the risk of groundwater contamination, but new methods to refine the understanding of hydrogeology are emerging. Brightwell says: "Fugro is deploying downhole magnetic resonance logging to great effect. Used historically in the oil and gas sector, the technique is proving effective in civil engineering applications where the continuous and precise record of permeability and porosity can help foundation design and reduce the risk of groundwater contamination."

Proper site investigation
Site investigation for pile design should follow a similar pattern to most other types of projects; starting with a desk study is thus essential, explains Rogers. "A simple desk study will relate the proposed development with all available information to anticipated probable ground conditions, ground risks and geotechnical design requirements. From the desk-study findings, a concise and efficient site investigation can be specified."

Methodology will obviously vary from site-to-site, but significant benefit can often be derived from geophysical screening before initiating the main intrusive investigation and detailed ground modelling. Furthermore, ensuring that information is flowing swiftly and efficiently to appropriate project teams before, during and after the site investigation is key to its success, adds Rogers. "A demonstrable bugbear of mine in this regard is when site investigation - decent or poor in its scope - is issued without simple yet crucial information such as positions or levels of the boreholes. In this example, borehole positions and levels are key in incorporating geotechnical designs (such as pile toe levels) with the superstructure's layout and can be the difference of metres per pile length if the designer knows where the load bearing strata is found."

Brightwell agrees: "Time is money in construction and it is vital to keep feedback loops as short and fast as possible. Any medium- to large-scale investigation will benefit from data delivery via a web-based platform to provide shared, secure and timely access to the most up-to-date information."

Meanwhile, Reynolds stresses that the intrusive campaign should also be used to constrain any geophysically-derived ground model to ensure congruency between the two sets of information. "A 3D ground model can be updated by including any subsequent test data, which can help improve the spatial resolution of the model as well as reduce uncertainties in derived engineering parameters, all of which helps to save money," he points out.

Indeed, by carefully planning the project timeline, the initial site investigation can be planned so that the right information is provided at the right time. It is not unusual, says Reynolds, for this interactive approach to deliver cost savings of 10-15 per cent of an entire site investigation campaign. This, of course, only being a fraction of the saving a proper site investigation can secure for the entire project.

 
 
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