Lesson 1
How Do Soils Form? Engineering Geology for Civil Engineers
Almost every soil you will ever build on used to be rock. Knowing which rock it came from and how it got to your site tells you a lot about how it will behave, before you have run a single lab test.
1. Why should a civil engineer care about geology?
Geology is the science of the Earth’s materials (rocks, minerals, soils and groundwater) and how they formed. Engineering geology is the part of it that answers one practical question: what is this ground made of, and can we safely build on it, dig into it or build with it? (Bell, 2007; Waltham, 2009).
All construction happens in the Earth’s thin outer layer, the crust. The crust is only about 5–70 km thick (thinnest under the oceans, thickest under mountain ranges), which is at most about 1% of the Earth’s radius of 6371 km (Kious and Tilling, 1996). Most civil engineering projects only disturb the top few tens of metres of the crust.
A soil’s origin (the rock it came from and how it travelled) controls its properties: grain size, shape, layering and strength. Geology gives you a first guess. Lab testing then confirms it.
2. The rock cycle
Rocks and soils are constantly being recycled into each other. It happens far too slowly to notice in a human lifetime (the Earth is about 4.5 billion years old), but over geological time it never stops. Geologists summarise all these processes in one diagram called the rock cycle (or geological cycle) (Bell, 2007; Waltham, 2009):
Follow the arrows and you’ll see there isn’t just one loop. There are several routes:
- Magma → igneous rock (cooling). Molten rock rises towards the cooler surface and hardens.
- Any rock → residual soil (weathering). Igneous, sedimentary and metamorphic rock can all be broken down at the surface. Soil still sitting where it formed is residual soil.
- Residual soil → deposition → transported soil. Water, wind, ice or gravity carry the soil away and drop it somewhere else, forming transported soil.
- Volcanic ash → deposition (a shortcut). Ash is made of tiny fragments of magma blasted out of volcanoes. It skips the solid-rock stage and settles straight out as a soil-like deposit.
- Transported soil → sedimentary rock (lithification). Buried layers are compacted by the weight above and cemented back into rock.
- Igneous or sedimentary rock → metamorphic rock (metamorphism). Intense heat and pressure, from deep burial, earth movements or contact with nearby hot magma, transform the rock without melting it.
- Igneous or metamorphic rock → magma (melting). Taken deep enough, rock melts and the cycle starts again.
Like every model, this diagram leaves some routes out. In nature, sedimentary rock can also melt (although it usually becomes metamorphic first), and metamorphic rock can be metamorphosed again. The diagram shows the main routes that matter for engineering.
For civil engineers, the top half of the diagram matters most. Natural ground is either rock, residual soil or transported soil, and the route a soil took to reach a site shapes how it behaves. Many sites also have made ground (fill placed by people) on top, but that is not part of the natural cycle.
3. The three rock types, and what they mean for construction
Rocks are grouped by how they formed. That origin sets their grain structure, which in turn sets their engineering behaviour (Bell, 2007; Waltham, 2009).
Igneous rocks: born from magma
- Intrusive (cooled slowly underground): time for large crystals to grow, so they are coarse-grained. Examples: granite, diorite, gabbro. Magma can also squeeze into cracks and harden as thin sheets, called sills (along the layers) and dykes (cutting across them).
- Extrusive (cooled quickly at the surface, from volcanoes): fine-grained or even glassy. Examples: basalt, andesite, rhyolite, obsidian (volcanic glass). Trapped gas bubbles make light, holey (vesicular) rocks such as pumice and scoria.
On site: fresh (unweathered) igneous rock is usually very strong, so it’s excellent to found on. The catch is cracks: shrinkage during cooling and later earth movements fracture it, and blocks can slide along those cracks even though the rock itself is strong.
Sedimentary rocks: soil turned back into rock
- Clastic: made from compacted and cemented fragments of older rock. Gravel becomes conglomerate (rounded pieces) or breccia (angular pieces), sand becomes sandstone, silt becomes siltstone, and clay becomes mudstone, or shale if it splits easily into thin layers.
- Chemical: minerals that came out of solution in water (e.g. rock salt, chert).
- Organic: built mainly from the remains of living things (e.g. most limestone, chalk, coal).
