Site Investigation, Preparation and Temporary Works

Introduction and Context

Construction begins long before concrete is placed. The success, safety, cost and programme of a project depend heavily on understanding the site, preparing it correctly and providing temporary facilities that remain stable throughout construction. Site investigation establishes the ground and environmental conditions. Surveying and setting out transfer the design from drawings to the land. Site preparation creates a safe and workable platform. Excavation, earthwork and dewatering modify the ground while attempting to control settlement, instability and water inflow. Temporary works—such as shoring, scaffolding, access roads, ramps, hoarding and formwork—support construction activities until the permanent works can perform their intended function.

Errors at this stage are particularly dangerous because they may remain hidden until excavation, foundation construction or heavy rainfall exposes them. A competent construction technician must therefore interpret basic soil information, coordinate surveys, identify underground services, plan safe access and recognise when specialist geotechnical or temporary-works design is required.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Explain the purpose and stages of a site investigation.
2. Interpret basic soil, groundwater and site survey information.
3. Plan surveying and setting-out operations using appropriate control points.
4. Describe site clearance, grading, access and earthwork procedures.
5. Explain excavation stability, support systems and safe working practices.
6. Select suitable basic dewatering methods and identify their limitations.
7. Recognise utility, environmental, settlement and temporary-works risks.
8. Apply these principles to a practical construction scenario.

1. Site Information and Site Investigation

1.1 Sources of site information

Before fieldwork begins, the project team should assemble available information, including:

• topographic and cadastral surveys;
• geological and published soil maps;
• previous site investigation reports;
• planning, drainage and environmental records;
• records of existing buildings, roads and retaining structures;
• utility-owner drawings and service records;
• aerial photographs and satellite imagery;
• flood, contamination and mining records; and
• information from adjacent landowners and local authorities.

Existing records are useful but must not be treated as conclusive. Utility drawings may be incomplete, geological maps are regional rather than site-specific, and old structures may have undocumented foundations or basements. Desk information must therefore be verified by field investigation.

1.2 Purpose and scope

A site investigation provides evidence for decisions about foundations, earthworks, slopes, retaining structures, pavement construction, drainage, temporary works and environmental protection. It should answer questions such as:

• What materials occur at and below the site?
• Are there weak, compressible, expansive or collapsible layers?
• What is the groundwater level and how does it vary?
• Can excavations stand temporarily, or will support be required?
• Are contamination, buried obstructions, cavities or unexploded hazards present?
• What effect will construction have on neighbouring structures?

The extent of investigation should reflect project size, ground variability, risk and the consequences of failure. A small low-risk structure may require limited trial pits and in-situ testing, while a deep basement, bridge or high-rise building requires a carefully designed ground investigation by competent specialists.

1.3 Stages of investigation

A practical investigation commonly includes four stages:

Desk study. Existing information is reviewed and a preliminary ground model is developed.

Reconnaissance. The site is inspected for slopes, seepage, vegetation, cracks, filled areas, settlement, access constraints, overhead lines, drainage paths and signs of contamination.

Field investigation. Trial pits, boreholes, probing and in-situ tests are used to examine subsurface conditions. Boreholes are appropriate for deeper or inaccessible strata; trial pits provide direct visual inspection at shallow depth but are limited by depth, groundwater and safety requirements.

Laboratory testing and reporting. Samples are classified and tested. The report should describe methods, logs, groundwater observations, test results, limitations and design recommendations.

Investigation is often iterative. If excavation reveals conditions different from the model, work should pause where necessary and the risk assessment and design should be reviewed.

1.4 Basic soil description

Soils are commonly described by particle size, grading, plasticity, density or consistency, colour, moisture condition and origin. Broad groups include gravel, sand, silt, clay, organic soil and made ground.

• Gravel and sand are coarse-grained soils. They usually drain relatively quickly, although loose saturated sand may be vulnerable to instability or liquefaction under particular seismic conditions.
• Silt has small particles and may lose strength rapidly when wet or disturbed.
• Clay has cohesive behaviour and may be plastic, compressible and sensitive to changes in moisture.
• Organic soil and peat are generally highly compressible and unsuitable for supporting substantial loads without treatment or removal.
• Made ground may contain variable fill, rubble, voids and buried obstructions. It should never be assumed uniform merely because it appears level.

