Uncertain knowledge about what lies under the surface is a major cause of delays and budget overruns on construction projects. It is well known that not knowing the location of utilities and other infrastructure is a major cause of delays, but unforeseen ground conditions; soil conditions, ground water and underlying geology, is also a major cause of large budget overruns on construction projects, especially for projects involving deep excavation and construction below the seabed. Advanced remote sensing, 3D digital ground modeling and visualization technologies are changing dramatically how we capture, analyze, visualize and share information about the subsurface.
Background
Every construction project devotes considerable effort to discovering subsurface geotechnical conditions and structural geological setting. The traditional approach involves boreholes, CPT and test pits. Regardless of the process, all geotechnical acquisition results in point data. This data is used by the engineering consultant for a single project and rarely shared.
In the Netherlands an attempt is being made to address the problem by the Key Registry for the Subsurface (BRO) which came into force in January, 2018. The BRO registry ultimately will record 26 geotechnical data types. In the UK the Dig to Share project, supported by Atkins, the British Geological Survey (BGS) and Morgan Sindall, is also attempting to address this problem. Its aim is to develop a fully digital workflow, which is accessible to the whole industry, to upload and access data from a web-based system.
The traditional approach to data collection about the structural geological setting involves shallow seismic reflection, seismic refraction, and legacy resistivity methods. These methods provide information about the different layers underlying a site though not always as accurate as required. Typically, this type of data is low resolution, and will usually not be useful in defining key sub-bottom variations such as the transition between compacted sands and rock, vertical features or variations in rock strengths across a site. In addition, legacy qualitative methods rarely provide the quality and accuracy to generate reliable and useful high-resolution models of the variations in each stratum across a site.
Three-dimensional geological modeling has developed dramatically over the past 30 years from contouring and gridding techniques using mainframe computers through to PC based geological modeling software. These tools were developed for certain industries such as mining and oil and gas and deal with very specific geological scenarios and data types. Many geological survey organizations worldwide have started to implement varying software systems and methodologies to facilitate a migration, from a 2D paper-based survey to a 3D digital service provider of geoscientific information. Rapid advances in gaming technology has enabled a renaissance in ground modelling and particularly in 3D visualizations.
Advanced remote sensing technologies
Another approach relies on remote sensing technologies that are designed to acquire large volumes of high-resolution data about the structural geology over a large area, quickly. These advanced geophysical methods are digital and allow for determination of thickness and depth of geological layers as well as layer and intra-layer information from the one sensor. Shot intervals are better than one per second make it possible to collect large volumes of high-resolution data in a short period of time. The objective is to understand subsurface complexities in detail at a site which makes it possible to target boreholes to elucidate geological anomalies. I had the opportunity to talk to Jason Errey, Director OEMG Global, about one of these advanced technologies.
High resolution resistivity technology
One such method is Aquares, an advanced resistivity technology which enables the capture of high resolution quantitative and qualitative information about the strata under a site. Quantitative data includes depth and thicknesses of sub-bottom structures. Qualitative data includes type of sediment; sand, silt or clay and rock characteristics such as fresh or weathered. High resolution combined quantitative and qualitative data is extremely useful for successful planning construction activities including, dredging, trenching, under bores, tunneling, or piling. With this information project planners and engineers have a complete picture of the existing subsurface environment. This allows designs to take into account subsurface structures such as weathered and unweathered rock outcrops or buried river valleys to considerably reduced dredging and construction costs.
Modern digital ground modelling practice is essential for planning and asset management. The results of high resolution Aquares surveys can be combined with other data in an Integrated Digital Ground Model (IDGM) via the MyGeoTwin product. Data is acquired for the IDGM in a staged manner and stored in a data lake for use in future projects.
Sydney Harbour Metro tunnel
An example of where the Aquares high resolution resistivity technology was deployed is the Metro tunnel under the Sydney harbour in Australia. In late 2016, Sydney Metro commissioned an Aquares advanced geophysical survey. The Aquares survey was undertaken in two stages over two nights. On the first night a high-resolution survey targeted the top 15 metres below seabed. On the second night the survey extended this to a further 40 metres below seabed. A week after acquisition, a four-dimensional model covering the top 40 metres below seabed comprising one million data points was delivered to the Sydney Metro Authority. When compared to the legacy below seabed information, the new geophysical information showed substantial differences in the rock levels in addition to previously undetected remnants of a river channel following a geological fault line.
The new technology is also more efficient. For the initial design for the tunnel created in 2015 two years was spent conducting and analyzing multiple legacy geotechnical and geophysical surveys, costing millions of dollars. Due to site complexities, these early surveys failed to define the correct rock level and the associated geological risks. The Aquares survey was completed in 2 nights, processed in a week and combined with all other available data in a 4D ground model a week after that, at a fraction of the cost.
Buenaventura dredging
The Port Authority in Buenaventura needed to undertake maintenance and capital dredging along a 10 km section of a 180 m wide channel leading to the port. Based on limited seismic results that were unable to identify intra-layer variability and five boreholes, dredging contractors assumed approximately 30% sediment and 70% hard rock and estimated an average dredge cost of $17.6 per cubic metre.
The results of a more advanced Aquares geophysical survey method provided crucial information about the level of the sediment-rock interface and the differentiation of harder and softer rock types. The survey found that the channel bottom was composed of 70% soft sediments, 25% rock and only 5% hard rock. As a result, cost estimates were lowered to $7.9 per cubic metre. For the dredge volumes on this particular 10 km section of the access channel, the cost savings were estimated at $34 million.
Conclusion
Modern digital ground modelling practice is essential for planning and asset management. Advanced remote sensing, digital data acquisition and management tools now available reduce the risk of unforeseen geological conditions delaying and significantly increasing the cost of major construction projects. Advanced digital geophysics provides a reliable source of 3D data that can be integrated into an integrated ground model to provide asset owners with a digital tool to manage and control risks relating to subsurface ground conditions.
Subscribe
Comments