Site Investigation, Soils and Ground Conditions

Introduction

A foundation transfers structural loads to the ground. The safety, serviceability and economy of a foundation therefore depend not only on the building but also on the nature and variability of the soil beneath it. Site investigation is the systematic process of collecting, interpreting and communicating information about ground conditions before design and construction.

For a civil draftsman, geotechnical information has direct practical importance. Drawings must show existing levels, geological strata, groundwater, excavation limits, foundation depths, ground-improvement measures and precautions for adjacent structures. A draftsman does not replace the geotechnical engineer, but must understand the meaning and limitations of geotechnical information well enough to prepare accurate, coordinated and safe documentation.

This chapter explains site surveys, soil classification, geotechnical reports, bearing capacity, settlement, groundwater, expansive and frost-susceptible soils, contamination, excavation and safe working requirements.

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 survey plans, borehole logs, trial pits and geotechnical reports.
3. Classify soils using particle size, plasticity and engineering behaviour.
4. Distinguish bearing-capacity failure from excessive settlement.
5. Recognise the effects of groundwater, expansive soils, frost and contamination.
6. Identify excavation hazards and appropriate control measures.
7. Translate geotechnical recommendations into coordinated foundation and flooring drawings.

1. Site Investigation and Survey Information

1.1 Desk study and preliminary assessment

A desk study is undertaken before intrusive work. It may include:

• topographic and boundary surveys;
• geological and soil maps;
• previous site investigation reports;
• records of mines, quarries, wells, landfills and underground services;
• historical maps and aerial photographs;
• flood, drainage and groundwater records;
• planning, environmental and contamination registers; and
• information about neighbouring foundations and retaining structures.

Historical land use is particularly important. A former fuel station, factory, dry-cleaning facility, landfill or agricultural chemical store may leave contaminants or buried obstructions. A site that appears vacant may contain demolition rubble, tanks, old foundations or uncontrolled fill.

1.2 Walkover survey

A walkover survey verifies desk-study information. The investigator records slopes, vegetation, surface water, cracks in nearby buildings, retaining walls, made ground, outcrops, drainage channels, signs of erosion and access restrictions. Trees may indicate desiccated clay in dry seasons, while lush vegetation or damp ground may suggest shallow groundwater or poor drainage.

1.3 Topographic and setting-out surveys

A topographic survey establishes horizontal positions and elevations. Important items include the site boundary, benchmarks, existing buildings, roads, utility covers, trees, walls, watercourses, spot levels and contours. Levels are usually related to an accepted vertical datum. A foundation drawing should not use unexplained or inconsistent levels.

The draftsman should distinguish:

• existing ground level (EGL): the surveyed level before construction;
• formation level: the level to which soil is excavated or prepared;
• finished floor level (FFL): the completed floor surface level;
• underside of foundation (USF): the lowest level of the foundation element; and
• reduced level (RL): an elevation referred to a stated datum.

A site plan must also show the direction of slope. Contours that are close together indicate a steeper slope than contours spaced widely apart. Incorrect interpretation of levels can produce inadequate excavation, ponding, loss of cover or foundations placed at different depths without proper design.

1.4 Intrusive investigation

Common methods are:

No investigation method describes every point on a site. Ground is naturally variable, and made ground can be especially irregular. The investigation must therefore be proportionate to the structure, site history and consequences of failure.

2. Soil Composition and Classification

Soil is a natural or placed material consisting of mineral particles, organic matter, water and air. In a simplified three-phase model, the solid particles form the soil skeleton, while the voids may contain water, air or both.

2.1 Particle-size groups

Soils are commonly described as gravel, sand, silt and clay. Boundaries vary between classification systems, so the adopted standard must be stated in a report. Particle size strongly influences drainage and strength, but particle size alone does not determine behaviour.