A soil profile is represented by a log showing layer boundaries, descriptions, sample depths, groundwater observations and test results. The profile is a model between investigation points, not a continuous certainty.

1.5 Important soil parameters

Several parameters are used in preliminary interpretation.

Water content is defined as:

where w is water content, m_w is the mass of water and m_s is the mass of dry soil solids. The equation applies to a soil sample tested by mass; all masses must use the same units.

Bulk density is:

where ρ is bulk density, m is total wet or moist mass and V is total volume. Dry density uses dry soil mass instead. Density is expressed commonly in Mg/m³ or kg/m³.

Degree of saturation is:

where S_r is degree of saturation, V_w is water volume and V_v is void volume. It ranges from 0% for completely dry soil to 100% for fully saturated soil. In practice, reliable values require representative sampling and accurate volume or index data.

For preliminary bearing and slope assessments, effective stress is important:

where σ′ is effective normal stress, σ is total normal stress and u is pore-water pressure. The relationship is used for saturated or partially saturated soil analyses under the assumptions of conventional soil mechanics; it does not by itself provide a complete strength or settlement calculation.

Shear strength is often represented using the Mohr–Coulomb model:

where τ_f is shear strength at failure, c′ is effective cohesion, φ′ is effective friction angle and σ′ is effective normal stress. This is an idealised model. Values must come from appropriate laboratory or field testing and must be selected for drained or undrained conditions as applicable.

Worked example: effective stress

Suppose the total vertical stress at a point is 180 kPa and the pore-water pressure is 70 kPa. Then:

The effective stress is 110 kPa. If groundwater rises and pore pressure increases while total stress remains constant, effective stress decreases, potentially reducing soil shear resistance.

2. Surveying and Setting Out

2.1 Survey control

A survey establishes horizontal position and elevation. Control points should be stable, protected from construction damage and referenced to an agreed coordinate and level datum. At least two independent checks are desirable so that a single disturbed point does not control the entire project.

A topographic survey records contours, spot levels, boundaries, trees, roads, drainage, structures and visible services. The survey should identify its accuracy, equipment, date, coordinate system and benchmark reference.

2.2 Setting out procedure

Setting out transfers grid lines, building corners, excavation limits, column centres, levels and offsets from the design to the site. A typical sequence is:

1. Review the latest approved drawings and coordinate schedule.
2. Confirm site boundaries, datum and control points.
3. Check instruments and calibration status.
4. Establish a site grid and independent reference points.
5. Mark primary axes and use offset pegs or profiles outside excavation zones.
6. Set out foundation lines, reduced levels and excavation limits.
7. Check dimensions, diagonals, coordinates and levels independently.
8. Record observations and obtain inspection approval before excavation.

Profile boards or batter boards are placed outside the excavation so that lines can be restored after excavation. Pegs must be protected and clearly labelled. Setting out should not rely on dimensions scaled from a drawing; written dimensions and surveyed coordinates take precedence.

2.3 Basic level calculation

Using a levelling instrument, the height of instrument is:

and the reduced level of a point is:

where HI is height of instrument, RL_BM is the reduced level of the benchmark, BS is backsight staff reading, RL_P is the reduced level of point P and FS_P is its foresight reading. All readings must use the same length unit and datum.

Example. If a benchmark has RL = 100.000 m and the backsight is 1.250 m, then HI = 101.250 m. If the foresight at a foundation point is 2.030 m, its level is RL = 99.220 m. A check through a closing benchmark is required to detect observational or booking errors.

3. Site Preparation and Earthwork

3.1 Preparation sequence

Site preparation should be planned as a controlled sequence rather than as general clearance. It normally includes:

• fencing, gates, warning signs and security;
• welfare, first aid, fire protection and emergency arrangements;
• identification and isolation of services;
• ecological and contamination controls;
• removal or protection of vegetation and topsoil;
• demolition and removal of obstructions;
• temporary drainage and sediment control;
• formation of access roads and working platforms;
• stripping, stockpiling and testing of suitable topsoil;
• cut-and-fill operations; and
• compaction and verification of the final formation.

Topsoil should generally be stripped from building platforms because it contains organic matter and is compressible. It may be stored in low, stable stockpiles for later landscaping. Stockpiles should not obstruct drainage, access or emergency routes.

