How do we investigate the ground before building, and what conditions cause problems?
Engineering geology: the engineering properties of rocks and soils (strength, jointing and discontinuities, weathering and the behaviour of clays, sands and gravels); the purpose and methods of site investigation (desk study, boreholes, trial pits and core logging); the ground conditions that cause problems for foundations (weak or compressible soils, swelling clays, solution cavities in limestone, made ground and high groundwater); the role of foundations and the ground model.
A focused answer to the OCR H414 dot point on engineering geology. Covers the engineering properties of rocks and soils, the purpose and methods of site investigation (desk study, boreholes, trial pits, core logging), the ground conditions that cause foundation problems (weak or swelling soils, solution cavities, made ground, groundwater), and the role of foundations and the ground model.
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What this dot point is asking
OCR wants you to describe the engineering properties of rocks and soils, to explain the purpose and methods of site investigation, to identify the ground conditions that cause problems for foundations, and to explain the role of foundations and the ground model.
The answer
Engineering properties of rocks and soils
The behaviour of the ground under a structure depends on its properties:
- Rock strength depends on the rock type and, critically, on discontinuities (joints, bedding planes and faults), which weaken the rock mass and can act as sliding surfaces. Weathering further reduces strength.
- Soils behave very differently by grain size: gravels and sands are strong and free-draining but have little cohesion; clays are cohesive but weak, can be compressible (settling under load) and may swell and shrink with changes in water content.
Site investigation
A site investigation determines the ground conditions before construction so that hazards are identified and safe, economical foundations can be designed:
The stages are:
- Desk study. Review existing information: geological maps, previous reports, old mine and quarry records, and aerial photographs.
- Ground investigation. Drill boreholes and dig trial pits, log the cores (recording the rock and soil at depth), find the depth to bedrock (rockhead) and the water table, and take samples for laboratory strength tests. Geophysical surveys may help.
- Ground model. Integrate all the data into a conceptual model of the subsurface, on which the foundation design is based.
Problem ground conditions
Certain conditions cause problems for foundations:
- Weak or compressible soils (for example soft clays) settle under load, sometimes unevenly, cracking the structure.
- Swelling (shrink-swell) clays expand and contract with water content, heaving or subsiding.
- Solution cavities in soluble rock (limestone), which can collapse as sinkholes.
- Made ground (former waste or fill), which is variable, loose and compressible.
- High groundwater, which lowers ground strength, complicates excavation and can cause uplift.
Foundations and the ground model
Foundations transfer the load of a structure to the ground. Shallow foundations suit strong, near-surface ground; deep (for example piled) foundations carry the load down to stronger strata where the surface ground is weak (soft clay, made ground) or where cavities are a risk. The ground model from the site investigation determines which foundation is appropriate.
Examples in context
Example 1. Sinkholes over limestone. Where soluble limestone has developed solution cavities, the ground can collapse as a sinkhole, so a site investigation must detect cavities (by boreholes or geophysics) before foundations are placed, often requiring deeper or reinforced foundations.
Example 2. Settlement on made ground. Buildings placed on poorly compacted made ground (old fill) have settled unevenly and cracked; logging the made ground in trial pits and designing piled foundations to the firm stratum below avoids this.
Try this
Q1. State the purpose of a site investigation. [2 marks]
- Cue. To determine the ground conditions (rock, soil and groundwater types, properties and arrangement) so hazards can be identified and safe, economical foundations designed.
Q2. Explain why swelling clay is a problem for foundations. [2 marks]
- Cue. Swelling clay expands and contracts with changes in water content, so it heaves and subsides, which can crack or distort a structure built on it.
Q3. Describe one method used in a ground investigation and what it provides. [2 marks]
- Cue. For example boreholes: drilling to recover and log the rock and soil at depth, find the depth to bedrock and the water table, and take samples for strength testing.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/02 20206 marksA building is to be constructed over an area of limestone bedrock partly covered by made ground (former quarry waste). Explain the ground problems that should be investigated and how a site investigation would assess them.Show worked answer →
A level-of-response answer; identify the hazards, then the investigation methods.
- Problem 1, solution cavities in limestone
- Limestone is soluble, so groundwater can dissolve it to form underground cavities (and sinkholes). A cavity beneath a foundation could collapse, so its presence must be investigated.
- Problem 2, made ground
- Quarry waste (made ground) is variable, often loose and poorly compacted, so it is weak and compressible and may settle unevenly under a building, causing cracking. Its thickness and properties must be assessed.
