Why do engineers need to understand the geology before building?
Engineering geology assesses the ground before construction: foundations, tunnels, dams and reservoirs must suit the rock and soil present; geologists check the strength and stability of rock, the presence of faults, the slope stability, the permeability of the ground (for a reservoir to hold water or a tunnel to stay dry), and the hazards of weak, soluble or swelling materials; poor ground investigation can lead to subsidence, leakage, collapse or failure, so a site investigation is carried out first.
A focused answer to the Eduqas GCSE Geology statement on engineering geology. Covers why the ground must be assessed before construction, the factors checked (rock strength, faults, slope stability, permeability, weak or soluble materials), and how poor investigation leads to subsidence, leakage or collapse.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Eduqas wants you to explain engineering geology: why the ground must be assessed before construction, so that foundations, tunnels, dams and reservoirs suit the rock and soil present. You need to know the factors geologists check (rock strength and stability, faults, slope stability, permeability, and the hazards of weak, soluble or swelling materials) and to explain how poor ground investigation leads to subsidence, leakage, collapse or failure, which is why a site investigation is always carried out first. This applies the whole course (rock properties, structures, slope stability, groundwater) to a practical problem.
The answer
Why the ground matters
A structure is only as good as the ground it stands on, so engineers must understand the geology before they build. Different structures place different demands on the ground:
- Foundations must rest on ground strong enough to bear the load without settling.
- Tunnels must pass through rock that is stable (will not collapse) and ideally dry (low permeability, so water does not flood in).
- Dams and reservoirs must sit on impermeable ground so the water is held rather than leaking away, and the dam must rest on strong, sound rock.
The job of engineering geology is to check whether the ground meets these demands, and to design around it where it does not.
The factors geologists check
A ground assessment looks at several linked factors:
- Rock strength and stability. Is the rock strong enough to bear the structure, or weak and likely to fail? Strong, unweathered rock is a good foundation; soft clay or heavily weathered rock is not.
- Faults. Faults are zones of weakness and can provide leakage paths or be sites of movement, so their presence must be mapped.
- Slope stability. On or near slopes, the risk of mass movement (landslides, slumps) into a cutting, against a structure, or into a reservoir must be assessed.
- Permeability. For a reservoir to hold water the ground must be impermeable; for a tunnel to stay dry the rock should not let water flood in. Permeable or fractured rock is a problem for both.
- Weak, soluble or swelling materials. Soft ground settles; soluble rock (limestone) can dissolve to open cavities and sinkholes; swelling clay expands and shrinks as it wets and dries, cracking foundations.
What goes wrong if the ground is unsuitable
Ignoring the geology leads to expensive, sometimes dangerous, failures:
- Subsidence and settlement. Weak or soluble ground lets a structure sink, often unevenly, cracking it.
- Leakage. A reservoir on permeable or soluble rock simply will not hold water.
- Collapse. Tunnels in weak rock, or buildings over hidden cavities, can collapse.
- Failure. A dam on weak rock or crossed by a fault, or a slope above a reservoir, can fail catastrophically.
The site investigation
Because these problems are hidden underground, a site investigation is carried out before construction. It typically uses boreholes to sample the rock and soil at depth, find the water table and the depth to sound rock (rockhead), and test the ground's strength and permeability, together with geological maps and cross-sections of the area. The results let engineers design suitable foundations, choose a safe route or site, or move the project if the ground is too poor.
Examples in context
Example 1. The leaking reservoir. Reservoirs built without checking for soluble or fractured rock have failed to fill because the water drained away underground, a costly reminder that permeability is the first thing to check.
Example 2. Building over old mine workings. Construction over hidden cavities, whether natural sinkholes or old mine shafts, risks sudden subsidence. A site investigation with boreholes is designed to find such voids before foundations are laid.
Try this
Q1. State why the ground beneath a reservoir must be impermeable. [1 mark]
- Cue. So that the water is held in the reservoir rather than leaking away through the rock.
Q2. Explain why building on soluble limestone can be risky. [2 marks]
- Cue. Water dissolves limestone to open cavities and sinkholes, which can collapse and cause subsidence, and which leak water from a reservoir.
Q3. Name the investigation carried out before construction and one thing it reveals. [2 marks]
- Cue. A site investigation (using boreholes); it reveals rock and soil strength, the water table, the depth to sound rock, cavities, or faults.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20206 marksA company plans to build a reservoir to store water in a valley. Explain the geological factors that must be checked before construction, and explain what could go wrong if the ground is unsuitable.Show worked answer →
Identify the factors that matter for a reservoir, then the consequences of getting them wrong.