3.2 Cut, fill and compaction

Earthwork balances excavation from high areas with filling in low areas where material quality and moisture conditions permit. Unsuitable material may include peat, contaminated soil, highly organic soil, excessively wet clay or material containing large obstructions.

Fill should be placed in controlled layers, moisture-conditioned where necessary and compacted using equipment suited to the soil. Quality control may include layer thickness records, moisture checks, density tests and proof-rolling. The acceptance criteria must come from the project specification; a visually firm surface is not sufficient evidence of adequate compaction.

A basic volume relationship for mass conservation is:

where V_cut is the in-situ cut volume, C_r is the applicable conversion factor and V_fill,available is the approximate compacted fill volume available. The factor depends on soil type, bulking, shrinkage and compaction; it must be obtained from project or laboratory data rather than assumed universally.

3.3 Access and working platforms

Construction traffic requires routes that can carry expected axle loads in all reasonably foreseeable weather. Access planning considers gradients, turning radii, visibility, pedestrian segregation, edge protection, drainage, dust, mud and emergency access. Temporary roads may use compacted granular material, stabilised soil, concrete or other engineered systems.

Working platforms for cranes, piling rigs and heavy plant require particular attention to bearing capacity, drainage, thickness, reinforcement and edge distances. A platform that supports a light vehicle may not safely support a crane outrigger. Platform design and inspection should be undertaken by competent persons where heavy plant or variable ground is involved.

4. Excavation, Stability and Ground Support

4.1 Excavation hazards

Excavation hazards include collapse, falls into openings, plant overturning, falling materials, flooding, hazardous atmospheres, undermining of adjacent structures and contact with services. Collapse can occur suddenly even in apparently cohesive soil. Vibration, rainfall, surcharge loads and groundwater can accelerate failure.

Spoil heaps, materials and vehicles should be kept away from excavation edges unless the support design explicitly allows the surcharge. Safe access and egress must be provided, and excavations should be inspected after rain, vibration, alteration or other conditions that may affect stability.

4.2 Sloping, benching and shoring

The principal methods of protecting excavation sides are:

• Sloping or battering: the sides are cut back to a safe angle appropriate to the soil and groundwater.
• Benching: the sides are formed in horizontal steps where geometry and soil conditions permit.
• Shoring: structural support such as timber, hydraulic frames, trench boxes, sheet piles or soldier piles resists ground movement.
• Specialist retaining systems: diaphragm walls, contiguous piles or secant piles may be used for deep excavations and restricted sites.

There is no universal safe angle for every soil. The choice depends on soil strength, layering, water, depth, weather, surcharge, nearby foundations and construction duration. Temporary works should be designed, checked, installed and inspected under an appropriate temporary-works management system.

A trench box protects workers inside a supported zone but does not necessarily prevent ground movement outside the box or protect nearby structures. It must be installed and used according to its design and manufacturer’s instructions.

4.3 Influence on adjacent structures

Excavation can remove lateral support from neighbouring foundations, cause ground loss, lower groundwater and induce settlement. Before work begins, the team should record the condition of adjacent buildings, locate their foundations where practicable, establish monitoring points and define trigger levels. Monitoring may include survey prisms, crack gauges, inclinometers, piezometers and vibration monitors.

Trigger-action plans should state who receives readings, what constitutes an alert, what immediate action is required and who can authorise changes. Monitoring without predetermined action is not an adequate control.

5. Groundwater and Dewatering

5.1 Groundwater risks

Groundwater may enter through permeable strata, joints, utility trenches or perched water zones. It can soften formation soils, erode fines, flood excavations and reduce effective stress. Rapid pumping may cause settlement outside the site by lowering groundwater levels, particularly where compressible layers or shallow foundations are present.

Water management begins with diversion and prevention: maintain surface drainage, seal unnecessary inflows, protect slopes from runoff and avoid allowing water to stand on formation levels.

5.2 Common dewatering methods

• Sumps and pumps: suitable for modest inflows where soil will not be carried into the excavation.
• Wellpoints: closely spaced shallow wells connected to a header pipe and vacuum pump; useful for lowering groundwater around relatively shallow excavations.
• Deep wells: larger-diameter wells used for deeper drawdown and higher permeability soils.
• Cut-off systems: sheet piles, slurry walls or grout reduce inflow rather than simply pumping water away.
• Eductor or ejector systems: may be used where low-permeability soils require specialised drawdown.