On site: behaviour varies widely. Well-cemented sandstone and conglomerate can be strong but hard to excavate. Sedimentary rocks are usually layered, and the layer boundaries (bedding planes) can act as weak slip surfaces. Limestone and chalk slowly dissolve in slightly acidic water, which can create hidden voids and sinkholes (Waltham, 2009). Site investigation must look for these.
Metamorphic rocks: transformed by heat and pressure
- Foliated (minerals lined up in parallel sheets): slate and phyllite (from shale or mudstone), schist (from mudstone, shale or some igneous rocks), gneiss (often from granite).
- Non-foliated (no layering): marble (from limestone or dolomite), quartzite (from sandstone), hornfels (from rocks baked by nearby magma).
On site: metamorphism generally makes rock harder and stronger. Unweathered non-foliated types are excellent ground. Foliated types can split and slide along their sheets. The same property is why slate splits so neatly into roofing tiles.
| Rock type | Formed by | Examples | Main engineering concern |
|---|---|---|---|
| Igneous | Cooling magma | Granite, basalt | Fractures and joints |
| Sedimentary | Compressed and cemented sediment | Sandstone, shale, limestone, chalk | Weak bedding planes, dissolution (sinkholes) |
| Metamorphic | Heat and pressure | Marble, quartzite, slate, schist | Slip along foliation |
4. Weak spots in rock: joints, faults, bedding and folds
A small lump of rock can be incredibly strong. A whole rock mass is usually only as strong as its weakest planes (Goodman, 1993; Waltham, 2009). These are the four features to look out for:
| Feature | What it is | Why it matters |
|---|---|---|
| Joints | Cracks where the rock has split but the two sides have not slid past each other | Let water in; blocks can slide or topple out of slopes and tunnels |
| Faults | Cracks where the two sides have slid past each other | Often crushed, weak rock along the fault; possible further movement |
| Bedding planes (and foliation in metamorphic rock) | Boundaries between layers laid down at different times | Natural slip surfaces, especially if they dip towards a cutting |
| Folds | Layers bent by compression: upward arches (anticlines) or downward troughs (synclines) | Layers that were laid down roughly flat may now be steeply tilted |
A joint or bedding plane dipping out of a slope, towards the road or building below, can act as a ready-made sliding surface. Planes dipping steeply into the slope are unlikely to cause sliding, but they can let tall, thin blocks topple forwards (Hoek and Bray, 1981). Either way, engineers always record the orientation of these planes (their dip and direction), not just whether they exist.
5. Weathering: how rock turns into soil
Weathering is the breakdown of rock into smaller, softer material. There are two kinds, and they usually work together (Bell, 2007; Waltham, 2009).
Physical (mechanical) weathering: breaking it up
The rock is broken into pieces, but its minerals don’t change.
- Freeze–thaw: water in cracks expands by about 9% when it freezes, wedging the cracks wider.
- Unloading: as the rock above erodes away, the pressure on the rock below is released, so it expands and cracks open.
- Plant roots growing into cracks.
- Abrasion by wind, water or ice carrying grit, plus rock falls and landslides.
Typical result: gravel, sand and silt-sized fragments made of the same minerals as the parent rock. Fresh fragments are usually angular. They only become rounded if they are later carried and worn down, for example by a river.
Chemical weathering: changing what it’s made of
Minerals react with water, oxygen and carbon dioxide and are dissolved or turned into new minerals.
- Solution: minerals dissolve in water (this is how caves and sinkholes form in limestone and chalk).
- Hydrolysis: minerals react chemically with water. This is the main way feldspar, a common mineral in granite, turns into clay minerals.
- Hydration: minerals take water into their structure.
- Carbonation: reaction with carbon dioxide dissolved in rainwater, which forms weak carbonic acid (H₂CO₃).
- Oxidation: reaction with oxygen. With iron-bearing minerals this is the same process as rusting, and it gives many soils their red, orange or brown colour.
- Reduction: the reverse of oxidation, where oxygen is removed. It happens in waterlogged ground with little air and often leaves soils grey or blue-grey.
Typical result: clay minerals, plus dissolved material carried away in water. This is where the fine, sticky, water-sensitive soils come from. Chemical weathering is fastest in warm, wet climates, which is why tropical regions often have very deep weathered soil profiles.
Physical weathering makes smaller pieces of the same minerals. It mainly produces coarse soils (gravel and sand) and silt.
Chemical weathering makes new minerals, and it is the process that produces clay minerals.
In nature the two work together. Cracks from physical weathering let in water, which speeds up chemical weathering.
