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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 failures in the recent past due to lack of stability of soil. The entire building can come down if this aspect is not given due importance.

Though many times this is mis understood as a structural scope, all these are specialist geo technical roles.


Though all these are roles and responsibilities of a soil consulting firm, in this structural blog am explaining a soil report so that young and aspiring civil engineers can understand the importance.

Also knowing how to interpret the

The soil report that I have chosen to represent in this structural design tutorial is on compressive soils and it has some difference and importance. That also underlines the reason that a specialist should do the job. See the video



GENERAL POINTS COVERED IN A SOIL REPORT

A soil report is a document that conveys the entire soil testing and the parameters in a way that all stake holders of the project can understand and interpret it. This is a part of any soil investigation and is submitted by the geotechnical consultant as a deliverable. It will contain all the details about the methods carried out, the reasons to choose the method, the results the recommendations and lot more. I have listed the general points covered below. Please ensure to see the above video too.


Soil exploration method:

The soil report generally contains a section that explains the site conditions, project location and the kind of soil exploration done. You will get the preliminary idea about the project and the methods and the reasons from this section.


Objective of soil exploration:

The objective of the exploration, scope of soil exploration work etc also will be explained in a soil report. It is always better to co-ordinate this before so that what you need as a structural consultant is conveyed in the right way. Sometimes, there will be site specific or project requirements that you have.


Laboratory test results:

Physical properties of soil and Engineering properties of the soil like shear strength parameters are some times handy to understand the soil behaviour. Generally, a table with properties like dry density, bulk density of soil and even grain size distribution is provided in the table.


Bore hole location Plan:

The number of bore holes and its location is important and a soil consultant decides this based on geotechnical codes and his experience. A plan locating the bore hole will be there in the report. This will be handy especially when the site is vast and you have different SBC for soils and you need this info to size the footings.


Bore Logs:

Grain size distribution curves in the bore hole also will be provided along with the plan. Bore logs and this curve gives us a very good idea on the soil conditions and N value of soil will be represented in the bore log. We also get a very detailed understanding if the soil is compressive and decide the restriction in width of footings etc from this data.


Recommendations

Soil recommendations include the SBC, Modulus of subgrade and lot more. Minimum depth needed will be mentioned here for normal footings. If it is pile foundations, the type of piles, its depth and the pile capacity will be mentioned in the recommendations.

In many cases the recommendations also include precautionary measures to be adopted in certain situations. One example is the need to provide soling below the footing if the soil is poor.


Summary of this blog

Soil exploration is a mandatory step in any building project. Many think it is needed only in larger and complicated projects. However, it is important for even a small project like a residential building. The money one thinks is saved by not doing a soil test may have to be spent for repairing later. Some times the assumed SBC by the designer will be less than the real and hence the size of footing can be more than needed and one end up spending more than what would have been spent for a soil test. Let us all practice and preach design with no compromise. If you need some more tips on structural points related to footings, I have another blog here.

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