Pumping should discharge to an approved location after considering sediment, contamination, flow rate and environmental permits. Filter systems may be required to prevent piping of soil particles. Dewatering design must account for neighbouring wells, settlement, recharge, rainfall and pump failure.

5.3 Simple pumping estimate

For a simple steady radial flow condition in a confined aquifer, an idealised relationship may be written as:

where Q is pumping flow rate, k is hydraulic conductivity, b is aquifer thickness, s is drawdown, R is radius of influence and r_w is well radius. This equation assumes homogeneous, isotropic, confined and steady-state conditions with appropriate boundary conditions. It is not a substitute for a project-specific hydrogeological assessment, especially for layered or unconfined ground.

6. Temporary Works and Construction Logistics

Temporary works are structures or measures needed to enable construction but not intended to remain as part of the completed building. Examples include excavation support, access scaffolds, falsework, formwork, temporary bridges, haul roads, cofferdams, working platforms, hoarding and temporary drainage.

A temporary-works process should include:

1. Identify the temporary work and its failure consequences.
2. Define design conditions, loads, sequence and interfaces.
3. Appoint a competent designer and checker where required.
4. Prepare drawings, calculations, method statements and inspection requirements.
5. Control materials, installation and changes.
6. Inspect before loading and after events such as heavy rain or impact.
7. Obtain formal approval before use or removal.

Temporary works must be designed for construction loads, not merely permanent structural loads. Loads may include self-weight, workers, stored materials, wind, plant, impact, water, vibration and accidental actions. The sequence of installation and removal is part of the design.

A temporary works register helps identify status, ownership, design responsibility, inspection dates and release conditions. Unauthorised field alterations—such as cutting a brace to make room for a pipe—can invalidate the design and create immediate danger.

7. Utilities, Environment and Safety Governance

Utility strikes are among the most preventable high-consequence incidents. A reliable process combines records, site detection and controlled excavation. Records should be obtained from each utility owner, but drawings must be treated as approximate. Detection methods may include electromagnetic locating, ground-penetrating radar and trial holes. Services should be marked on the ground, isolated where possible and exposed using safe methods near the expected alignment.

Environmental controls may address dust, noise, vibration, contaminated soil, protected habitats, sediment-laden water, waste classification and public nuisance. Soil movement across a site can spread contamination. Waste should be segregated, documented and transported to authorised facilities.

Safety controls should follow the hierarchy of control: eliminate the hazard where possible, substitute a safer method, apply engineering controls, establish administrative controls and use personal protective equipment as the last line of defence. Risk assessments and method statements must be specific to the actual site and revised when conditions change.

Ethically, personnel must report unsafe ground, missing services, unexpected water or discrepancies between drawings and field conditions. Production pressure never justifies entering an unsupported excavation or bypassing a permit, inspection or isolation procedure.

8. Technology and Contemporary Practice

Total stations, robotic instruments, GNSS receivers, laser scanners and unmanned aerial surveys improve speed and spatial control. Building information modelling can link survey coordinates, utilities, excavation limits and temporary works. Digital models are valuable only when their coordinate system, accuracy, revision status and source data are controlled.

Sensors can provide near-real-time information about settlement, groundwater, vibration and structural movement. Automated alerts support early intervention but do not replace professional interpretation. Artificial intelligence may assist with image classification, survey comparison, anomaly detection and document review; however, it should not independently approve excavation support, interpret uncertain ground conditions or replace statutory competence. All AI-assisted outputs require verification against field evidence and approved design information.

9. Illustrative Case Study: Basement Excavation beside an Existing Building

Background. A contractor is constructing a two-storey basement beside a masonry building founded on shallow footings. The site contains made ground over dense sand, and groundwater is recorded near the proposed basement level.

Problem. The initial proposal uses open sloping excavation. The boundary is too close to the neighbouring building for the required batter, and uncontrolled pumping could lower groundwater beneath the neighbour’s footing.