6. Residual vs transported soils
Once rock has weathered into soil, it either stays where it is or gets carried away (Coduto, Yeung and Kitch, 2011; Knappett and Craig, 2019).
Residual soils: stayed at home
A residual soil sits directly on top of the rock it came from. It builds up where weathering is faster than erosion. It often keeps the colour, minerals and even the original texture of the parent rock. It usually becomes gradually less weathered and stronger with depth until it becomes rock, so there is often no sharp boundary between soil and rock.
Transported soils: moved and dumped
A transported soil was carried from somewhere else and deposited. The transporting agent leaves a signature on the soil, and that signature is very useful to an engineer (Bell, 2007; Waltham, 2009):
| Carried by | Name | Typical character |
|---|---|---|
| Rivers and streams | Alluvial | Rounded grains, sorted into layers of gravel, sand, silt and clay. Often loose or soft, with a high water table, and very variable over short distances |
| Lakes | Lacustrine | Mostly fine silts and clays settled in still water; often soft. Lakes fed by glacier meltwater can leave varved clays: thin alternating layers of light silt (laid down in summer) and dark clay (laid down in winter) |
| Sea | Marine | Recent coastal and estuary deposits are often soft, compressible clays and silts. Much older marine clays, such as London Clay, can be stiff |
| Wind | Aeolian | Very uniform fine sand (dunes) or silt (loess). Loess can collapse suddenly when wetted |
| Ice (glaciers) | Glacial till (boulder clay) | Unsorted mix of every size from clay to boulders; very common across the UK, northern Europe and North America |
| Glacier meltwater | Glaciofluvial | Sorted, often rounded sands and gravels, because running water carried them |
| Gravity (often helped by water) | Colluvial | Loose, mixed, angular debris at the foot of slopes. It may contain old slip surfaces |
Water sorts, ice doesn’t. As moving water slows down, it drops the largest, heaviest particles first and carries fine ones further, so river and lake deposits tend to be layered by size. A glacier carries everything together, so glacial till is a jumble of every size at once. When glacier meltwater carries the material, it gets sorted again.
7. Rock or soil? The practical differences
In the field the boundary between “weak rock” and “strong soil” can be blurry. Rather than worrying about definitions, it’s more useful to know how the two typically behave differently (Goodman, 1993; Knappett and Craig, 2019):
| Rock | Soil | |
|---|---|---|
| Grains held together by | Natural cement or interlocking crystals | Friction and interlocking between grains, plus attractive forces between particles and water in clays. Soils are rarely cemented |
| Pore space | Usually low (with exceptions such as chalk and some sandstones) | High, typically about 25–60% of the volume (more in very soft clays and peat) |
| Continuity | Broken up by joints and faults, so behaves as blocks | Can usually be treated as one continuous material |
| Weathering effects | Can dramatically change its properties | Changes are more gradual |
| What controls design | Usually the discontinuities | Usually the material itself (grading, water content, density) |
8. Worked examples: reading a site
Geology questions don’t have numerical answers. Instead, you read the clues and predict what could go wrong. Here’s how to think them through.
Example 1: A housing site on a river floodplain
A developer plans houses on flat land beside a river. Trial pits show layers of rounded sand and gravel, with bands of soft grey silt and clay. Groundwater appears 1.5 m below the surface. What type of soil is this, and what should the engineer look out for?
Flat land by a river, rounded grains and neat layers sorted by size all point to material carried and dropped by water.
This is a transported soil, specifically an alluvial deposit.
- The soft clay and silt bands may compress under load, so check for settlement.
- A high water table means excavations may flood or have unstable sides, and may need dewatering.
- Alluvial layers change quickly over short distances, so a single borehole is not enough.
- Floodplains, by definition, flood, so check the site’s flood risk.
Answer: Alluvial soil. Investigate the soft layers and groundwater carefully before choosing foundations.
Example 2: A road cutting through tilted sandstone
A new road will be cut through a hillside of sandstone. The design shows a cut face at 60°. The geological survey shows the bedding planes dip at about 35° towards the road, roughly parallel to the cut face. Is this a concern?
Sandstone is sedimentary, so it has bedding planes, which are natural surfaces of weakness.
The planes dip out of the slope, towards the road. They are also flatter (35°) than the cut face (60°), so they will be exposed, or “daylight”, in the face of the cutting. Once the cutting removes the rock at the toe, the blocks above have nothing supporting them from below.
