Ground Investigation and Geotechnical Characterization

Introduction and Context

Ground investigation is the systematic process of obtaining, interpreting and communicating information about the ground conditions that influence the planning, design, construction and performance of civil engineering works. It includes desk study, site reconnaissance, intrusive investigation, geophysical surveying, sampling, in situ testing, laboratory testing and geological–geotechnical interpretation.

Unlike many structural materials, soil and rock are natural materials formed by geological processes. Their properties vary spatially, may change with groundwater conditions and are often observed only at discrete locations. Consequently, a ground investigation cannot “prove” the ground condition everywhere. Its purpose is to develop a defensible ground model, reduce uncertainty to an acceptable level and provide parameters appropriate for engineering decisions.

An inadequate investigation may lead to unexpected weak strata, excessive settlement, instability, groundwater inflow, construction delay, environmental damage and disputes. An investigation that is excessive or poorly targeted may waste resources without materially improving design reliability. Professional investigation is therefore an exercise in risk management as well as data collection.

Learning Objectives

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

1. Plan a staged desk study, reconnaissance survey and intrusive investigation.

2. Select borehole locations, depths, sampling methods and investigation spacing.

3. Explain the principles, uses and limitations of geophysical methods.

4. Distinguish disturbed, undisturbed and representative samples.

5. Specify and interpret SPT, CPT, pressuremeter and vane shear tests.

6. Select laboratory tests for classification, strength, compressibility, permeability and chemical characterization.

7. Construct a geological and geotechnical ground model.

8. Quantify spatial variability and distinguish variability from epistemic uncertainty.

9. Communicate investigation findings, limitations and residual risks in a professional report.

1. The Ground Investigation Process

A robust investigation is normally developed in stages. The stages are iterative rather than strictly linear.

1.1 Investigation objectives

The investigation should begin with engineering questions, not with a predetermined list of tests. Typical questions include:

• What materials and geological structures are present?
• Where is competent founding material located?
• Is the ground susceptible to bearing failure, excessive settlement, collapse, liquefaction or heave?
• What groundwater levels, pressures and seasonal changes may occur?
• Can excavations and slopes remain stable?
• What materials can be reused as fill?
• Are contamination, aggressive groundwater or buried obstructions present?
• What ground risks could affect construction methods, programme and cost?

The investigation scope should reflect the structure, consequence of failure, construction method, geological complexity and uncertainty. A high-risk retaining wall, deep excavation or major bridge foundation normally requires more extensive characterization than a lightly loaded low-rise structure.

1.2 Desk study

A desk study assembles existing information before fieldwork. Sources may include geological and hydrogeological maps, topographic maps, aerial photographs, remote-sensing data, previous site investigation reports, well records, mining records, historical maps, flood records, utility drawings, environmental databases and records of nearby foundations or excavations.

Historical land use is particularly important. Former quarries, landfills, industrial sites, fuel stations, railway yards and bomb-damaged areas may contain variable fill, buried structures or contamination. Historical photographs can reveal former channels, ponds, embankments and changes in drainage.

The desk study should produce preliminary geological sections, a list of anticipated hazards and a statement of uncertainties to be tested in the field. It should not be treated as a substitute for site investigation: maps may generalize boundaries, and published information may not represent the exact site.

1.3 Site reconnaissance

A walkover survey verifies desk-study information and records observable conditions. Important observations include terrain, outcrops, scarps, springs, seepage, drainage, vegetation changes, cracking, settlement, fill placement, erosion, adjacent structures and access constraints. The survey should also identify overhead and underground services, restricted areas, environmental sensitivities and safe locations for drilling equipment.

Photographs and georeferenced field notes are valuable. Observations should distinguish fact from interpretation. For example, “water emerging at the base of the slope” is an observation; “a perched water table exists” is an interpretation requiring confirmation.

1.4 Investigation design

Investigation locations should be selected to test the ground model and the engineering hypotheses. Boreholes and soundings should cover the building footprint, foundation zones, slopes, retaining structures, borrow areas and proposed temporary works. Locations should target geological boundaries, suspected filled ground, low points, anomalous geophysical results and areas of high consequence.

Investigation depth should extend through all strata that could influence the proposed works. For foundations, this includes strata within the zone of significant stress increase and any deeper compressible, weak or dissolvable layer that could control performance. For excavations, investigation should extend below excavation formation and sufficiently behind and below retaining systems to characterize groundwater and stability mechanisms.

A uniform grid is not always efficient. Adaptive investigation uses early results to refine later locations and depths. The investigation should include contingency resources for unexpected conditions.

