Format Contents of Soil Investigation Report

The results of soil exploration, including field investigation and testing, as well as the laboratory test results and their analysis, along with suitable recommendations, are presented in the form of a soil investigation report. The soil investigation report is an important legal document that is used as the basis for design of foundations or earth structures as well as their construction. The report also forms the basis for investigating any potential instability or failure of the structure during its life. As such, the soil investigation report should be prepared with utmost care.

The purpose of soil investigation report is to provide data of the soil and site characteristics and conditions to fulfill the following objectives:

i. To ensure a safe and economical design of foundation.

ii. To minimize settlement and heave or any such problems during the life of the structure.

iii. To predict and adopt measures to control possible construction problems.

iv. To investigate into failures or potential failure of existing structures to evolve an alternate design.

Preparation of a geotechnical investigation report is an engineering art, and professional experience plays its due role in documentation.

The soil investigation report should be clear and precise, without any scope for ambiguity in the estimate of soil parameters, analysis, and interpretation as well as recommendations. The quality of a soil investigation report essentially depends on the geotechnical investigation – a well-planned and properly executed geotechnical inves­tigation with necessary field and laboratory testing and sampling coupled with judicious analysis is a prerequisite for a good geotechnical investigation report.

The following data is essential for the preparation of a good geo technical investigation report:

i. Details of the proposed structure.

ii. Site conditions, including geological and topographical features.

iii. History of land use.

iv. Details of the adjacent structure(s).

v. Details of field investigations, including field testing and sampling.

vi. Laboratory test results.

A good soil investigation report should be prepared in the following standard format:

1. Introduction.

2. Objectives of the geotechnical investigations.

3. Details of the proposed structure.

4. Site conditions.

5. Field investigations.

6. Laboratory test results.

7. Soil profile.

8. Allowable bearing capacity.

9. Analysis and interpretation of the results.

10. Foundation alternatives.

11. Recommendations.

12. Limitations and uncertainties of soil exploration.

13. Annexure.

If the report is voluminous, it may be prepared in two volumes. Volume I is the main body of the report and con­tains the main text, summarized data sheets, location maps indicating the geographical location of the site, pro­posed structure, various field tests and sampling, soil profiles, etc. Volume II contains the supporting calculations pertinent to the data in Volume I, data of field, laboratory tests and investigations, etc.

The various elements of the soil investigation report are briefly explained below:

1. Introduction:

This section indicates the client company and the project for which geotechnical investigations are carried out, its location, the name(s) of the case investigator(s), the main objective or context of the geotechnical investigations, and the reference to the work order by the client company.

2. Objectives of the Geotechnical Investigations:

This section gives the detailed objectives for which the geotechnical investigations are being carried out. If the case investigator feels that the scope is inadequate, the same should be clearly stated, which will become important when the soil or foundation is found to behave differently from the predictions of the report due to inadequate geotechnical investigations.

3. Details of the Proposed Structure:

This section gives the type and number of the proposed structure(s) in the site, along with the extent of its area. This also specifies the nature of the structure, that is, whether sensitive or non-sensitive, or light or heavy, and the nature of its use and possible loads and discharges that the underlying formation will be subjected to from the proposed structure. The location of the proposed structure should also be shown in the site plan.

4. Site Conditions:

This section gives the nature of the ground surface at the site, whether undulating or flat. If the ground is highly uneven, reference should be made to a relatively permanent datum from which the depth to various soil strata and the sampling are measured.

This section presents a brief account of the geology and morphology of the area, including the presence of a hill or valley water courses or ponds, and cultivated land within and outside the site.

It also gives information about the land use history of the site, existence of any old structure or mines within or in the neighborhood of the site, and also any other observations or findings during field visit which is useful in deciding the design parameters. This section also provides a survey map showing the geographical location of the site and the proposed structure.

5. Field Investigations:

This section reports details of field investigations, including date of commencement and completion, as well as the type of exploratory methods adopted. The layout of the test pits and/or deep borings should be provided in rela­tion to the plan of the proposed structure.

Details of field tests conducted, such as SPT, SCPT, DCPT, PLT, vane shear tests, or PMT, should be specified. Specific observations of site conditions, including color of soil, groundwater seepage, speed of progress at different depths during the boring operations, and field tests, should be clearly reported. The depth interval at which the field tests are conducted in each borehole should be specified.

The type of samplers used for collection of undisturbed soil samples, including the inside clearance, area ratio of the samplers, and the recovery ratio obtained, should be included. The degree of disturbance that occurred in collection of soil samples should be indicated, which is very important information, especially for sensitive soils. The method adopted for collection of rock core samples as well as the RQD should be included.

The details of the field tests conducted, if any, and their results with special remarks on their limitations and applications are presented in this section. If any nonstandard tests or equipment is specially devised for the inves­tigations, the details of the equipment and the test procedure adopted should be described in the report.

The maximum depth of exploration in each borehole should be recorded. The reference datum from which the depth to various strata is recorded should be reported. The depth of GWT in each borehole should be recorded and included in the report.

Specific comments and observations of the case investigator on the overall performance and progress of the field investigations should be included in this section.

6. Laboratory Test Results:

This section presents the results of various laboratory tests conducted on undisturbed soil samples collected from the field. The date of collection of the samples and the date of test conduction are to be specified to indicate the time lag between the two and its possible effect on the test results. The method used for transportation and preser­vation of the soil samples and the method and tools used for recovery of soil specimens for each test are specified. For shear tests, drainage conditions such as UU, CU, or CD, with special reference to the range of confining pressure used, are indicated.