• Gravel: coarse particles; generally free-draining and capable of high frictional resistance when dense.
• Sand: granular soil with drainage and strength strongly affected by density and saturation.
• Silt: fine soil that may be compressible, frost-susceptible and vulnerable to erosion or loss of strength when saturated.
• Clay: cohesive fine-grained soil whose volume and strength can change with water content.
• Organic soil and peat: soils containing decomposed vegetation; usually highly compressible and unsuitable for direct support without special treatment.
• Made ground or fill: placed or disturbed material with potentially unpredictable composition and compaction.

2.2 Grading and plasticity

A particle-size distribution is obtained by sieve analysis for coarse particles and sedimentation methods for fine particles. Well-graded granular soil contains a broad range of particle sizes and may compact efficiently. Poorly graded soil has a narrow range of sizes or a gap in the distribution.

Plasticity describes how fine-grained soil changes consistency with water content. The principal Atterberg limits are:

• liquid limit, w_L;
• plastic limit, w_P; and
• plasticity index, I_P = w_L - w_P.

Here, w_L and w_P are water contents, normally expressed as percentages, and I_P is also expressed as percentage points. The relationship is applicable to fine-grained soils for which these limits can be measured. A high plasticity index commonly indicates greater potential for volume change, although mineralogy, structure and stress history must also be considered.

2.3 Water content and density

Water content by mass is:

where m_w is the mass of water and m_s is the mass of dry soil solids. The formula assumes that the masses are measured from a representative sample and that the dry mass is determined reliably.

Bulk unit weight is:

where γ is bulk unit weight in kN/m³, W is total soil weight in kN and V is total volume in m³. For saturated soil, the total unit weight includes both solids and water. Effective stress concepts require separating total stress from pore-water pressure.

2.4 Effective stress

The effective stress principle is central to soil mechanics:

where σ′ is effective normal stress, σ is total normal stress and u is pore-water pressure. All quantities must have the same stress units, commonly kPa. The relationship is most directly applied to saturated soils under conditions where pore pressure is meaningfully defined. Effective stress controls much of the frictional strength, compressibility and deformation of soil.

An increase in groundwater level generally increases pore pressure and reduces effective stress. This can reduce short-term stability in excavations and affect the resistance of saturated granular soils. Lowering groundwater may increase effective stress but can also cause consolidation settlement in compressible layers.

3. Reading a Geotechnical Report

A geotechnical report normally contains the investigation scope, site description, exploratory logs, laboratory results, groundwater observations, interpretation, design parameters, limitations and recommendations. The report date, borehole coordinates and vertical datum should be checked before information is transferred to drawings.

A borehole log may show depth, stratum description, sample type, in-situ test results, groundwater observations and remarks about refusal or obstructions. Descriptions such as “stiff brown slightly sandy clay” are engineering observations, not a guarantee that the entire layer is uniform.

The draftsman should extract and coordinate:

• recommended founding stratum;
• minimum foundation depth;
• allowable or design bearing pressure, including its basis;
• expected settlement or movement limits;
• groundwater level and seasonal uncertainty;
• excavation support and dewatering requirements;
• concrete exposure or chemical-attack classification;
• requirements for removal or improvement of fill;
• floor slab subgrade and sub-base requirements; and
• testing, inspection and hold points.

A report may use allowable bearing pressure, which commonly incorporates safety factors and serviceability considerations, or ultimate bearing capacity, which is a failure-level value. These values must never be interchanged without professional confirmation. A draftsman should reproduce the report terminology exactly and refer ambiguous values to the engineer.

4. Bearing Capacity and Foundation Performance

4.1 Meaning of bearing capacity

Bearing capacity is the ability of soil to support foundation pressure without unacceptable shear failure or deformation. Failure modes include general shear failure, local shear failure and punching failure. The actual mode depends on soil density, foundation geometry, embedment, loading and drainage conditions.