2. Developing the Ground Model

2.1 Geological and geotechnical models

A geological model describes the origin, sequence, structure and geometry of natural and anthropogenic materials. A geotechnical ground model translates this description into engineering units with characteristic behaviour, design parameters, groundwater conditions and potential failure mechanisms.

A useful model contains:

• geological strata and formation boundaries;
• made ground and buried structures;
• weathering profiles and discontinuities in rock;
• groundwater levels and pressure regimes;
• material descriptions and index properties;
• strength, stiffness, compressibility and permeability ranges;
• spatial trends and anomalous zones;
• geological hazards and construction implications; and
• confidence levels and unresolved uncertainties.

The model should be three-dimensional where the project warrants it. Cross-sections should show interpreted boundaries, borehole data and the degree of confidence. Interpolation between boreholes must be identified as interpretation rather than direct observation.

2.2 Geotechnical units

Materials are grouped into geotechnical units when they have sufficiently similar origin, composition and engineering behaviour. A geological formation may contain several geotechnical units, while material from different geological origins may behave similarly. For example, heterogeneous fill should not be treated as one uniform unit merely because it occupies one mapped layer.

Descriptions should use standardized soil and rock terminology, including grading, plasticity, consistency, density, colour, structure, weathering, discontinuities and strength. Terms such as “hard” or “good material” are inadequate unless linked to observable criteria or measured parameters.

2.3 Groundwater model

Groundwater investigation should consider the elevation of water, pore-water pressure, hydraulic gradients, permeability contrasts, recharge, seasonal variation, tidal effects and construction-induced changes. A single water strike during drilling is not necessarily an equilibrium groundwater level. Standpipes, vibrating-wire piezometers or other monitoring instruments may be required.

Hydraulic conductivity can vary by orders of magnitude and is often directional. In layered deposits, horizontal flow may be much greater than vertical flow. Perched water can occur above low-permeability lenses, while confined aquifers may produce artesian pressures. These distinctions influence excavation support, basal heave, piping, uplift and dewatering.

3. Boreholes, Trial Pits and Sampling

3.1 Borehole methods

Common methods include cable percussion, rotary open-hole drilling, rotary coring, sonic drilling and hollow-stem augering. Method selection depends on soil and rock type, groundwater, access, required sample quality, depth and environmental constraints.

Cable percussion drilling can be effective in many soils and permits sampling and Standard Penetration Testing, but borehole walls may collapse and samples may be highly disturbed. Rotary drilling is adaptable and can penetrate rock; however, fluid circulation may wash out fines or alter groundwater observations. Continuous coring provides information about rock recovery, discontinuities and weathering, but core quality depends on equipment, drilling practice and operator skill.

Trial pits provide direct visual examination and relatively large bulk samples at shallow depth. They are useful for fill characterization, pavement investigations and shallow foundations, but are limited by groundwater, unstable sides, access and safe excavation depth.

3.2 Sampling quality

Sampling quality should be matched to the decision. Disturbed samples are suitable for visual classification, water content, particle-size distribution and some compaction tests. They are not generally suitable for determining in situ fabric-dependent strength or compressibility.

Undisturbed samples aim to preserve the soil structure, water content and stress history sufficiently for advanced laboratory testing. Perfectly undisturbed sampling is rarely achieved. Thin-walled piston or fixed-piston samplers are generally preferable in soft cohesive soils; thick-walled samplers may be appropriate where greater robustness is needed. Sample disturbance increases with excessive clearance, poor cutting geometry, vibration, rapid extraction, improper sealing and inadequate transport.

Block samples can provide high-quality material in suitable cohesive soils, but they are labour-intensive and difficult below the water table. In granular soils, obtaining truly undisturbed samples is difficult; in situ testing and laboratory reconstitution may therefore be more reliable for some purposes.

Samples should be labelled with location, depth, orientation where relevant, date, method and recovery. They must be sealed against moisture loss, protected from temperature extremes and transported with minimal disturbance. Chain-of-custody procedures are essential where contamination or legal evidence is involved.

4. Geophysical Investigation

Geophysical methods provide indirect, spatially continuous information between intrusive points. They are particularly valuable for identifying boundaries, cavities, buried objects, rockhead and variations in stiffness or density.

4.1 Seismic methods

Seismic refraction and reflection use travel times of elastic waves. P-wave and S-wave velocities can indicate changes in material stiffness, degree of weathering and rippability. Surface-wave methods, including MASW, estimate shear-wave velocity profiles from dispersive surface waves. Seismic methods are useful for mapping rockhead and dynamic properties, but interpretation can be affected by velocity inversions, lateral heterogeneity, cultural noise and non-unique solutions.

