Adaptation of a ground investigation technique from the oil and gas industry is offering improved detection of geological anomalies in the Middle East.
The Middle East poses numerous challenges for ground engineering consultants due to the region’s variable and challenging geology.
The presence of karstic cavities - features resulting from the dissolution of soluble rocks that are either infilled or void - as well as fracture zones and areas of anomalous stiffness
Even a small void at a depth of tens of metres can be significant to tunnelling or foundation works
They are all found at relatively shallow depths - up to 100m below ground level - precisely where foundations, tunnels and other underground civil engineering structures need to be built.
These geological features are risks that need to be managed ahead of any form of construction disturbing the ground.
“Undetected cavities during construction can lead to delays and expensive, and unplanned works,” says ground investigation consultant Fugro geophysics manager for the Middle East region Rod Eddies.
“Investment in timely and robust ground investigation can allow targeted remediation and uncover opportunities to ‘design out’ problems by positioning structures away from identified subsurface hazards before work begins on site.”
He continues: “Around a quarter of construction projects encounter significant delays due to unforeseen ground problems, so there is a very strong case for investing typically less than 1% of project budget in ground risk mitigation.”
Three component seismic surveying: how it works
Fugro’s three component (3C) approach to seismic surveying provides the clarity to detect cavities of relatively small dimensions as well as having deeper penetration capability, beyond the depths of most foundations or metro tunnels.
It uses a state of the art vibroseis acoustic source to transmit a highly controlled wave field into the ground. A series of geophones detects signals reflected and refracted from boundaries and, critically, scattered and diffracted from discontinuities within the ground.
Unlike other methods generally used in geotechnical applications, the 3C seismic technique analyses three components of the wave field - the vertical, and two orthogonal horizontal and crossline components - not just one component. It has the potential to greatly improve relatively shallow ground models through better imaging of voids, fracture zones and areas of anomalous stiffness properties.
But the presence of karst and other geological features is very difficult to detect, since boreholes may not pick up every anomaly.
“Boreholes and lab analysis give a very sparse understanding of the problem [particularly] where ground characteristics vary more [frequently] than one can afford to drill boreholes,” says Eddies.
“Even a small void at a depth of tens of metres can be significant to tunnelling or foundation works.”
To optimise a borehole programme, a seismic survey can be carried out beforehand, he explains, to map out ground conditions and locate any anomalies that require further investigation.
The process involves striking the ground to produce seismic waves that propagate through the ground and are eventually picked up by receivers.
However, this technique is less reliable at shallow depths due to surface noise disturbing the quality of the data received.
Also, many common forms of seismic surveying are not accurate where there is a high water table and the presence of saline water, as is the case in the Middle East.
Seeking a solution
To accurately locate geological features, Fugro decided to look to the oil and gas industry for a solution.
But the technique used there was found to be only suitable for deeper investigations where the ground is more stiff.
At shallow depths, the technique would not yield sufficiently accurate results.
The firm then decided to adapt the technique to suit the accuracy required to pick up anomalies at shallow depth for construction projects.
“We have considered a way of receiving information using very sophisticated three component receivers rather than one-component as is usually the case in the oil and gas industry,” explains Eddies.
“We looked at the way work is carried out in the exploration [oil, gas, minerals] sector so that, rather than capturing a small part of the [seismic] energy coming back, we tried to capture all the wave field - that is the x, y and z components.”
He explains that the data received using this method is more reliable in locating geological features, clearly identifying where follow-up intrusive investigation is required.
“Our system optimises the intrusive programme of where boreholes need to be drilled and offers a way of filling in the gaps between these points of control - highlighting where there is a continuity and where there is variability,” Eddies says.
The three component seismic survey is also quick to undertake, which befits the fast pace of construction commonly associated with the Middle East and leads to fewer costly boreholes needing to be drilled.
Eddies concludes: “Having a full picture of ground stratigraphy and structure, together with engineering properties, allows foundation design to be tailored to actual, rather than assumed, conditions.
“This can potentially deliver big savings by removing the need for overly conservative design.”
The ‘whole life’ approach: Performance modelling and future proofing
Today, we live, work and play in a world where much of the infrastructure is increasingly influenced by sophisticated asset management systems and processes.
With demands for increased performance and capacity and ever narrower maintenance windows, asset owners are under pressure to deliver more for less on behalf of their customers and stakeholders.
Amey’s asset management approach and expertise allows us to trade-off the optimisation of cost, risk and performance in order to maximise the whole-life value of a wide range of assets.
Previously, industry practice has been to give priority to asset condition surveys and measurements, with less focus on forecasting how condition and performance will change over time.
Good decision processes are forward-looking, ensuring that we are focused on addressing tomorrow’s issues within a future operating environment - not solving yesterday’s problems.
Change is a constant feature of this future environment, being driven by a mix of economic austerity, the growing demands of regulators and owners, and more powerful analytical techniques.
We should recognise that forecasting models are an aid to decision making and that any discrete forecast is not an end in itself
Responses to this change require the exercise of choice - the generation and evaluation of options within a scientific and well-informed process.
For example Amey uses a “hard systems” decision process based on a clarity of objectives, alternative ways of meeting these goals, smart measures of performance and a transparent decision framework.
Amey has been developing and applying predictive techniques for some time to help evaluate the long-term performance of its solutions. Forecasting models allow it to evaluate the performance of different approaches to asset management in a variety of operating environments.
This is aided by our investment in sophisticated modelling techniques and access to ever-increasing banks of data. This strengthens our ability to optimise performance, minimise risk across a wide range of services and to support our clients’ infrastructure investment plans.
We should recognise that forecasting models are an aid to decision making and that any discrete forecast is not an end in itself. Amey understands the need to ask the right question of any modelling activity and to apply experience and judgement to any outputs.
An analytical approach to asset management creates long-term value across a range of infrastructure sectors and increases owners’ understanding of their infrastructure and management of their physical assets. This in turn drives reduced whole-life costs, enhanced performance and more transparent and better-managed asset risks.
The whole-life approach will continue to be Amey’s priority as we continuously improve our capabilities, and welcome new business growth at home and abroad.
Dr Mark Brown is business development director for the consulting, rail and strategic highways division, Amey