Analysis. The desk study identifies historical fill and an old drainage line. Boreholes confirm variable made ground, dense sand and groundwater. A condition survey finds existing cracks that must be distinguished from construction-induced movement. The risk assessment identifies ground loss, settlement, utility damage, flooding and public access risks.

Approach. The team adopts a designed perimeter retaining system with internal bracing. A temporary works designer checks wall stiffness, anchors or braces, surcharge loads and construction sequence. Groundwater is controlled using a combination of cut-off measures and carefully monitored pumping. Survey prisms, crack gauges and piezometers are installed. A permit-to-dig system verifies utilities, and spoil is tested before disposal. Excavation proceeds in stages, with inspections after each stage and a trigger-action plan for movement or drawdown.

Outcome. The excavation remains stable, groundwater drawdown outside the site is limited and monitoring shows movement below the agreed alert level. The project experiences a short delay while an unexpected concrete obstruction is removed, but no neighbouring damage occurs.

Learning points. The case demonstrates that investigation must consider existing structures and groundwater, not just foundation soil. A technically feasible permanent design can still require substantial temporary works. Monitoring is most effective when linked to predefined actions, and a controlled change process is essential when field conditions differ from records.

Challenges and Limitations

Site information is always incomplete. Boreholes sample discrete locations, groundwater varies seasonally, laboratory specimens may not represent field fabric, and construction can change drainage paths. Survey accuracy may be reduced by poor visibility, unstable control points or incorrect datum assumptions. Dewatering predictions are uncertain where geological boundaries and hydraulic connections are unknown.

These limitations are managed through appropriate investigation density, conservative assumptions, verification during excavation, competent review, monitoring and a clear response plan. No single test or technology eliminates uncertainty. The objective is not to claim perfect knowledge but to reduce uncertainty to a level compatible with safe and economical construction.

Best-Practice Checklist

Before excavation, the project team should be able to confirm that:

• the latest ground investigation and drawings have been reviewed;
• boundaries, levels, control points and services are verified;
• temporary works have an identified designer, checker and approval status;
• excavation support or battering is suitable for actual soil and water conditions;
• access, egress, edge protection and emergency arrangements are in place;
• groundwater discharge is permitted and monitored;
• adjacent structures have been surveyed and monitoring points installed where needed;
• spoil, contamination, sediment and dust controls are active; and
• inspection, hold-point and change-control procedures are understood by the workforce.

Chapter Summary

Site investigation develops a usable ground model from records, reconnaissance, fieldwork and laboratory testing. Soil descriptions, groundwater conditions, density, water content, effective stress and shear strength influence foundation, excavation and dewatering decisions. Survey control and independent checks are essential for accurate setting out and level transfer.

Site preparation includes service verification, clearance, topsoil management, drainage, access, cut-and-fill and controlled compaction. Excavations must be protected by suitable battering, benching or designed support, with attention to surcharge, groundwater, adjacent foundations and changing weather. Dewatering methods range from sumps to wellpoints, deep wells and cut-off systems; pumping must be designed to avoid flooding, piping and external settlement.

Temporary works require the same professional discipline as permanent works: defined design conditions, competent responsibility, checking, inspection, sequencing and change control. Utility protection, environmental management, monitoring and ethical reporting are central to safe construction. Modern digital tools and AI can improve information management and monitoring, but they supplement rather than replace engineering judgement and field verification.

Key Terms

Site investigation: Systematic examination of ground, groundwater and environmental conditions for construction decisions.

Ground model: An interpreted representation of soil, rock, groundwater and site conditions.

Made ground: Soil or material placed or significantly altered by human activity.

Trial pit: A shallow excavation made to inspect and sample near-surface ground.

Borehole: A drilled hole used to investigate deeper subsurface conditions.

Reduced level: Elevation of a point relative to an agreed vertical datum.

Setting out: Transferring design positions, lines and levels to the construction site.

Topsoil: Organic-rich surface soil generally unsuitable beneath structural foundations.

Compaction: Mechanical densification of soil by reducing air voids.

Formation level: Prepared surface on which a foundation, pavement or structural layer is constructed.

Surcharge: Additional load applied near an excavation or retaining structure.

Effective stress: Stress carried by the soil skeleton, equal to total stress minus pore-water pressure.

Dewatering: Controlled removal or exclusion of groundwater from a construction area.

