Tips to interpret a Soil Report

Geotechnical explorations are mandatory for any building projects. However, soil testing and soil reports are not given the required importance by many clients who builds buildings. Many thinks that soil exploration is a structural engineering scope. While it is very important that structural engineers and consultants are aware of the basic and fundamentals of soil mechanics, soil mechanics or Geotechnical engineering is a separate discipline and a specialist soil consultant or a geotechnical consultant should be on board for any project.

In this blog am writing about the role of a soil consultant and what you will find in a soil report. However please note that I am explaining a very important point on compressive soil in the structural video. You need to note that the content in video and blog is related but different. Please ensure you first read the blog and then see the structural design tutorial video for full understanding.

Role of a Soil Consultant

Many think that the role of a soil consultant is limited to testing soil and providing a report in the form of a pdf booklet. This is not really correct. A soil consultant will have a very important role in deciding various things including stability of slopes and soil structure interaction. In the below section, I have only mentioned some basic and generic role of a soil consultant.

To decide the method of soil exploration

Method of soil exploration depends on the type and size of the project and the general profile of the soil. Based on these parameter and experience, the geotechnical engineer decides the method of soil exploration. There are dedicated codes of practice on type of soil investigations and that gives guidelines on which soil exploration needs to be adopted based on specific soil conditions.

To decide the number of test pits/ boreholes

The number of bore holes and depth of exploration etc also needs careful understanding and experience. This also depends on the kind of project and its size and the general soil profile. I have a separate blog here which indicates how to decide the number of bore holes.

To collect samples and test the soil parameters

Soil samples should be collected very carefully and it should be undisturbed if the result need to be correct and accurate. A wrong soil sample can result in a poor soil analysis and result in many issues

Analyse and interpret the soil properties

Once the soil sample is collected, A geo technical lab will conduct tests and analyse its properties. This is a very important process to establish all the soil properties that will help us to decide its strength and behaviour.

Convey the site conditions as a report

This is done to ensure that the report is interpretable to other stake holders like site engineer, structural consultant, architect and the client.

Recommend the type of foundations

Soil report contains a recommendation on type of footings specific to a project. This is done after analysing the soil conditions and the behaviour and the test results. All the above steps are also as important as any wrong information can result in recommendation of a wrong type of footing.

Recommend alternative foundation systems when needed

In some cases, there can be more than one solution. The client and the construction team may need options to decide based on various other conditions like skillset available, a later change to add a basement, site conditions and so on.

Recommend the SBC of soil and the modulus of subgrade reaction

SBC or soil bearing capacity of soil is mandatory information needed to decide the size of the footing. Analysing the soil properties, a specialist geo tech engineer can use various formula and code provisions to arrive at the SBC of soil. Subgrade modulus also known as spring constant is also needed for the structural analysis of rafts and piled rafts.

Recommending methods to improve soil bearing capacity

Some times the soil may be really poor and it may need some improvements in properties to enable the client to build there. There are various methods that can be adopted. A soil consultant will recommend this based on the type of soil and the conditions. Some times sand piling can improve the soil conditions. In black cotton soils, some times soil replacement is suggested. All these are decisions of a soil expert.

Deciding stability of slopes

It is in fact a soil consultant who should decide the stability of slopes. If a site can be excavated vertically or not. If there need to be soil protection by shoring. If there need to be soil nailing and so on. We have seen many structural failure