For a preliminary, simplified calculation for a strip footing on level ground, a commonly used total-stress or effective-stress form is:

where q_ult is ultimate bearing capacity, c′ is effective cohesion, q is effective surcharge at foundation level, γ is soil unit weight, B is footing width, and N_c, N_q and N_γ are bearing-capacity factors dependent mainly on friction angle. This expression assumes a simplified homogeneous soil, a strip footing, appropriate drainage conditions, level ground and a defined failure mechanism. Real design may require shape, depth, inclination, eccentricity, water-table and layered-soil corrections. It is not a substitute for the adopted design code or a geotechnical engineer’s assessment.

If a global factor of safety is used in a preliminary allowable-capacity approach:

where q_allow is allowable pressure and F is the specified factor of safety. Modern limit-state design often uses partial factors rather than one global factor, so the method must be stated.

4.2 Bearing pressure under a simple footing

For a concentric axial load on a rectangular footing:

where P is service load in kN, A is loaded area in m² and q is average pressure in kPa, since 1 kN/m² = 1 kPa. The equation assumes uniform contact pressure and does not include moment, eccentricity or redistribution.

For a footing of length 2.0 m and width 1.5 m carrying 300 kN:

A = 2.0 × 1.5 = 3.0 m²

q = 300 / 3.0 = 100 kPa.

Whether 100 kPa is acceptable depends on the report’s design pressure, settlement criteria and load combinations. The calculation alone does not establish adequacy.

4.3 Settlement

A foundation may be safe against shear failure but still perform poorly because of settlement. Settlement includes immediate distortion, primary consolidation and, in some soils, secondary compression. Differential settlement is usually more damaging than uniform settlement because it causes cracking, racking of frames, uneven floors and service-pipe distress.

Settlement is influenced by load magnitude, footing size and stiffness, soil compressibility, drainage, groundwater, construction sequence, adjacent excavations and variability of strata. For this reason, a report’s predicted settlement and differential-movement limits are as important as bearing pressure.

5. Groundwater, Drainage and Flooring

Groundwater may be perched above a low-permeability layer, seasonal, artesian or connected to a nearby watercourse. A single observation in a borehole is not necessarily the maximum groundwater level. Construction drawings should identify the need for waterproofing, drainage, sump pumping or a designed tanking system where appropriate.

Water affects foundations by:

• reducing effective stress when pore pressure rises;
• causing piping, erosion or instability in excavations;
• increasing uplift on basements and slabs;
• softening fine-grained subgrades;
• transporting contaminants; and
• promoting dampness, mould and deterioration in floors.

A ground-bearing floor generally requires a competent, uniform subgrade, controlled fill where necessary, suitable sub-base, damp-proof membrane and insulation or vapour control as required by the design. The membrane must be continuous and properly connected to wall damp-proofing. A suspended floor may be preferable where ground is weak, variable, contaminated or persistently wet.

6. Expansive, Collapsible and Frost-Susceptible Soils

6.1 Expansive soils

Expansive clay swells when wetted and shrinks when dried. Seasonal moisture changes may be caused by drought, leaking drains, vegetation removal or tree growth. Distress may include stepped masonry cracks, distorted openings and uneven floors.

Risk reduction may involve deeper foundations, reinforced ground beams, suspended floors, moisture management, root barriers, removal and replacement of unsuitable soil, or engineered ground improvement. The correct solution depends on clay mineralogy, volume-change data, trees, climate, structure and local practice. Standard shallow footings should not be selected solely from visual soil description.

6.2 Collapsible soils

Some loose, dry or metastable soils appear strong when unsaturated but collapse upon wetting or loading. Water leakage, rising groundwater and poor drainage can trigger sudden settlement. Site investigation should identify collapsible structure and require controlled filling, pre-wetting, densification or a foundation system that bypasses the vulnerable layer where appropriate.

6.3 Frost action

Frost-susceptible soils, water and freezing temperatures can produce frost heave. Fine silts are often vulnerable because they allow capillary water movement while retaining sufficient permeability. Thawing can produce softening and loss of support.

Protection may require founding below the local frost depth, using non-frost-susceptible granular material, providing drainage and insulation, and preventing water accumulation. Local building regulations and climatic data govern the required depth; a universal frost depth should not be assumed.

