Knowledge Base

How to Read Geological Reports: Understanding Soil Conditions and Their Impact on Design and Construction.

When planning a construction project, whether a new home, a basement extension, or a commercial building, one of the most critical yet often overlooked steps is understanding the geological report. These reports provide information about the ground beneath your feet and significantly determine the design and construction techniques needed to ensure a safe, stable, and cost-effective project.

In this article, we’ll break down how to read geological reports, explain key terms, and explore how soil conditions can influence your project. Whether you’re a homeowner curious about your property’s foundation, an architect designing a structure, or a builder planning excavation, this guide will help you make sense of the technical details and their real-world implications.
What is a Geological Report?

A geological report is a detailed document prepared by geotechnical engineers or surveyors after investigating a site. It typically includes:

  • Soil and rock layers (stratigraphy).
  • Groundwater conditions.
  • Strength and stability of the ground.
  • Recommendations for design and construction.

These reports are based on data from boreholes, trial pits, and laboratory tests. They help identify potential challenges and opportunities, ensuring the project is built on a solid foundation.

Key Elements of a Geological Report

Let’s take a closer look at three extracts from a typical geological report and unpack what they mean:

Extract 1: Soil Layers and Their Properties

“The made ground is variable, consisting of brick rubble, ash, and clay outside original basement areas, and loose sand and gravel beneath the original basement. The gravel is generally dense to very dense floodplain gravel with a perched water table at a level of approximately 10.5 m OD, giving between 1 m and 4 m of water over the London Clay.”

What This Means:

  • The site has variable soil conditions, with layers of made ground (artificially placed material like brick rubble and ash) and natural deposits like sand, gravel, and clay.

  • A perched water table is a localized zone of trapped groundwater that forms above an impermeable layer, such as London Clay, rather than connecting to the main groundwater table. This isolated reservoir can cause water ingress and instability during excavation.
In areas like London, where London Clay is present, a perched water table forms because:
  • London Clay has very low permeability, preventing water from draining through it. Water that infiltrates more permeable layers above, such as sand or gravel, becomes trapped, accumulating and creating a localized saturated zone.
  • The main groundwater table lies deeper, within highly permeable layers like Thanet Sand or Chalk, while the perched water table remains separate. This makes it temporary and unstable, fluctuating with rainfall and drainage conditions.
  • Thanet Sand is a geological formation found in southeast England, particularly beneath London and Kent. It consists of fine-grained quartz sand with some silt and clay, often appearing light grey, bluish-grey, or yellowish, and contains glauconite, giving parts of it a greenish tint. Located beneath London Clay and above the Chalk Formation, it can be up to 30 metres thick. Thanet Sand is highly permeable, allowing water to flow through easily, making it an important aquifer and influencing groundwater behaviour. In construction, it presents challenges, as its instability and water retention can require careful foundation design, water management, and dewatering techniques, especially in deep excavations, basements, and tunnels
Impact on Design and Construction:
  • Excavation: Loose sand and gravel can collapse during digging, so shoring or sheet piling may be needed.
  • Waterproofing: A perched water table means waterproofing measures (e.g., drainage systems, tanking) are essential to prevent flooding.
  • Foundations: The dense gravel and London Clay provide a stable base, but the variable made ground may require ground improvement techniques.
Extract 2: Soil Behaviour and Groundwater

"The London Clay contains three basic zones: (a) the top zone is stiff to very stiff fissured clay; (b) the middle zone is very stiff to hard fissured clay with sand partings; and (c) the bottom zone is hard silty clay with local fissuring. The Woolwich and Reading Beds were proved in three boreholes, commencing as hard, highly fissured clay. The pore water pressure profile indicates underdrainage due to the historic lowering of the water table in the underlying Thanet Sand/Chalk aquifer."
What This Means:
Fissuring:
  • Fissures are small cracks or fractures in the clay. While clay is generally impermeable, fissures can allow water to seep through, affecting strength and stability.
  • The top and middle zones of the London Clay are fissured, which means they may be prone to water movement and localized instability. This can complicate excavation and foundation design, as fissured clay may require additional support or stabilization.

The Woolwich and Reading Beds:
  • These are geological layers found beneath the London Clay. They consist of hard, highly fissured clay that becomes multi-coloured with depth.
  • The Woolwich and Reading Beds are dense and strong, but their highly fissured nature adds complexity to the subsurface conditions. Fissures can create pathways for water, leading to potential instability or the need for specialized construction techniques.

