The Lifecycle of Geology in a Civil Engineering Project (Part 3)
Alex Petty - Chartered Geotechnical Professional
I Challenge the Norm to Change the Industry and Help Save my Clients Millions of $$$ | Geotechnical Director | Adjunct Associate Lecturer
Do Civil Engineers need to know any geology? Find out in our latest blog series which walks you through the lifecycle of a project and just why geology is key:
Geology for Initial Planning and Scoping
Geology for Construction Materials
Geology for Foundation Design
Geohazard Identification
Impacts of Geology on Construction
Geology for Foundations (Part 3)
Geology is a very important factor is designing and constructing foundations. After all every foundation sits on the soil or rock that exists beneath a structure!
The soil or rock conditions (i.e. the geological materials) beneath the site often govern the type of foundation that is chosen.
For example, a structure placed on a site that is underlain by soft alluvial clays will most likely require piled foundations, whereas one that is underlain by heavily over consolidated soils or rock will likely only require shallow foundations.
Typical foundations are shown in the figures below.
Typical pile foundation configuration
Pile configurations for a structure
A shallow foundation:
A typical shallow foundation?
A typical shallow pad foundation
The above scenario provides a basic example of why the geology is so important for civil engineering.?But, with detailed engineering geological assessment, potential issues can be raised and addressed.
Undertaking an intrusive investigation using boreholes may provide some geological information of the foundation subgrade materials. But it is the geological interpretation that identifies things that weren’t encountered or than cannot be seen with the investigation. The project examples below display the importance of geological knowledge in informing the foundation design
Project Example 1
For example, a project for a road bridge in the eastern Kimberley in WA identified the presence of metamorphosed sedimentary rocks along the proposed bridge alignment. However, the geological mapping, knowledge, experience and interpretation identified the presence of other igneous rocks in the immediate area.
Although these were not specifically encountered during the borehole drilling, it was noted that it was likely that they would be present in the area.
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During construction of one of the bridge pad footings, a contact between the two rock types was encountered with an associated shear zone approximately 0.5m wide (shown as red dashed in figure below)
Contact between igneous and meta-sedimentary rock present in pad footing excavation with shear zone (red dashes)
The foundation design required the highly fractured shear zone to be excavated to reach competent rock.
Because a geological interpretation of the area was undertaken, this scenario did not come as a surprise and was easily dealt with during construction.
Project Example 2
Another example is shown below where a borehole was drilled to assess the ground conditions beneath a site. The borehole encountered sand until refusal on limestone.
Excavation in typical pinnacle Tamala Limestone
Now, without geological knowledge of the area, the boreholes may just encounter sand to a few metres depth. Whereas in some locations, limestone is very near the surface. Knowing that the geological processes that form and shape the Tamala Limestone in this area means that it was anticipated that pinnacles such as these would be encountered in the excavation.
This is particularly important for assessing the excavatability and allowable bearing pressures for the shallow foundations at this site.
Project Example 3
A final example for the importance of geological context related to a marine project with driven piled foundations.
This complexity was not specifically identified using the geotechnical investigation data alone.
Two separate ground models can be created based on the ground conditions encountered. The ground conditions encountered can be summarised as follows:
Ground Model Based Solely on Geotechnical Investigation Points
Now, if we just look at the boreholes and CPTs (additional locations were also completed and not displayed), a simple ground model can be created. The below cross section image displays a ground model that could be interpreted by just linking up strata from investigation points.
Cross section using just investigation points
This is a reasonable interpretation if solely looking at the site area and not looking externally to the site. However, the next section shows the importance of using engineering geology/geomorphology to assist in ground model interpretation.
Ground Model Based on Geotechnical Investigation Points AND Engineering Geological Interpretation
Looking outside the bounds of the site, a couple of easily identifiable things can be interpreted. A palaeochannel is known to be present in this area and it is possible that it would roughly follow the meandering of the existing Swan River. In addition, the cutting of the channel created a steep sided channel creating high angle cuts within the limestone. It is possible that the steep limestone cliffs that are present on the river shore immediately adjacent to this site were formed by the palaeochannel.
These couple of interpretations together with the knowledge that the limestone is typically deeper than other areas surrounding the site, it is possible to interpret a different ground model, as shown on the cross section below.
Cross section using investigation points AND Geological Interpretation
This model indicates the potential for a steep sided slump within an historical channel. The driven piles had to be carefully designed to ensure they didn’t penetrate deeply into the potential slumped limestone features.
So, hopefully the case studies presented above display the importance of looking outside the site and of using engineering geology and geomorphology skills and interpretation to establish ground models for civil engineering projects.