- Problem 3, groundwater
- High groundwater can reduce ground strength, complicate excavation and accelerate solution of the limestone.
- Site investigation
- Start with a desk study (geological maps, old quarry records, aerial photos) to identify the limestone and the made ground. Then carry out a ground investigation: drill boreholes and dig trial pits to log the made ground and bedrock, detect cavities, and find the depth to rockhead and the water table; take samples for strength testing; and build a ground model of the subsurface to design suitable (for example deeper or piled) foundations.
Top-band answers identify the specific hazards (solution cavities, weak made ground, groundwater) and describe a logical investigation (desk study then boreholes and trial pits) that assesses each and informs foundation design.
OCR H414/01 20194 marksExplain why a site investigation is carried out before construction, and describe two methods used to investigate the ground.Show worked answer →
Give the purpose, then two methods.
- Purpose
- A site investigation determines the ground conditions (the types, properties and arrangement of the rocks, soils and groundwater) so that engineers can identify hazards (weak ground, cavities, high water table) and design safe, economical foundations. Without it, unexpected ground conditions could cause settlement, cracking or failure.
- Method 1, desk study
- Reviewing existing information (geological maps, previous site reports, old mine or quarry records, aerial photographs) to build an initial picture of the likely geology and hazards before going on site.
- Method 2, boreholes (and trial pits)
- Drilling boreholes (or digging trial pits) to recover and log the rock and soil at depth, find the depth to bedrock and the water table, and take samples for laboratory strength tests.
Markers reward the purpose (determine ground conditions to identify hazards and design foundations) and two valid methods (desk study, boreholes, trial pits, core logging, geophysical survey).
Related dot points
- Mass movement: the types of slope failure (rockfall, translational and rotational slides, slumps and debris flows); the balance of driving and resisting forces on a slope; the factors that trigger failure (slope angle, rock and soil type, water content, discontinuities, weathering, earthquakes and human activity); the recognition of warning signs; the engineering methods used to stabilise slopes and reduce risk.
A focused answer to the OCR H414 dot point on mass movement. Covers the types of slope failure (rockfall, translational and rotational slides, slumps, debris flows), the balance of driving and resisting forces, the triggers of failure (slope angle, rock type, water, discontinuities, earthquakes, human activity), warning signs, and the engineering methods used to stabilise slopes.
- Structural measurement: the definition and measurement of true dip, apparent dip and strike; the recording of orientation data; the calculation of the true (vertical and stratigraphic) thickness of a bed from its outcrop width, dip and the slope of the ground; the use of trigonometry in structural calculations.
A focused answer to the OCR H414 dot point on structural measurement. Covers true dip, apparent dip and strike, recording orientation data, and the calculation of the true thickness of a bed from its outcrop width and dip using trigonometry, with the common traps.
- Fieldwork and maps: the recording of field observations (field sketches, measurements and logged sections); the interpretation of geological maps (outcrop patterns, the rule of Vs and the relationship between topography and dip); the construction of a geological cross-section from a map; the recognition of structures (folds, faults and unconformities) on maps and cross-sections.
A focused answer to the OCR H414 dot point on geological maps and fieldwork. Covers recording field observations (sketches, measurements, logs), interpreting outcrop patterns and the rule of Vs, the relationship between topography and dip, constructing a cross-section from a map, and recognising folds, faults and unconformities on maps.
- Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.
A focused answer to the OCR H414 dot point on groundwater. Covers porosity and permeability and how they vary between rock types, aquifers, aquitards and the water table, confined and unconfined aquifers, calculating porosity, using a simple form of Darcy's law for groundwater flow, and the issues of over-abstraction and contamination.
- Earthquake hazards: the primary and secondary hazards (ground shaking, surface rupture, liquefaction, landslides and tsunamis); the distinction between hazard, vulnerability, exposure and risk; the factors that determine the impact of an earthquake (magnitude, depth, ground conditions, population density, building design and preparedness); monitoring and mitigation (building codes, land-use planning, early-warning systems and education); the limits of earthquake prediction.
A focused answer to the OCR H414 dot point on earthquake hazards. Covers primary and secondary hazards (shaking, surface rupture, liquefaction, landslides, tsunamis), the distinction between hazard, vulnerability, exposure and risk, the factors controlling impact, monitoring and mitigation, and the limits of earthquake prediction.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)