Underdrainage:
  • Underdrainage happens when the main groundwater table (in the deep aquifer) has been lowered over time, usually due to human activities like pumping groundwater for industrial or city use. This creates a pressure imbalance, causing water from the upper soil layers to drain downward, affecting soil stability and excavation projects.
How Does Underdrainage Happen?
  • The water table (the level where the soil is fully saturated with water) usually is stable within an aquifer. The groundwater in deeper layers supports the water in the layers above, maintaining pore water pressure (the water pressure within soil pores).
  • The water table drops when industries, cities, or construction projects pump large amounts of groundwater from the deep aquifer. This creates a gap between the main water table and the water in the upper soil layers.
  • Because water always moves from high to low pressure, water from upper layers (sand, gravel, silt) starts seeping down to refill the deeper aquifer. This reduces the water content in the upper soil, making it drier and less stable.
Usually, pore water pressure helps support the weight of the soil, preventing it from collapsing or compacting. As water drains downward, this pressure decreases, which can cause soil shrinkage or settlement (the ground slowly sinks), instability in excavations (walls may collapse more easily), and foundation problems (if the ground dries and shifts).

Real-Life Example (Making it Simple!)
Imagine you have a sponge soaked with water (the soil with normal groundwater levels). If you squeeze water out from the bottom (like pumping water from the deep aquifer), the top layers start drying as water moves downward. Over time, the sponge loses its moisture, shrinks, and becomes less stable—just like soil undergoing underdrainage.
Historical Groundwater Extraction:

  • Over time, large-scale groundwater pumping from the Thanet Sand/Chalk aquifer has lowered the water table. This has created a "drawdown" effect, causing water from the London Clay and other overlying layers to drain into the aquifer.
  • This historical extraction has long-term effects on the site’s hydrogeology, including changes in pore water pressure and soil stability.

Pore Water Pressure:

  • Pore water pressure is the pressure groundwater exerts within the soil’s pores. It plays a critical role in soil stability.
  • In this case, the underdrainage caused by historical groundwater extraction has reduced pore water pressure in the London Clay. This can lead to soil consolidation (compression) and potential settlement over time.

Rapid Rise in Groundwater Levels:

The report also mentions a rapid rise in groundwater levels in one borehole (from -28 m OD to -17 m OD), which could indicate fluctuations in pore water pressure due to changes in the aquifer or local groundwater conditions.
  • OD (Ordnance Datum) is a standard reference point for elevation measurements in the UK. It represents height relative to the mean sea level at a specific location. The most used reference in the UK is Ordnance Datum Newlyn (ODN), based on the mean sea level at Newlyn, Cornwall. When a measurement is “-28 m OD” or “-17 m OD”, the groundwater level is 28 metres or 17 metres below the standard sea level reference point.
What Does the Rapid Rise in Groundwater Levels Mean?
In the borehole, the groundwater level rose quickly from -28 m OD to -17 m OD, meaning water levels increased by 11 metres. This suggests:
  • A change in local groundwater conditions, possibly due to recent heavy rainfall or underground flow changes.
  • Fluctuations in pore water pressure could affect soil stability.
  • Potential dewatering issues in excavation, such as a sudden rise in water level, can lead to unexpected water ingress.
Impact on Design and Construction:
  • Foundation Design: The fissured clay layers may require deeper foundations or ground improvement to ensure stability.
  • Groundwater Management: Underdrainage and fluctuating water levels mean dewatering systems and waterproofing are critical.
  • Monitoring: Regular monitoring of groundwater levels is necessary to address any sudden changes, such as the rapid rise observed in one borehole.
Extract 3: Soil Layers and Their Properties

"The subsoil consists predominantly of sandy clay up to a depth of 10 meters. Standard Penetration Tests (SPT) indicate N-values ranging from 3 to 50, suggesting variable soil density. Groundwater was encountered during drilling."

What This Means:
  • Soil Composition: The site has a layer of sandy clay extending to 10 meters. Sandy clay is a mixture of sand and clay particles, which can affect its load-bearing capacity and drainage properties.
  • Soil Density (SPT N-values): The Standard Penetration Test (SPT) measures soil resistance to penetration and provides an N-value.
N-value of 3 indicates very loose soil.
N-value of 50 indicates very dense soil. The wide range of N-values suggests that the soil density varies significantly across the site, which can impact foundation design.

  • Groundwater Presence: Encountering groundwater during drilling indicates that the water table is within the depth of interest. This can influence excavation methods and the need for dewatering.
Impact on Design and Construction:
  • Foundation Design: Variable soil density means some areas require deeper foundations or soil improvement to ensure stability.
  • Excavation: The presence of groundwater necessitates planning for dewatering to keep the excavation dry and stable.
  • Soil Improvement: In areas with very loose soil (low N-values), compaction or grouting may be needed to enhance soil strength.

How Soil Conditions Affect Design and Construction

Soil conditions are a major factor in determining a project's feasibility, cost, and timeline. Here’s how they influence key decisions:

Foundation Type:
  • Stable soils (e.g., dense gravel, hard clay) allow for shallow foundations like strip or pad footings.
  • Unstable or variable soils (e.g., loose sand, made ground) may require deep foundations like piles or raft foundations.

