Compiled Soil/Rock Profile(CSP) is nothing but the cross section across various individual boreholes carried out as part of the detailed Geotechnical Investigation. The importance of which is yet to be realised by the design fraternity. I urge the institutes to look into this aspect and encourage the students in adopting the same.
Even till now, many of the soil testing agencies give only individual soil profiles in their geotechnical report, whereas it is very essential to compile the same into a CSP ie., Compiled Soil/Rock Profile (as the name itself suggests), the importance of which needs to be understood.
From the drawing shown below, we can see a Raft foundation resting on soil strata with varying depths of compressible soil strata below the founding level. You can see the dip in the rock level below the raft, the realisation of which is not possible without drawing the CSP.
Notably, in this case, it is to be realised by the Geotechnical Engineer that the allowable settlement of the Raft foundation needs to be restricted to about 25mm(in this case), even though the BIS code allows for more ie., 50 to 75 mm(refer IS 1904-1986). Ultimately, the soil design should be carried out in such a way that the safety of the structure is ensured.
If only individual soil/ rock profiles are drawn by the Geotechnical Engineer, he/she shall not be able to understand the actual profile at the site, as a whole. The above is just an instance for us to understand the importance of a CSP. I urge any geotechnical engineer who reads this article to adopt it in their reports as an SAP (Standard Operating Procedure). This will definitely for the future develop a philosophy in soil design that will help the engineers and also the Structural Consultant to issue the GFC (Good For Construction) drawings. Notably, the same would also give some idea to the client as well when they try to visualise the site conditions in terms of its soil profile.
Recall the above figure depicting the condition of a Raft foundation resting on soil strata with a steep dip in the rock level encountered below the founding level. The effect of differential settlement on the structure can be understood by noting the causes for the same as given below:
The foundations of different elements of a structure may have unequal settlements and the difference between such settlements will cause differential settlement. Some of the causes for differential settlements are as follows:
- Geologic and physical non-uniformity or anomalies in type, structure, thickness, and density of the soil medium (pockets of sand in clay, clay lenses in sand, wedge like soil strata, that is, lenses in soil), an admixture of organic matter, peat, mud, etc;
- Non-uniform pressure distribution from foundation to the soil due to non-uniform loading and incomplete loading of the foundations, now in this case is due to the
- Varying water regime at the construction site
- Over stressing of soil at adjacent site by heavy structures built next to light ones;
- Overlap of stress distribution in soil from adjoining structures
- Unequal expansion of the soil due to excavation for footings
- Non-uniform development of extrusion settlements; and
- Non-uniform structural disruptions or disturbance of soil due to freezing and thawing, swelling, softening and drying of soils.
Let me conclude by saying that CSP is very important for all civil engineers.
N. Sudhindra, Executive Consultant, Nagadi Consultants Private Limited, Bangalore- nagadi.co.in