Small-strain shear modulus may be estimated from shear-wave velocity using

G_max = ρ V_s²,

where G_max is the maximum or small-strain shear modulus, ρ is mass density and V_s is shear-wave velocity. The relationship assumes an elastic wave propagating through a continuum and is most appropriate for very small strains. It should not be confused with the much lower secant or tangent stiffness used in foundation settlement or deformation analyses at engineering strain levels.

4.2 Electrical and electromagnetic methods

Electrical resistivity methods measure apparent resistivity, which depends on pore-fluid conductivity, saturation, mineralogy, porosity and temperature. They can help identify groundwater, saline intrusion, voids and material boundaries. Electromagnetic and ground-penetrating radar methods are useful for locating services, buried objects and shallow interfaces, although clay, conductive fill and wet ground can strongly attenuate signals.

4.3 Integration with intrusive data

Geophysical data should calibrate against boreholes, trial pits or soundings. A resistivity anomaly is not automatically a void, and a seismic boundary is not automatically a geological contact. The strongest practice combines methods with different physical sensitivities and expresses interpretations probabilistically or with confidence ratings.

5. In Situ Testing

5.1 Standard Penetration Test

The Standard Penetration Test (SPT) drives a split-spoon sampler into the ground using a specified hammer and drop arrangement. The reported penetration resistance is conventionally based on the number of blows required for the final 300 mm of penetration after an initial seating drive, although equipment and reporting practices must be stated.

The raw blow count is affected by hammer energy, rod length, borehole diameter, sampler configuration, drilling method, overburden pressure and operator practice. For comparison, the measured value is often normalized to an energy ratio, commonly expressed as N_60. A general form is

N_60 = N_m (E_R/60) C_B C_S C_R,

where N_m is the measured penetration resistance, E_R is delivered hammer energy as a percentage of the theoretical free-fall energy, and C_B, C_S and C_R are corrections for borehole diameter, sampler and rod length. Exact correction conventions must follow the adopted standard or correlation source.

SPT correlations with relative density, friction angle, liquefaction resistance and bearing capacity are empirical and soil-dependent. They should not be applied blindly to gravel, cemented soils, very soft clay, highly unusual fills or tests with poor energy control. Refusal must be recorded with penetration and blow count; it is not automatically proof of sound rock.

5.2 Cone Penetration Test

The Cone Penetration Test (CPT) pushes an instrumented cone at a constant rate, typically measuring cone tip resistance q_c, sleeve friction f_s and, in piezocone testing, pore-water pressure u. A friction ratio is commonly defined as

R_f = 100 f_s/q_c,

where R_f is in percent when q_c and f_s use consistent stress units. CPT data provide a nearly continuous profile and are highly effective for detecting thin layers, soil transitions and variability.

CPT interpretation may use normalized parameters such as net cone resistance and corrected cone resistance. Corrections are important in soft soils and where unequal end areas cause pore-pressure effects. CPT does not usually recover a physical sample, so classification should be checked with boreholes or sampling. It may be difficult in coarse gravel, cemented layers, very dense soils or obstructions. Seismic CPT adds shear-wave velocity, while dissipation tests provide information about pore-pressure equalization and consolidation characteristics.

5.3 Pressuremeter testing

A pressuremeter expands a cylindrical probe in a borehole and measures pressure–volume or pressure–displacement response. The test can provide deformation modulus, limit pressure and information about in situ horizontal stress, depending on the instrument and interpretation method. It is valuable for foundation and retaining-wall deformation analysis, especially where stiffness is critical.

Results are sensitive to borehole quality, probe seating, membrane condition, loading rate and interpretation procedure. Self-boring pressuremeters can reduce disturbance but require specialized equipment. Pressuremeter parameters should be reported with test preparation and correction details.

5.4 Field vane shear test

The field vane test measures the undrained shear strength of soft, saturated cohesive soil by rotating a four-bladed vane. The peak torque is converted to an undrained strength using the assumed failure-cylinder geometry and a torque correction. After remoulding, a remoulded strength can be measured and sensitivity estimated as

S_t = s_{u,peak}/s_{u,remoulded}.

The test is most appropriate for soft cohesive soils without significant sand lenses, gravel or fissures. Rod friction, anisotropy, progressive failure, rate effects and partial drainage can influence results. Corrections may be needed for plasticity and testing conditions. The test should not be used indiscriminately in fissured or highly layered clay.

6. Laboratory Characterization

Laboratory testing should be selected to answer design questions and should use samples of appropriate quality. A large number of low-value tests does not compensate for a few unsuitable samples.

6.1 Classification and index testing

Water content, particle-size distribution, Atterberg limits, density, specific gravity and organic content support classification and correlations. The plasticity index is

I_P = w_L - w_P,

where w_L is liquid limit and w_P is plastic limit, both expressed as water contents on a consistent basis. Index tests are useful for identifying trends but are not direct substitutes for strength, stiffness or permeability testing.

