Soil Site Investigation Report

After the drilling rigs leave and the laboratory finishes testing, the client receives a thick document called a site investigation report, also known as a geotechnical or soil report. For many owners, developers, and even some contractors, it looks intimidating: pages of tables, logs, graphs, and technical terms. Yet this report is one of the most valuable documents in a construction project. It tells you what lies under your site and how your building should be founded.

This article explains the typical contents of a site investigation report and how to read it with confidence.

Purpose of the Report

The report documents the ground conditions and converts them into engineering recommendations. It is written for the structural engineer, architect, contractor, and client. It should answer these questions: What is the ground like? How strong and compressible is it? Where is the groundwater? What foundation is suitable? What risks need to be managed?

A good report is not just a data dump. It combines facts (what was found) with interpretation (what it means) and recommendations (what to do).

Typical Structure of a Report

Although formats vary, most reports follow a similar order.

1. Introduction and scope. This section states the client, site location, project description, the purpose of the investigation, the scope of work, and the standards followed. Check that the structure described matches your actual plans.

2. Site description and geology. It describes the topography, current land use, vegetation, drainage, neighbouring structures, and the regional geology. In Kenya, this might mention volcanic rocks, lake sediments, black cotton soil, or weathered basement.

3. Field investigation. It describes the methods used: trial pits, boreholes, drilling method, SPT, sampling, and any other in-situ tests, with a location plan showing where each exploration point lies.

4. Laboratory testing. It lists tests performed and the standards, with summary tables and, often, appendices of detailed results.

5. Ground conditions. The engineer interprets the borehole logs and tests to describe the soil profile, layer by layer, along with groundwater.

6. Analysis and recommendations. It presents foundation options, founding depth, allowable bearing pressure, expected settlement, and special precautions.

7. Appendices. Borehole logs, test sheets, graphs, photographs, and maps.

The Site Plan and Exploration Locations

Start with the location plan. It shows the boundary of your site and where each borehole or pit was placed. Check that the exploration points are distributed sensibly across the building footprint rather than clustered in one corner. If the building layout changed after the investigation, tell the engineer, because the data may no longer represent the new footprint.

Reading a Borehole Log

The borehole log is the heart of the report. It is a vertical record of what was found at each location. Typically, it shows depth on the left, followed by the soil description, symbols representing soil types, sample and test locations, SPT N-values, and groundwater observations.

Soil descriptions follow a standard format: consistency or density (for example, “stiff” or “dense”), colour, soil type (clay, silt, sand, gravel), and notes such as “with gravel” or “weathered tuff.” Pay attention to changes in layer, the depth at which firm material begins, and the depth at which drilling stopped. Note any remarks about water inflow, caving, or refusal.

Understanding SPT and In-Situ Test Results

Beside the log or in a table you will see SPT N-values at different depths. Higher values generally mean denser or stiffer soil. Look for patterns: a steady increase with depth is typical, while low values beneath a stronger layer signal a possible weak zone. Refusal indicates very hard ground or rock, though the engineer should explain what it represents. If other tests such as DCP or vane shear were used, their results may be graphed with depth.

Interpreting Laboratory Results

Laboratory tables summarise properties for each sample. Some key ones:

• Moisture content: high values in clays may indicate soft or weak soil.
• Liquid limit, plastic limit, plasticity index: high plasticity index suggests a clay that may shrink and swell, such as black cotton soil.
• Particle size distribution: shows proportions of gravel, sand, silt, and clay.
• Shear strength (cohesion and friction angle): used to calculate bearing capacity.
• Consolidation parameters: used to estimate settlement in clays.
• Chemical tests (pH, sulphates, chlorides): indicate whether special concrete or protection is needed.
• CBR and compaction: relevant for floors, pavements, and fill.

Ask the engineer to flag results that influence design decisions rather than reading each number in isolation.

Groundwater Information

Look for the water level recorded in boreholes or pits, the date of measurement, and any comments on seasonal variation. Groundwater affects bearing capacity, excavation, basement waterproofing, and concrete durability. Measurements taken in the dry season may understate the wet-season level, so a good report discusses likely seasonal changes.