7. Contaminated Land and Buried Hazards

Contamination may be chemical, biological, radiological or physical. Common contaminants include hydrocarbons, solvents, heavy metals, asbestos-containing materials, methane and aggressive chemicals. The risk depends on the contaminant, concentration, pathway and receptor. A contaminant can affect workers, occupants, groundwater, construction materials and neighbouring land.

A proper response includes historical assessment, sampling by competent specialists, laboratory analysis, risk assessment, remediation or containment, waste classification and verification testing. Excavated soil must not be moved around the site casually; it may become regulated waste or spread contamination.

Drawings should show exclusion zones, capping layers, membranes, gas protection, clean cover, temporary works and monitoring requirements where specified. Suspected asbestos, buried tanks, unexploded ordnance or unusual odours require work to stop and the relevant competent authority or specialist to be consulted.

8. Excavation and Safe Working Requirements

Excavations can fail through collapse, surcharge, vibration, water ingress, undermining of nearby structures or falling materials. A safe system of work should be based on competent assessment and applicable occupational-safety law.

Essential controls include:

• locating and isolating underground services;
• providing safe access and egress;
• battering, benching or shoring excavation sides;
• keeping spoil, plant and materials away from edges;
• controlling vehicles and surcharge loads;
• inspecting excavations after rain, vibration or other changes;
• managing groundwater and surface runoff;
• preventing falls with barriers and edge protection;
• testing atmospheres where hazardous gases may occur; and
• preventing unauthorised entry.

Never assume that a shallow excavation is automatically safe. Soil type, weather, depth, neighbouring loads and water conditions must all be considered. Temporary works drawings and calculations should be prepared by competent persons where required.

9. Translating Investigation Results into Drawings

A coordinated foundation package commonly includes a location plan, setting-out plan, foundation layout, sections, schedules, floor build-up, drainage details, excavation notes and construction specifications. The draftsman should:

1. Confirm the survey datum and coordinate system.
2. Overlay borehole and trial-pit positions on the site plan.
3. Mark the assumed founding stratum and foundation levels.
4. Show changes in foundation depth and stepped footings clearly.
5. Detail interfaces between foundations, slabs, retaining walls and services.
6. Identify required subgrade preparation and testing.
7. Include groundwater, contamination and excavation notes from the report.
8. Coordinate structural, architectural, drainage and utility information.
9. Flag discrepancies rather than silently resolving them.
10. Record revisions and the geotechnical basis of changes.

Computer-aided drafting and building information modelling can link ground models, borehole data and foundation elements. However, a digital model is only as reliable as its survey, assumptions and data coverage. AI-assisted interpretation may help organise logs or detect inconsistencies, but professional review is essential; AI must not be allowed to invent strata or approve a foundation.

Applied Case: Illustrative Redevelopment of a Former Workshop

Background

A two-storey masonry building is proposed on a gently sloping site formerly occupied by a vehicle-repair workshop. The topographic survey shows a 1.2 m level difference across the footprint. A desk study identifies possible underground fuel storage.

Problem

Trial pits reveal 0.8 m of variable gravelly fill over soft clay. Boreholes show a deeper stiff clay layer, but groundwater is encountered within the fill after rainfall. Laboratory results indicate hydrocarbon contamination in one area.

Analysis

The fill is not accepted as a uniform founding stratum. The soft clay could produce differential settlement, while the water level may destabilise open excavations. The contaminated zone cannot be treated as ordinary spoil. A ground-bearing floor over uncontrolled fill would be inappropriate without a designed treatment scheme.

Approach and solution

The geotechnical team recommends removing unsuitable fill within the building footprint, placing tested compacted engineered fill, and founding reinforced strip footings on the stiff clay where practical. In the contaminated area, soil is segregated and managed under an approved remediation plan. The floor is designed with a verified sub-base, continuous damp protection and gas protection where required. Because of the slope, stepped foundations are detailed with clear vertical transitions and drainage is directed away from the excavation.