Sliding will happen if the bedding dips more steeply than the friction angle of the bedding surface. For sandstone surfaces this angle is typically somewhere around 25–35°, so a 35° dip is a real risk (Hoek and Bray, 1981). Water pressure in the joints makes it worse. This is called planar sliding. The friction angle must be confirmed by testing. Options include a flatter cut, rock bolts or anchors, drainage, or re-routing.
Answer: Yes, this is a real concern, even though the sandstone itself may be strong. It needs a proper slope stability assessment.
Example 3: A warehouse on a chalk area
A warehouse is planned on a site where the geological map shows chalk under a thin soil cover. A local resident mentions “a hole that suddenly opened in a field nearby a few years ago”. What should the engineer do?
Chalk is an organic sedimentary rock made almost entirely of calcium carbonate (from the skeletons of microscopic marine algae). It slowly dissolves in slightly acidic rainwater (chemical weathering by solution).
The “hole” was very likely a sinkhole: a buried void that collapsed. There may be others under the site that haven’t collapsed yet.
Use a closer grid of probes or boreholes, geophysical surveys to spot voids, and control drainage so water isn’t concentrated into the ground (leaking drains and soakaways commonly trigger collapses). Plan the investigation in line with BS 5930 (BSI, 2020), with specialist input for dissolution features (Waltham, 2009).
Answer: Treat dissolution features as a real hazard and design the site investigation to find them.
9. Try it yourself
Have a go before opening the answers.
Q1. Granite and basalt are both igneous rocks. Why does granite have large visible crystals while basalt is very fine-grained?
Granite cooled slowly, deep underground (intrusive), giving crystals time to grow. Basalt cooled quickly at the surface after a volcanic eruption (extrusive), so crystals had almost no time to form.
Q2. A borehole finds a stiff clay containing a random mix of sand, gravel and large boulders, with no layering. What is the most likely origin?
Glacial till (boulder clay). A glacier carries and dumps every particle size together without sorting, so you get a jumble with no layers. On site, watch for boulders obstructing piling and excavation, and remember that one borehole may miss them.
Q3. Which type of weathering mainly produces clay minerals: physical or chemical?
Chemical. Physical weathering just breaks rock into smaller pieces of the same minerals. Chemical weathering changes the minerals themselves, for example hydrolysis turning feldspar into clay minerals.
Q4. What is the difference between a joint and a fault?
Both are cracks in rock. Across a joint, the two sides have not slid past each other; at most the crack has opened slightly. Across a fault, the rock on each side has slid relative to the other.
Q5. A soil at a hillside site has the same reddish colour and minerals as the weathered granite directly beneath it, and gets gradually harder with depth. Residual or transported?
Residual. It matches the rock below and grades gradually into it, so it formed in place rather than being carried in from elsewhere.
10. Key takeaways
- Soil comes from weathered rock (plus volcanic ash, which skips the rock stage), as part of the rock cycle. All three rock types can weather into soil.
- Igneous rock forms from cooled magma, sedimentary from compacted and cemented sediment, and metamorphic from existing rock changed by heat and pressure.
- Rock masses are controlled by their weak planes: joints, faults, bedding and foliation. Check their direction.
- Physical weathering breaks rock into smaller pieces of the same minerals. Chemical weathering creates new minerals, including clay minerals.
- Residual soils stay on their parent rock. Transported soils carry the signature of how they moved: water sorts, ice doesn’t.
- Knowing a soil’s origin gives you an early warning of problems like soft layers, sinkholes and sliding planes.
11. Assessment
Test yourself properly. Unlike the practice questions, this assessment has no answers on the page. Attempt it on your own, as if it were an exam.
- Total: 40 marks. Suggested time: 60 minutes.
- Answer all questions. The marks for each question are shown in brackets.
- You may refer to the lesson notes, but write your answers in your own words.
- Where a question asks you to justify or explain, marks go to your reasoning, not just the final answer.