- Permeability of the valley floor and sides
- To hold water, the ground must be impermeable; if the rock is permeable (porous sandstone, fractured or soluble limestone), the water will leak away through it and the reservoir will not fill. This is the most important factor.
- Soluble rock
- If the rock is limestone, water can dissolve it, opening cavities and sinkholes through which the reservoir leaks and the ground subsides.
- Faults and rock strength
- Faults can provide leakage paths and zones of weakness; the rock must be strong enough to bear the dam and resist the water pressure.
- Slope stability
- Steep, unstable reservoir slopes can fail into the water; a large landslide into a full reservoir can send a wave over the dam.
- What could go wrong
- Leakage (the reservoir will not hold water), dam failure or collapse, and slope failure. Markers reward the permeability and soluble-rock checks (the reservoir must hold water), faults and rock strength, slope stability, and consequences such as leakage, subsidence or dam failure.
Eduqas 20185 marksExplain why a geological site investigation is carried out before constructing the foundations of a large building, and give two ground problems it might reveal.Show worked answer →
State the purpose of the investigation, then two specific ground problems.
Purpose. A site investigation checks the rock and soil so the foundations can be designed to suit the ground, avoiding subsidence, cracking or collapse later. It typically uses boreholes to sample the ground and find the water table and the rockhead.
Two ground problems (any two). Weak or soft ground (such as soft clay or made ground) that cannot bear the load and lets the building settle unevenly. Soluble rock (limestone) with hidden cavities or sinkholes that could collapse. Swelling or shrinking clay that moves as it wets and dries, cracking foundations. A high water table or buried faults are also valid.
Markers reward the purpose (match the foundations to the ground and avoid failure) and two genuine ground problems, each linked to how it threatens the building.
Related dot points
- Hydrocarbons (oil and gas) form from buried organic matter, then migrate from a source rock into a porous, permeable reservoir rock where an impermeable cap rock and a trap (for example an anticline or fault) hold them; groundwater is stored in a porous, permeable aquifer beneath the water table and supplied to wells; both depend on the rock properties porosity (storage) and permeability (flow), and both can be over-exploited or polluted.
A focused answer to the Eduqas GCSE Geology statement on hydrocarbons and groundwater. Covers how oil and gas form, migrate and are trapped by a reservoir, cap rock and structure, how groundwater is stored in aquifers below the water table, and the key rock properties porosity and permeability.
- Mass movement is the downslope movement of rock and soil under gravity; it includes slow creep, slides, slumps, flows and rockfalls; failure is triggered when the driving force (gravity, increased by steep slopes, heavy rain, loading and undercutting) exceeds the resisting force (friction and cohesion, reduced by water and weak or weathered rock); the risk is reduced by improving drainage, reducing slope angle, building retaining structures and avoiding building on unstable ground.
A focused answer to the Eduqas GCSE Geology statement on mass movement. Covers the types of mass movement, why slopes fail (the balance of driving and resisting forces and the role of water), the triggers, and how landslide risk is reduced by drainage, regrading, retaining structures and avoidance.
- An ore is a rock from which a metal can be extracted economically; ore minerals (for example galena for lead, haematite for iron) are concentrated by geological processes such as hydrothermal veins, magmatic settling and weathering and deposition; whether a deposit is worked depends on its grade, size, depth and the metal price; extraction by surface or underground mining has environmental costs, so it is balanced against the need for the metal and is followed by site restoration.
A focused answer to the Eduqas GCSE Geology statement on mineral resources. Covers the definition of an ore, the named ore minerals, how ore deposits are concentrated (hydrothermal, magmatic, weathering), the economic factors that decide whether a deposit is mined, and the environmental costs of extraction.
- Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.
A focused answer to the Eduqas GCSE Geology statement on folds and faults. Covers how compression produces folds (anticlines and synclines) and reverse faults, how tension produces normal faults, the parts of a fold and fault, and how these structures record the direction of past Earth movements.
- Geological history is reconstructed from a cross-section using the principles of superposition (younger beds lie above older), original horizontality, cross-cutting relationships (a fault or intrusion is younger than the rocks it cuts) and included fragments; the order of deposition, deformation, intrusion, erosion (unconformities) and faulting is deduced to give a relative sequence of events.
A focused answer to the Eduqas GCSE Geology statement on reading cross-sections. Covers the principles of superposition, original horizontality, cross-cutting relationships and included fragments, and how to combine them to deduce the relative order of deposition, intrusion, deformation, erosion and faulting in an area.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)