The drawings show borehole references, formation levels, excavation support notes, engineered-fill testing requirements and a hold point for inspection of the founding stratum.

Outcome and learning points

The project avoids relying on variable fill, recognises groundwater and contamination risks, and converts geotechnical recommendations into buildable details. The main lesson is that foundation selection follows a chain of evidence: survey, investigation, interpretation, design and verification. Omitting any link can create unsafe assumptions.

Challenges, Limitations and Professional Responsibilities

Ground investigation is subject to sampling disturbance, limited exploration points, seasonal groundwater variation, scale effects and uncertainty between investigation locations. Report recommendations may rely on assumptions about loads, construction methods or finished levels. Changes to the building footprint, loading, drainage or excavation depth can invalidate those assumptions.

Professionals must communicate uncertainty honestly. Drawings should not imply greater precision than the evidence supports. Safety-critical information must be checked against the current report and applicable codes, planning conditions, environmental requirements and occupational-safety regulations. Where information conflicts, the discrepancy must be referred to the responsible engineer or project manager before construction information is issued.

Contemporary Practice and Best-Practice Checklist

Modern practice increasingly uses digital terrain models, geographic information systems, CPT data, remote sensing, environmental sampling and BIM coordination. Sustainable practice favours reuse of suitable excavated material, reduced unnecessary excavation, protection of groundwater and careful remediation. These objectives must never override verified safety or contamination controls.

Before issuing foundation drawings, confirm that:

• the latest survey and geotechnical report have been used;
• all levels have a stated datum;
• founding strata and depths are clear;
• bearing and settlement criteria are identified;
• groundwater and drainage measures are coordinated;
• expansive, frost, collapsible or contaminated ground is addressed;
• excavation support and safe-access requirements are stated;
• service locations and adjacent structures are considered; and
• unresolved assumptions are formally recorded.

Chapter Summary

Site investigation provides the evidence on which foundation and flooring decisions are based. Desk studies, walkover surveys, topographic surveys, trial pits, boreholes, in-situ tests and laboratory testing each provide different information and have limitations. Soil behaviour depends on grading, plasticity, density, water content, effective stress and geological history.

Bearing capacity concerns resistance to shear failure, while settlement concerns deformation and serviceability; both must be considered. Groundwater changes pore pressure, effective stress, excavation stability and floor performance. Expansive, collapsible and frost-susceptible soils require specific assessment rather than generic foundation details. Contaminated ground demands controlled investigation, risk management and lawful handling of materials. Excavations require competent temporary works and safe systems of work.

For the civil draftsman, the central professional task is to convert reliable survey and geotechnical information into accurate, coordinated and transparent drawings. Digital tools improve coordination, but engineering judgement and verification remain essential.

Key Terms

Site investigation: Systematic study of ground conditions for design and construction.

Desk study: Review of existing records and historical information before fieldwork.

Trial pit: Shallow excavation used to inspect and sample near-surface ground.

Borehole: Drilled investigation hole used to examine deeper strata and groundwater.

Made ground: Soil or rubble placed or disturbed by human activity.

Plasticity index: Difference between liquid and plastic limits, I_P = w_L - w_P.

Water content: Mass of water divided by dry mass of soil solids.

Effective stress: Stress carried by the soil skeleton, σ′ = σ − u.

Bearing capacity: Ability of soil to support foundation pressure.

Allowable bearing pressure: Permitted pressure incorporating stated safety and serviceability criteria.

Settlement: Downward movement or deformation of the ground-foundation system.

Differential settlement: Unequal settlement between parts of a structure.

Groundwater: Water occupying pores or discontinuities below the ground surface.

Expansive soil: Soil that changes volume significantly with moisture variation.

Collapsible soil: Soil liable to sudden volume reduction when wetted or loaded.

Frost heave: Upward movement caused by freezing and growth of ice within soil.

Contaminated land: Land containing substances presenting an unacceptable risk.

Formation level: Prepared level at the base of an excavation or construction layer.

Engineered fill: Selected and compacted material placed to specified requirements.

