This article helps structural engineers read geotechnical reports critically. It explains what matters, why it matters, and how to use findings to drive safe, economical design.
Understanding the ground beneath a structure remains as crucial as the structure itself. Every building, bridge, or retaining wall interacts with the soil. Structural engineers must understand what lies beneath to make good design decisions. A sound foundation depends on more than formulas—it depends on how we interpret ground data.
Geotechnical reports contain the story of the soil. Yet many engineers treat them like formality, skimming through pages for bearing capacity values. This approach invites risks. Missing a subtle soil behaviour or groundwater note can lead to under performance or failure. Structural design must align with real ground conditions, not assumptions.
This article helps structural engineers read geotechnical reports critically. It explains what matters, why it matters, and how to use findings to drive safe, economical design. It offers a step-by-step breakdown that aligns with practical workflows. Understanding begins not with formulas, but with soil data in context.
Understand the Purpose of the Geotechnical Reports
A geotechnical report tells you how the ground behaves at the project site. It reflects the findings of site investigation and testing. Structural engineers rely on it to guide foundation choice, depth, and capacity.
The report includes details about soil type, strength, density, moisture, groundwater, and bearing capacity. It also provides recommendations about allowable pressures, expected settlements, and risk zones. Sometimes, it flags issues like collapsible soils, expansive clays, or contamination.
The report gives data and interpretation. The data includes borehole logs, laboratory results, and field test outputs. The interpretation links those findings to real engineering choices. As a structural engineer, both parts matter. You must understand the numbers, and how the geotechnical engineer arrived at their recommendations.
Start with the Project Overview and Scope
Every report begins with a summary of the project and investigation scope. This section tells you what the geotechnical team considered.
Check that the project description matches your structural plans. Was the investigation based on a two-storey building, but you now plan five storeys? Did they assume a raft foundation, while your plan includes pad footings?
Review how many boreholes or trial pits were used. See their depths and spacing. Ask whether the extent of the investigation suits the footprint and loads. A sparse borehole layout may hide problem zones.
Always consider investigation scope in context. Limited investigations can’t justify far-reaching assumptions. Match foundation decisions to the certainty of the ground model.
Examine the Ground Conditions Closely
This section forms the heart of most reports. It describes the subsurface layers identified across the site.
Pay attention to how the report describes each layer. You will see terms like firm clay, loose sand, or weathered rock. Understand these descriptions in engineering terms. For example, loose sand often means low shear strength and high risk of settlement.
Note the sequence of layers and their depth ranges. For shallow foundations, the top few metres are critical. For piles or rafts, deeper conditions matter. Look out for inconsistent layering across boreholes. Uneven conditions often demand tailored foundation solutions.
Geotechnical engineers sometimes use regional soil names. Always translate those into technical properties: bearing capacity, angle of friction, compressibility, cohesion, and permeability.
Interpret the Borehole Logs and Test Results
Borehole logs appear as graphical or tabular records. They show layer depths, soil types, SPT or DCPT values, moisture contents, and other key data.
Start by reading each borehole log vertically, then compare across boreholes. Look for sudden changes in strength or material. Soft layers beneath stiff strata can lead to punching failures or settlement.
SPT (Standard Penetration Test) values offer insight into soil strength. Low values suggest weak or compressible soil. High values suggest stiff or dense layers. For example, an SPT N-value of 5 in clay indicates very soft soil.
DCPT (Dynamic Cone Penetration Test) values serve similar purposes. Focus on abrupt changes in resistance. These often hint at potential failure planes or layering issues.
Review moisture content and plasticity results. Soils with high plasticity may shrink, swell, or creep. These properties influence differential settlement risk and should guide design detailing.
Check the Groundwater Conditions
Groundwater often poses more risks than soil type. Engineers should check the reported water table level. A high water table may reduce bearing capacity and increase uplift or floatation risks.
Reports usually show initial water levels and readings after 24 hours. This matters because water tables can rise over time. Temporary dewatering during construction does not eliminate long-term hydrostatic pressure.
Groundwater also affects soil behaviour. Silts under water pressure may lose strength. Clays may soften and become more plastic. Sand near saturation can become unstable under load. Pay attention to reported permeability and seepage conditions.
Designers must consider groundwater not just for bearing, but also for durability. Deep footings in groundwater zones may require waterproofing or protective coatings.
Understand the Laboratory Test Results
Laboratory tests provide engineering values you can use directly in calculations. Reports often include:
- Grain size distribution: helps classify soils and predict permeability
- Atterberg limits: shows plasticity and potential shrink-swell behaviour
- Bulk density and dry density: needed for stress calculations
- Consolidation tests: shows settlement potential under load
- Shear box or triaxial tests: gives cohesion and angle of internal friction
Focus on how these values relate to design choices. For example, clay with high liquid limit and low shear strength may need larger footings or deeper foundations. Sandy gravel with good friction angle may support higher loads.
