Geotechnical Engineers and Geophysicists – the perfect match Part II
Do you remember the article regarding perfect match between geotechnical engineers and geophycisists? If not, click here for a quick reminder!
In this article we will continue to discuss the co-operation between geotechnics and geophysics by showing examples of successful co-work, where the conclusions from ground investigation projects for geotechnical purposes have benefited from the use of both traditional geotechnical methods and geophysical surveys.
If you need a short re-cap on geophysical investigation techniques, please read more by visiting our method pages on ground penetrating radar (GPR), resistivity/IP, seismics and transient electromagnetics.
Let’s start with some good examples of how we can be more efficient in subsurface investigations for geotechnical applications.
Highs and lows
When planning for a road, you most often have more than one plausible route to choose between. The choice can be based on housing areas, archaeological findings, rare flowers, and wildlife habitats, but the geological conditions will also have an impact. Where will it be most beneficial to construct the road, taking into account zones of instability or the easiest location to excavate a cutting or tunnel? In such early planning stages, the cost of geotechnical drilling is huge and might be impossible due to issues with landowner consent and permits. Instead, geophysical methods can be deployed as an efficient way of gathering subsurface information along all these different possible corridors – providing detailed enough results to use for a planning stage.
As you can imagine, road schemes will tend to stretch over several different geological conditions. In Sweden for instance, large parts of the country have low valleys with clay and high lands with till. This makes the combination of resistivity and GPR perfect. The resistivity measurements are made in the clayey lowlands, and GPR in the tilly highlands. Together the ‘GPR+RES’ approach will provide a good overview of the geological setting, especially the depth to bedrock, but also information on soil layer thicknesses and extent of potential problem areas such as localized peat deposits; all important factors when designing a road project. Above all, this first quick information can also be used for planning the more detailed and expensive geotechnical drill-holes.
Waves in Norway
A rail and road upgrade scheme required the creation of a number of new rock-cut tunnels to carry both the road and railway. As the knowledge of the bedrock level (indicating minimum tunnel depth) and quality (rock strength) is critical for tunnel constructions, these sections of the development needed to be investigated thoroughly. Seismic refraction, both on land and in water was used in combination with geotechnical drillings to get as complete a picture of the geology as possible prior to the construction work starting. The seismic results successfully identified the rock head (where seismic velocity suddenly jumps up), zones of slower seismic velocity (indicating weaker, fracture zones) and discontinuities (most likely fault planes). All of these are crucial information in the planning process for safe tunnels.
A bump in the road
In a forestry plantation, some sections of the haulage roads were continually sinking, becoming flooded and very, VERY muddy during the winters. This was causing problems for the timber company as trucks were getting stuck or having to take alternative routes which meant lengthy and costly diversions. The problem was identified as pockets of peat (a waterlogged, organic brown soil) within the glacial tills. With the heavy loads of trucks, these peat-filled hollows would compress quickly, but to predict where was more or less impossible. However, resistivity profiles could rapidly identify the low resistivity zones associated with the waterlogged peat and measures to prevent road collapse could be made in the right locations. Time and money saved.
Building foundations for a greener tomorrow
All buildings require foundations designed to support them come rain or shine, and wind turbines are no different. Moreover, these foundations need to withstand immense and constantly changing lateral pressure exerted by the wind. This factor necessitates an understanding of the geology across the whole, and often wide foundation footprints. In such cases, many boreholes would be prohibitive in both cost and duration. If only there were another way to extend the ground model either side of a central borehole for a more competitive and complete ground investigation.
Fortunately, there is! The solution is, again, geophysics. In this example a large windfarm was surveyed using seismic Multichannel Analysis of Surface Waves (MASW) and resistivity imaging. Linear arrays were undertaken in a cross configuration over a central borehole marking the middle of each proposed turbine. In doing so, each geophysical line encompassed the full width of the foundations and lateral changes in shear velocity across the foundation area mapped with data tied to ‘ground truth’.
Data processing involved modelling shear wave and Standard Penetration Test (SPT) data for estimates of dynamic and static geotechnical parameters along with production of 2-D shear velocity and resistivity profiles spanning the width of the foundation footprints.
The average survey rate for a team of three in challenging conditions and with poor access was two turbine locations per day. This was much quicker than drilling boreholes. In some cases it was taking up to a day to track drill rigs to each turbine location. The added value in continuous profiles of data coverage is evident and the associated geotechnical parameters derived from the geophysics meant a more competitive ground investigation tender for the drilling company and, of course, a happy end client.
All these sewage pipes
Holiday home areas, close by larger cities, tend to be inhabited not just for summer vacations, but the year round. This puts a huge pressure on the municipality in charge to install and maintain reliable systems for both drinking water and sewage.
A typical holiday home area, in Sweden, consists of an uncountable number of small houses along long and winding roads. This quickly becomes too large an area to grasp in the planning process of a new water and sewage system. Most often the aim is to plan the pipe network such that is as short and cost-effective as possible. The number one choice is of course to use an excavator, just get all pipe trenches ready and throw in the pipes. But when the construction work has started, you do not want to hit any unexpected subsurface features, with a risk of adding both time and costs to the project.
So, you need to do some ground investigations before you start to excavate.
Traditionally, in Sweden, this is done by drilling holes at around 20 to 50 meter centers, across the whole area. Sounds expensive; is expensive. So, instead, you should bring along your GPR in a cart or behind a car and map the roads. During a field day you can walk quite far and collect dense data (1 cm intervals along the line, if you please) and during the next day you will most likely get a good overview of the ground conditions. The results can easily be displayed as a map, with red as shallow bedrock and green for ‘ok-to-dig’ areas. Upon handing the map to the geotechnical engineers, they can plan a drilling program with fewer but intelligently-placed drill-holes. The drilling density can be reduced in areas where the GPR interpretations are certain and maintained in areas where good quality GPR results could not be achieved.
To wrap up
As you see, the match between geotechnical engineers and geophysicists continues to be imperfectly perfect: the result of collaboration is perfect but the frequency of it happening may not yet be. We have shown some successful applications, and there are plenty more out there, happening all the time. The more geotechnical and geophysical information we get on the subsurface conditions, the better the design process of new infrastructure and construction projects will be.
So, we hope you are convinced that the combination of expertise, rather than working in isolation, is what can make the geotechnical world more perfect. And that you will be the one who stops building 3D models of for example the bedrock from just a few and sparse geotechnical drillings. And, instead demand greater data coverage from applying geophysics first.
If you have any questions or concerns, please call or contact us.
Jaana Gustafsson, Applications Specialist, Phd
Harry Higgs, Application Engineer, BSc FGS