Temporary works: Temporary structures or measures supporting construction safety and stability.

Review Questions

Short Answer Questions

1. What is the purpose of a desk study in site investigation?
2. Distinguish existing ground level, formation level and finished floor level.
3. Define effective stress and state its basic relationship to total stress and pore pressure.
4. Why can made ground be unsuitable for direct foundation support?
5. State four hazards associated with construction excavations.

Long Answer Questions

1. Explain the stages of a proportionate site investigation and the information obtained at each stage.
2. Compare gravel, sand, silt, clay, peat and made ground in terms of engineering behaviour.
3. Explain the difference between bearing-capacity failure and settlement, including why both affect foundation design.
4. Discuss how groundwater can affect foundations, floors, excavations and contamination management.
5. Describe how a civil draftsman should interpret and coordinate a geotechnical report in a foundation drawing package.

Analytical / Application Questions

1. A rectangular footing is 2.4 m long and 1.6 m wide and carries a service load of 384 kN. Calculate its average contact pressure, state the units, and explain what additional information is needed before judging adequacy.
2. A proposed building is on 0.6 m of variable fill over expansive clay, with trees near one boundary and seasonal groundwater variation. Recommend an investigation and design strategy, explaining the reasons for each part.
3. During excavation for a basement, workers observe water inflow, cracking at the excavation edge and an underground service not shown on the drawings. Explain the immediate actions and the information that must be obtained before work resumes.

Multiple Choice Questions with Answers

Question 1

Which investigation method provides direct visual inspection of shallow soil and existing foundations?

A. Trial pit

B. Remote sensing only

C. Laboratory sieve analysis

D. Settlement monitoring

Correct Answer: A. Trial pit

Question 2

The plasticity index is calculated as:

A. wL + wP

B. wL − wP

C. wP − wL

D. wL × wP

Correct Answer: B. wL − wP

Question 3

If a footing carries 300 kN over an area of 3.0 m², its average contact pressure is:

A. 10 kPa

B. 30 kPa

C. 100 kPa

D. 900 kPa

Correct Answer: C. 100 kPa

Question 4

In the relationship σ′ = σ − u, u represents:

A. Unit weight

B. Pore-water pressure

C. Ultimate load

D. Footing width

Correct Answer: B. Pore-water pressure

Question 5

Which soil condition is most directly associated with shrinkage during drying and swelling during wetting?

A. Clean gravel

B. Expansive clay

C. Dense sand

D. Crushed rock

Correct Answer: B. Expansive clay

Question 6

Why is differential settlement particularly damaging?

A. It always increases soil density

B. It can distort and crack the structure

C. It eliminates groundwater

D. It prevents all foundation movement

Correct Answer: B. It can distort and crack the structure

Question 7

Which document is most important for identifying historical fuel storage before intrusive work?

A. Historical land-use and desk-study records

B. The concrete reinforcement schedule alone

C. The interior finish schedule

D. The door hardware schedule

Correct Answer: A. Historical land-use and desk-study records

Question 8

What is an appropriate control for an unsupported excavation?

A. Place spoil at the edge

B. Allow unrestricted vehicle access

C. Use suitable battering, benching or shoring

D. Ignore rainfall if the excavation is shallow

Correct Answer: C. Use suitable battering, benching or shoring

Question 9

A single groundwater observation in a borehole should be interpreted as:

A. The guaranteed maximum groundwater level

B. The guaranteed minimum groundwater level

C. A measurement requiring consideration of season and site conditions

D. Proof that no dewatering is needed

Correct Answer: C. A measurement requiring consideration of season and site conditions

Question 10

Which statement best describes the role of a civil draftsman in using a geotechnical report?

A. Invent missing soil data to complete drawings

B. Translate verified recommendations into coordinated drawings and flag uncertainties

C. Replace all geotechnical design calculations

D. Ignore groundwater unless it appears on the survey

Correct Answer: B. Translate verified recommendations into coordinated drawings and flag uncertainties