Recommendations: The Part Most Readers Need

This section tells you what to do. Check for the following:

• Recommended foundation type: pad, strip, raft, or piles.
• Founding depth or level: where the foundation should bear.
• Allowable bearing pressure: the safe pressure the soil can carry, usually in kN/m² or kPa, with an assumed foundation size and settlement limit.
• Expected settlement: total and differential values.
• Special measures: soil replacement, drainage, dewatering, chemical protection of concrete, or ground improvement.
• Construction advice: inspection of excavations, protection from rain, and compaction requirements.

Make sure the allowable bearing pressure is tied to a stated depth and footing size, because the number is meaningless without that context.

Caveats and Limitations

Every report includes limitations. Exploration points sample only a small fraction of the site, so conditions between points are inferred. If excavation reveals different ground, such as soft pockets, fill, or rock at a different depth, the engineer must be informed and the recommendations reviewed. Treat these limitations as an invitation to inspect, not as boilerplate to be ignored.

Red Flags in a Report

Be cautious if the report lacks borehole logs or laboratory data, gives bearing capacity with no supporting analysis, recommends a foundation without discussing soil type, ignores groundwater, uses only one shallow test point for a large building, or is unsigned by a registered engineer. Such weaknesses suggest the document may not be reliable for design.

How to Use the Report in Practice

Share the report with the structural engineer before foundations are designed. Give the contractor a copy so they understand groundwater and excavation conditions. Keep it with project records, because it is useful for approvals, financiers, and future alterations. When construction begins, compare what is exposed in the excavation with the report, and call the engineer if anything differs.

Frequently Asked Questions

Do I need to understand every number? No. Focus on the ground profile, groundwater, recommendations, and caveats, and ask the engineer to explain the rest.

Can the report be reused for another project on the same site? Only if the new loads and layout are similar and the engineer confirms suitability.

Who is responsible for acting on the recommendations? The design team and contractor, with the client ensuring they do.

A Quick Checklist for Reviewing Your Report

When your report arrives, run through a simple checklist. Does it clearly describe the project and the site? Is there a location plan showing all boreholes or pits? Are logs included for each point, with soil descriptions, depths, and test values? Are laboratory results provided with standards stated? Is groundwater recorded with dates? Does the report explain the ground conditions in plain language? Are foundation options and an allowable bearing pressure given, with stated assumptions? Are settlement estimates and special risks addressed? Does it list limitations and recommend inspection during construction? Is it signed by a registered professional? A “yes” to all these suggests a sound report. Gaps should prompt questions to the consultant.

Questions to Ask Your Geotechnical Engineer

Good questions include: What is the most critical finding on this site? How variable is the ground across the footprint? At what depth is competent material found? Is any part of the soil expansive, soft, or collapsible? How might groundwater change in the wet season? What risks remain after the investigation? What should the contractor look for in the excavation? Would you recommend additional testing if the building layout changes? Asking such questions turns the report from a document into a conversation, and ensures the important findings reach the people making decisions.

Keeping the Report Alive Through the Project

The soil report is a living reference. During design, the structural engineer should check assumptions against it. During construction, the site team should compare the exposed ground with the logs. After construction, the report forms part of the building records and helps with future alterations, repairs, or additional floors. Keep both paper and digital copies, and ensure they are passed on when ownership changes.

Key Takeaways

A site investigation report is most useful when it is read actively. Use the logs to understand the ground, the laboratory results to understand its behaviour, the groundwater information to understand risk, and the recommendations to guide design. Keep it close throughout construction, compare it with what you see in the excavation, and return to your engineer whenever conditions or plans change. A well-read report is one of the cheapest forms of insurance a building project can have.

Conclusion

A site investigation report is not just a formality for approval; it is a practical guide to your ground. By understanding its structure, reading the borehole logs, noting groundwater, and focusing on the recommendations and caveats, you can make better decisions and ask sharper questions. In Kenya’s varied geology, a well-read report helps you build safely and economically on the ground you actually have.