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Geomechanics: Rock Solid Simulation

Terra-firma, rock-solid and concrete are terms that all inspire images of stability. What could be more reassuring than the support of a good solid foundation? The truth of the stability of terra-firma, rock formations and geomechanics in general is not quite as clear cut as it seems.

As engineers everywhere push the limits of speed, power and capability of products we buy every day, there are also awe inspiring feats of engineering that go unseen to most eyes. Engineers working on civil, oil & gas and infrastructure projects that work on huge scales and push technology just as hard.

Occasionally, we see a glimpse into the world of these engineers but rarely are the complexities around the geomechanics given the air time they deserve. The key to successful projects like these is to have fully understood the materials. Excavation, tunneling or construction on rock and soil already under huge loads needs very careful consideration.

The way geomechanical materials (rock, soil, concrete etc.) behave is very different from those of more familiar materials like steel. A load applied to a material, generally speaking, eventually results in failure or yield at the same point no matter which direction you apply the load. For example, if you had a cube of steel you could squash in in any direction and expect more or less the same result. There will of course be some variations for materials which are rolled or drawn (like sheeting or wire) but for the purposes of a simple definition this is a reasonable starting point.

Geomechanical materials cannot be simplified quite so readily. Soil and rock behave differently and are far from uniform in each direction. Moisture content in soils can change and variations in the water table can change things further. Rock formations and soil structures can often also have layers or gaps in which complicate things even further. The yield behavior for such problems are much more complex. What makes things even more challenging is that unlike manufactured parts it’s very difficult, or often impossible, to prototype or test designs.

geomechanics rock formation

If the challenge of complex materials wasn’t quite enough then we have to remember that these materials which have often been in place for significant periods of time are already under immense loads. The boring of a tunnel for example might involve drilling through rock and soil with tens or even hundreds of meters of material on top of it and maybe even other construction.

Removing material to bore a tunnel or drill a subsea well means changing this balanced system of forces and pressures. Changes made need to ensure that existing structures, surrounding buildings and other tunnels for example aren’t effected and that drilling won’t result in collapse of materials.

Pre-existing tunnels in terrain

In subsea environments the depths and pressures involved are increases as our appetite to push limits further grows and the needs of transportation infrastructure mean that longer and more challenging, often already crowded, projects drive the need for simulation even higher.

The engineering tasks on these projects are vast and the role simulation could play is huge. Accurate material models and being able to properly simulate existing loads on systems mean that confidence in analysis increases. Beyond tunnelling and drilling, understanding what happens once a project is completed is just as important. Over time structures may settle due to compaction and consolidation and for construction above ground it’s often required to look at slope stability. Construction of a new road for example on a hillside requires that if cutting away the ground to make the road then the resulting topology is stable and safe.

As well as the rock, soil and loading of the existing structures engineers must also look at the materials they are using for construction. Concrete is a very common choice, and for good reason, it’s very strong and relatively easy to build with. The behavior of concrete is also interesting though, whilst compression it’s behavior in tensions is completely different.

Concrete is used for construction of damns, buildings and tunnel walls. It’s also employed in the fabrication of foundations and supporting structures. Loading from surrounding materials and events such as passing loads, trains over bridges for example, and singular events such as seismic activity are all vital to account for.

ANSYS has very good solution simulation of all of these challenges. By using simulation technology engineers can check that proposed designs can meet the needs project and that they can do so safely. Simulation of rock and soil formation relies quite a lot on surveys which might include core samples and physical site surveys to report back on the existing make-up of the ground. Uncertainty will always be present and being able to vary material properties easily to test out best and worst case scenarios is incredibly valuable.

  Watch the webinar 'Advancing Concrete and Soil Structural Simulation with ANSYS' to show you some of technology.