The laboratory test results may be presented in tabular forms, and it is desirable that the results on disturbed and undisturbed soil samples are separately presented or clearly marked.

Soil classification should be complete as per the IS – 1498-1970 code, and the properties used for classification should be clearly specified, for example, grain size distribution, index properties, SPT, SCPT, DCPT, or unconfined compressive strength (laboratory) or vane shear strength (laboratory/field), etc. All the test results should be used to carry out soil classification such as grain size analysis, consistency limits based on the plasticity chart, and field identification tests in addition to those mentioned above. The nomenclature and symbols used for soil classification should be clearly mentioned to avoid misinterpretation.

Soil Profile:

Soil stratification at each borehole, based on the color of the drilling fluid and soil fragments and observation of the soil samples as well as on the analysis of field and laboratory tests, is reported in this section. The depth to the top of each stratum and the thickness of each stratum for each borehole should be specified.

The nomenclature of soil strata used in the report should be in accordance with the IS –1498-1970 Indian Standard Code of Practice for classification of soils with or without local names (with the local names provided within parentheses).

The depth to the top of soft and hard rock should be reported in this section, along with the degree of weathering. The soil and rock profile should be presented in the form of a bore log for each borehole and enclosed as an annexure. The variation in the soil or rock profile across adjacent bore logs or test pits is brought out and the logs are presented such that they join the profiles at various borehole locations.

Comparing the soil strata at different depths in various boreholes, the nature of variation in the soil strata across the length and breadth of the site, with special reference to the built-up area, should be commented upon. The depth up to which the soil characteristics or strata that would have a significant effect on the strength and stability of the structure and a bearing on the selection and design of foundation for the structure should be clearly remarked.

8. Allowable Bearing Capacity:

Using the laboratory test results, the safe bearing capacity of the soil at various depths should be computed based on the shear failure criterion as per IS – 6403-1981. The allowable bearing capacity should be carefully decided con­sidering all the pertinent factors such as the settlement criterion as per IS – 6403-1981; field test results; site condi­tions; water table; long- and short-term conditions etc. The permissible settlement for which the safe settlement pressure is calculated should be specified. A model calculation showing the computation of safe bearing capacity and safe settlement pressure is to be included for future reference.

9. Analysis and Interpretation of Results:

The results obtained from the field investigations, laboratory testing, and safe bearing capacity (SBC) computation as well as specific observations during the field investigations are thoroughly reviewed in this section vis-a-vis the soil classification based on the laboratory tests and field observations. The general characteristics of the soil and the nature and extent of their variation over the site should be conclusively described. The anticipated design and construction problems and the influence of soil characteristics (such as shear strength, compressibility, swelling, and GWT) and seepage should be brought out in this section.

10. Foundation Alternatives:

The nature and magnitude of the loads from various portions of the proposed structure, the soil profile, and the soil characteristics at different depths, should be used as guidelines in working on suitable foundation alternatives for the structure and in taking a judicious decision based on the strength, economic, and long-term stability considerations.

Expected settlements of the structure under different foundation alternatives should also be computed and presented. Model calculations should be shown for all computations made in the report. Possible problems that may be encountered during construction are also brought out based on the analysis of results. When more than one type of foundation are to be recommended owing to their relative merits from conflicting criteria, the same should be clearly mentioned, indicating the personal choice of the case investigator, leaving the ultimate decision to the client.

11. Recommendations:

This is the most important section of the geotechnical investigation report and points made here need to be clear, pre­cise, and free from error and ambiguity.

The recommendations should, among other things, include the following:

i. The recommended type of foundation for different parts of the structure.

ii. The depth from the existing ground surface where the foundation is to be located.

iii. Allowable bearing pressure and other soil characteristics to be used in the design of foundation, including the minimum width for which the allowable bearing pressure is applicable.

iv. Specific guidelines on whether the excavated soil can be used for refilling, and if not, suitable alternate materials within the vicinity of the site.

v. Suitable ground improvement techniques, wherever required and where the suggested bearing capacity is based on the improved ground conditions. In the case of expansive soils, special foundation techniques to be used to ensure long-term stability of the structure.

vi. Suitable time for commencement and completion of the foundation and the structure from compressibility, shear strength, and seepage considerations.

vii. Where the GWT is likely to be at or near the ground surface, specific caution to the client for close supervision and quality control during construction.

viii. In an area where soil erosion conditions are high or where the terrain is highly uneven, recommendations for special measures to prevent loss of overburden.

ix. Where non-conventional foundations are suggested, inclusion of the design and construction procedure of such foundations.

x. Anticipated problems during construction of the structure and special measures to be adopted to facilitate rapid and hassle-free construction.

12. Limitations and Uncertainties of Soil Exploration:

This section clearly specifies the limitations and uncertainties of the soil exploration due to possible inadequate budget, inadequate scope and/or objectives of the soil exploration, or difficult or variable soil or site conditions. Specific recommendation of the case investigator for further investigation/exploration to address the cited limita­tions should be specified.

13. Annexure:

All the data related to the geotechnical investigation cited in the report as well as the supporting calculations, maps, drawings, and photographs are enclosed in the annexure in a logical sequential order, with clear reference number as cited in the report.