Where data appears incomplete, don’t guess. Contact the geotechnical team for clarification or request further testing. Never assume worst-case or best-case values without evidence.
Use the Design Recommendations Critically
Geotechnical reports often provide allowable bearing pressures, typical footing sizes, or settlement estimates. These help, but they are not final answers.
Check the assumptions behind every recommendation. Are allowable pressures based on ultimate or service limit states? Did they apply safety factors already? What depth are values valid for?
Some reports quote allowable bearing capacities without clear explanation. Ask whether those reflect drained or undrained conditions. Clay behaves differently under short-term versus long-term loading.
Settlement estimates also need scrutiny. Did the report assess immediate settlement only, or also long-term consolidation? Were loads based on your final structural design?
Good structural engineers never copy values blindly. Use recommendations as a starting point, then validate against your design and risk appetite.
Identify Potential Geohazards Early
Every structural engineer must watch for signs of geohazards. Geotechnical reports often mention them in passing. Your job is to spot those hints and act.
Common hazards include:
- Expansive clays that swell when wet and shrink when dry
- Collapsible soils that settle rapidly under load or saturation
- Loose fill or made ground that lacks strength or uniformity
- Ground prone to erosion or scour
- Liquefaction risk in seismic zones
Some reports include risk tables or hazard mapping. Others may only describe suspect layers without emphasis. Always scan for signs of inconsistency, instability, or prior land use issues.
Where risk exists, ask the geotechnical engineer for mitigation advice. This may include deeper foundations, ground treatment, or construction controls.
Match Foundation Options to Ground Conditions
Once you understand the ground profile, interpret the implications for structural design. Not every site suits pad foundations. Not all piles solve problems.
Shallow foundations suit strong, uniform soils near the surface. Watch for soft layers beneath, or lateral variability. Rafts work well in variable or weak soils when settlements are small.
Piles bypass weak upper layers but cost more. Use piles when bearing capacity improves with depth or when ground is unsuitable near surface. But confirm that deep layers can sustain pile loads.
Soil type also affects foundation detailing. Clays may cause skin friction loss. Sandy soils may not support tension piles. Rock layers may need socketed piles or anchoring.
Design choice must always reflect site realities—not structural preference alone.
Coordinate With the Geotechnical Engineer
The geotechnical report is not the final word. It’s the beginning of an engineering dialogue.
Where questions arise, contact the geotechnical engineer. Ask how they interpreted low SPT values or whether seasonal groundwater variation matters. Share your structural loads early so the foundation advice fits actual demand.
Many problems arise when structural and geotechnical teams work in silos. Prevent this by engaging with each other from day one.
Also consider site visits. Seeing borehole locations, spoil material, or water ingress firsthand improves your understanding. Experience strengthens engineering judgement.
Watch for Limitations and Assumptions
Geotechnical reports often include disclaimers. They may state that findings apply only at test locations, or only for the design loads available at the time. Respect these boundaries.
If your design changes significantly—more loads, different footprint, deeper basement—then the report may no longer apply. Do not stretch recommendations beyond their intended use.
Also watch for incomplete data. Sometimes groundwater wasn’t encountered due to dry season. Or tests were limited due to site access. Know what the report covers—and what it leaves out.
Your structural design must rest on solid understanding. Assumptions buried in appendices can affect structural safety.
Make Design Decisions That Reflect the Soil
Structural design always sits on the soil. Whether you use Eurocode, BS 8110, or ACI, remember that codes assume reliable ground inputs.
Good engineers match their foundation dimensions, depths, and reinforcements to real site data. They also allow for construction realities—tolerances, water ingress, or unexpected strata.
Use partial factors appropriately. Design both for strength and serviceability. Avoid bearing failure and also control settlement and tilt.
Where uncertainty exists, build in conservatism or call for further tests. The cost of more investigation is often less than the cost of foundation failure.
Conclusion
Reading a geotechnical report is not optional for structural engineers—it is essential. The soil beneath your structure holds more influence than any beam or slab.
This guide shows you how to read beyond the numbers. Understand the borehole logs. Assess the test results. Question the assumptions. And match every foundation decision to the real ground.
Don’t treat the report as paperwork. Treat it as a map. A good structural design starts with a clear view of what lies beneath your feet.
Also See: Foundation Design in Expansive Soils – Challenges and Strategies
Sources & Citations
- Craig, R. F. (2012). Soil Mechanics (8th ed.). CRC Press.
- BS EN 1997-1:2004 – Eurocode 7: Geotechnical Design – Part 1: General Rules.
- Tomlinson, M. J., & Woodward, J. (2014). Foundation Design and Construction (7th ed.). Pearson Education.