Section A: Concepts (13 marks)
- Name the three main groups of rock and state briefly how each one forms. [3 marks]
- Sedimentary rocks are divided into clastic, chemical and organic types. Describe what each type is made from and give one example of each. [3 marks]
- Explain the difference between physical and chemical weathering. Name two processes of each. [5 marks]
- Define a residual soil and a transported soil. [2 marks]
Section B: Reading the ground (15 marks)
A borehole is drilled on flat land about 50 m from a river. The simplified log reads:
Depth (m) Description 0.0 – 3.5 Loose grey fine to medium SAND with rounded grains, well sorted, with thin layers of soft grey silt. Groundwater met at 1.2 m. 3.5 – 9.0 Stiff brown sandy gravelly CLAY with many cobbles and boulders of mixed rock types. No layering. 9.0 + Sandstone. (a) Identify the most likely geological origin of each of the two soil layers. Give two pieces of evidence from the log for each. [4 marks]
(b) For each soil layer, give one engineering concern for the design or construction of foundations. [2 marks]
(c) The drilling crew report “rock at 5.2 m” in a second, nearby borehole. Why should the engineer be cautious before accepting this? [1 mark]
A railway cutting is to be made in slate. The cut face will be at 70°. A survey shows two sets of planes:
- Set 1: foliation dipping at 50° towards the track, running roughly parallel to the cut face.
- Set 2: joints dipping at 80° into the slope, also roughly parallel to the face.
(a) Name the most likely type of instability associated with each set, and explain why. [4 marks]
(b) State two pieces of information you would need to confirm whether Set 1 will actually slide. [2 marks]
(c) Suggest two measures that could reduce the risk to the railway. [2 marks]
Section C: Applied case (12 marks)
A developer plans a two-storey school on a site underlain by chalk, with a thin cover of soil. The desk study notes a circular hollow, about 4 m across, in the neighbouring field. The drainage design sends all roof water into soakaways close to the building.
Write a short technical note to the developer (maximum 300 words) that:
- explains the geological hazard and what the hollow may indicate [4 marks]
- explains why the proposed soakaways are a concern [3 marks]
- recommends appropriate investigation and design measures [3 marks]
Clear, professional communication suitable for a non-specialist client. [2 marks]
How your work is judged
| Criterion | What a strong answer shows |
|---|---|
| Technical accuracy | Correct terms, definitions and processes |
| Use of evidence | Conclusions backed by specific clues from the question (grain shape, sorting, dip direction, and so on) |
| Engineering judgement | Links geology to real consequences for design and construction |
| Communication | Clear, concise and well organised, in your own words |
You’re welcome to complete this assessment for your own practice. If you’d like me to mark it personally and send written feedback within 5 days, you can send it to me for £20. The marking & feedback page explains how.
- Upload one file (PDF, Word or a clear photo of handwritten work) with your name at the top.
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12. References
- Bell, F.G. (2007) Engineering Geology. 2nd edn. Oxford: Butterworth-Heinemann.
- British Standards Institution (BSI) (2020) BS 5930:2015+A1:2020 Code of practice for ground investigations. London: BSI.
- Coduto, D.P., Yeung, M.R. and Kitch, W.A. (2011) Geotechnical Engineering: Principles and Practices. 2nd edn. Upper Saddle River, NJ: Pearson.
- Goodman, R.E. (1993) Engineering Geology: Rock in Engineering Construction. New York: John Wiley & Sons.
- Hoek, E. and Bray, J.W. (1981) Rock Slope Engineering. 3rd edn. London: Institution of Mining and Metallurgy.
- Kious, W.J. and Tilling, R.I. (1996) This Dynamic Earth: The Story of Plate Tectonics. Reston, VA: U.S. Geological Survey.
- Knappett, J.A. and Craig, R.F. (2019) Craig’s Soil Mechanics. 9th edn. Boca Raton, FL: CRC Press.
- Waltham, T. (2009) Foundations of Engineering Geology. 3rd edn. Abingdon: Spon Press.
13. Further learning
- Read: Waltham’s Foundations of Engineering Geology is short, very visual and written for civil engineers. It is the best next step after this lesson.
- Explore your own area: the British Geological Survey (bgs.ac.uk) has free online geology maps of the UK. Look up the rock and soil beneath your home or campus and see which deposits from this lesson you can find.
- Identify rocks: geology.com has photo galleries of igneous, sedimentary and metamorphic rocks, which help you learn what each one looks like.
- Earth structure and plate movements: the U.S. Geological Survey (usgs.gov) publishes free educational material, including This Dynamic Earth.
- In the field: next time you pass a road cutting, quarry or cliff, look for bedding planes, joints and folds, and ask which way they dip.