Excavation and Shoring:
  • Loose or waterlogged soils can collapse during excavation, necessitating shoring, sheet piling, or dewatering systems.
  • Fissured clay layers may require careful handling to prevent instability.

Waterproofing and Drainage:
  • High groundwater levels or perched water tables demand robust waterproofing measures, such as tanking or cavity drainage systems.
  • Proper drainage design is essential to manage water flow and prevent flooding.

Ground Improvement:
  • Weak or compressible soils may need stabilization techniques, such as soil mixing, grouting, or the use of geotextiles.

Construction Techniques:

Soil conditions are crucial in determining the safest and most effective construction techniques. Below are some common methods used based on different ground conditions:

Excavation Support & Retaining Structures (For weak, loose, or waterlogged soils)
  • Diaphragm Walls – Used for deep excavations, basements, and underground structures, especially in urban areas with unstable soil.
  • Sheet Piling – Steel, timber, or concrete sheets are driven into the ground to prevent collapse in soft soils or near water bodies.
  • Secant Pile Walls – Overlapping concrete piles for water-tight retaining walls in sandy or clayey soils.
  • Soldier Piles & Lagging – H-beams with wooden or concrete panels for temporary excavation support.
  • Soil Nailing – Reinforcing slopes or excavation walls with anchored steel bars.
  • Ground Improvement Techniques (For soft, weak, or compressible soils)
  • Soil Compaction – Using rollers or vibratory equipment to densify loose sands and gravels.
  • Grouting (Permeation or Jet Grouting) – Injecting cementitious materials to strengthen weak soils or fill voids.
  • Soil Stabilisation – Mixing lime, cement, or fly ash into clay or silt to improve load-bearing capacity.
  • Deep Mixing Method (DMM) – Mechanical mixing of stabilising agents into weak soils to create solid columns.
  • Vibro Stone Columns – Installation of crushed stone columns in soft ground to improve bearing capacity and drainage.

Dewatering & Drainage Solutions (For high water table or perched water table conditions)
  • Wellpoint Systems – A series of small wells and pumps used to lower the groundwater table temporarily.
  • Deep Wells – Large-diameter wells with submersible pumps for major groundwater control in deep excavations.
  • Cutoff Walls (Slurry or Bentonite Walls) – Barriers that prevent water inflow into excavations.
  • Cavity Drainage Systems – Interior drainage channels used for waterproofing basements.

Foundation Methods (Based on soil strength & stability)
  • Shallow Foundations (Strip, Raft, Pad Foundations) – Suitable for dense, stable soils bearing structural loads.
  • Pile Foundations (Driven, Bored, CFA Piles) – Used in soft clay, loose sand, or waterlogged soils to transfer loads to deeper stable strata.
  • Micropiles – Small-diameter piles for sites with restricted access or weak surface soils.
  • Underpinning – Strengthening existing foundations by extending them deeper, often in renovation projects.

Tips for Reading Geological Reports

  • Start with the Summary: Most reports include an executive summary highlighting key findings and recommendations. This is a great place to start.
  • Understand the Terminology: Terms like stiff clay, fissured, perched water table, and underdrainage may seem technical, but they’re crucial for understanding soil behaviour.
  • Look for Visual Aids: Diagrams, cross-sections, and borehole logs can help you visualize the soil layers and groundwater conditions.
  • Focus on Recommendations: The report will often suggest specific design and construction techniques based on the soil conditions. These are invaluable for planning your project.
  • Consult Experts: If you’re unsure about any part of the report, don’t hesitate to consult a geotechnical engineer or construction professional.

Why This Matters for Homeowners, Architects, and Builders

Understanding the geological report can help homeowners make informed decisions about their property, such as whether a basement extension is feasible or if additional waterproofing is needed.

Soil conditions influence architects' and designers' structural design, material choices, and even a project's aesthetics. For example, a site with unstable soils may require a lighter structure or innovative foundation solutions.

The report provides a roadmap for excavation, shoring, and construction techniques, helping builders avoid costly surprises during construction.

Final Thoughts

Geological reports are more than just technical documents—they are a successful project’s foundation. Understanding the soil conditions and their implications allows you to make smarter decisions, avoid costly mistakes, and ensure your project is built to last.

So, don't be intimidated the next time you’re handed a geological report. Dive in, ask questions, and use it as a tool to create something remarkable. After all, every great project starts with a solid foundation—literally and figuratively.
Are you planning a residential renovation or home extension or dreaming of a stunning new basement or loft? At Masters, we bring your vision to life with precision, expertise, and a commitment to quality.
2025-02-05 19:39 Science behind building