Piping: Transport of soil particles by flowing groundwater.

Shoring: Temporary structural support provided to excavation sides.

Temporary works: Structures or measures that enable construction but are not normally part of the completed work.

Trigger-action plan: Predetermined monitoring thresholds linked to required responses.

Working platform: Prepared ground designed to support construction plant or equipment.

Permit to dig: Formal control authorising excavation after hazards and services are verified.

Review Questions

5 Short Answer Questions

1. What are the four principal stages of a site investigation?
2. Why should topsoil normally be removed from a building platform?
3. State two hazards associated with open excavations.
4. What is the purpose of a temporary-works register?
5. Why must utility drawings be verified in the field?

5 Long Answer Questions

1. Explain how soil type, groundwater and surcharge influence excavation stability and the selection of support.
2. Describe a complete surveying and setting-out procedure for a small building project.
3. Compare sumps, wellpoints, deep wells and cut-off systems for groundwater control.
4. Explain the responsibilities and approval stages in a temporary-works management process.
5. Discuss how site preparation, earthwork quality control and access planning influence project safety and cost.

3 Analytical / Application Questions

1. An excavation is planned beside an old shallow-founded building in variable made ground. Develop a risk-control plan covering investigation, support, monitoring, groundwater and emergency response.
2. A level book gives a benchmark reduced level of 52.400 m, a backsight of 1.650 m and a foresight of 2.115 m at a proposed foundation point. Calculate the height of instrument and foundation reduced level, then explain how you would check the result.
3. During excavation, groundwater rises rapidly and fine sand begins entering a sump. Analyse the likely mechanisms, identify immediate actions and recommend a revised water-control approach.

Multiple Choice Questions with Answers

Question 1

Which activity is the main purpose of a site investigation?

A. To select the contractor’s office furniture

B. To establish ground and groundwater conditions for design and construction decisions

C. To determine the final paint colours

D. To replace all surveying operations

Correct Answer: B. To establish ground and groundwater conditions for design and construction decisions

Question 2

A benchmark has a reduced level of 100.000 m and a backsight of 1.250 m. What is the height of instrument?

A. 98.750 m

B. 99.250 m

C. 101.250 m

D. 102.500 m

Correct Answer: C. 101.250 m

Question 3

If total stress is 180 kPa and pore-water pressure is 70 kPa, the effective stress is:

A. 70 kPa

B. 110 kPa

C. 180 kPa

D. 250 kPa

Correct Answer: B. 110 kPa

Question 4

Which material is generally most unsuitable for supporting substantial structural loads without treatment?

A. Dense gravel

B. Compacted granular fill

C. Peat

D. Dense sand

Correct Answer: C. Peat

Question 5

What is the primary function of a trench box?

A. To increase groundwater flow

B. To protect workers within a supported excavation zone

C. To measure reduced levels

D. To compact topsoil

Correct Answer: B. To protect workers within a supported excavation zone

Question 6

Which action best reduces the risk of utility strikes?

A. Relying only on old drawings

B. Excavating rapidly with heavy plant

C. Combining records, detection and controlled trial holes

D. Removing all warning signs

Correct Answer: C. Combining records, detection and controlled trial holes

Question 7

Why can uncontrolled dewatering damage neighbouring buildings?

A. It always increases soil saturation

B. It may lower groundwater and cause settlement

C. It eliminates all excavation loads

D. It prevents soil movement completely

Correct Answer: B. It may lower groundwater and cause settlement

Question 8

Which item is a temporary work?

A. Completed reinforced-concrete foundation

B. Permanent external wall

C. Temporary excavation bracing

D. Finished roof covering

Correct Answer: C. Temporary excavation bracing

Question 9

A level reading is 2.030 m and the height of instrument is 101.250 m. The point reduced level is:

A. 99.220 m

B. 100.780 m

C. 103.280 m

D. 101.250 m

Correct Answer: A. 99.220 m

Question 10

What is the most appropriate principle when field conditions differ significantly from the investigation report?

A. Continue without recording the difference

B. Hide the condition to protect the programme

C. Review the risk assessment and design before proceeding where necessary

D. Remove all monitoring equipment

Correct Answer: C. Review the risk assessment and design before proceeding where necessary