6.2 Strength testing

Triaxial tests may be consolidated drained, consolidated undrained with pore-pressure measurement, or unconsolidated undrained. The chosen procedure must reproduce the relevant drainage and stress path. Direct shear testing is useful for interfaces, granular soils and cases where a defined shear plane is appropriate, but displacement and stress distributions are less uniform than in a high-quality triaxial test. Unconfined compression testing is convenient for cohesive soils but assumes a meaningful undrained response and is sensitive to sample disturbance.

Effective-stress analysis commonly uses the Mohr–Coulomb relationship

τ_f = c' + σ'_n tan φ',

where τ_f is shear stress at failure, c' is effective cohesion intercept, σ'_n is effective normal stress and φ' is effective friction angle. This is an idealized linear envelope over a specified stress range. It should not be extrapolated beyond the tested range, and apparent cohesion in unsaturated or structured soils should not automatically be treated as a durable material parameter.

For undrained total-stress analysis, the parameter s_u is often used. Its value depends on stress history, anisotropy, strain rate, sample quality and test mode; it is not a universal constant for a soil layer.

6.3 Compressibility and consolidation

Oedometer testing provides the relationship between vertical effective stress and one-dimensional strain. It may identify preconsolidation stress, compression and recompression behaviour, coefficient of consolidation and secondary compression. Settlement predictions require consideration of stress history, drainage path, layer thickness, loading duration and three-dimensional effects.

6.4 Permeability and chemical tests

Hydraulic conductivity tests should reflect the expected flow direction and stress state. In layered soils, laboratory specimens may not represent field-scale preferential flow. Chemical testing may include pH, sulfate, chloride, organic compounds, metals and other contaminants. Results should be interpreted against the relevant environmental and construction standards, including requirements for worker safety, waste classification and concrete durability.

7. Spatial Variability and Uncertainty

7.1 Two forms of uncertainty

Aleatory variability is natural spatial or temporal variation in the ground. Epistemic uncertainty arises from limited observations, imperfect sampling, measurement error or model uncertainty. The distinction matters: additional boreholes can reduce epistemic uncertainty, but they do not remove natural variability.

Sources of uncertainty include sparse investigation points, scale effects, sampling disturbance, test repeatability, uncertain groundwater conditions, hidden geological structures and inappropriate correlations. A professional report should state these limitations explicitly.

7.2 Statistical description

Measured parameters should be summarized using appropriate statistics, including number of observations, minimum and maximum, mean, median, standard deviation, coefficient of variation and selected percentiles. The arithmetic mean is

x̄ = (1/n) Σ x_i,

where x_i is observation i and n is the number of observations. The sample standard deviation is

s = √[Σ(x_i − x̄)²/(n − 1)].

These statistics are meaningful only when observations are comparable and the sampling scheme is understood. Combining different geotechnical units or mixing corrected and uncorrected test results can produce misleading summaries.

Spatial correlation should be considered because nearby observations are often more similar than distant observations. Variograms, covariance models and kriging may be appropriate for sufficiently dense and reliable datasets, but geostatistical output remains model-dependent. A smooth interpolated surface can conceal abrupt geological boundaries and should not replace engineering judgment.

7.3 Characteristic and design values

A characteristic value is a cautiously selected representation of ground behaviour for a defined design situation; it is not automatically the mean, minimum or fifth percentile. The appropriate value depends on the failure mechanism, volume of ground involved, spatial averaging, consequence of failure and model uncertainty. Design values additionally reflect code-specified partial factors or reliability requirements.

For example, a foundation bearing mechanism involving a large soil volume may be influenced by spatial averaging, whereas a localized weak seam may be controlled by a low local value. The parameter selection must therefore be linked to the mechanism being checked.

7.4 Value of information

Additional investigation is justified when its expected reduction in uncertainty can change a decision or reduce risk by more than its cost and delay. This may involve targeting a suspected weak layer, installing groundwater instruments, performing CPT between boreholes or testing an alternative construction method. Investigation should stop when residual uncertainty is acceptable for the decision, not merely when a predetermined number of tests has been completed.

8. Applied Case: Illustrative Riverside Development

Background

A proposed six-storey building with one basement is located on a former riverside industrial site. Desk study indicates alluvial deposits over weathered mudstone, historical filling and possible buried tanks. The project has adjacent buildings and a low-lying river boundary.

Problem

The design team must assess foundation options, basement excavation stability, groundwater inflow, contamination and settlement risk. A simple regular borehole grid could miss buried obstructions, loose channel deposits and local contamination.

Analysis and investigation approach

The desk study identified former industrial use, historical river migration and a potential high groundwater table. Reconnaissance found localized settlement, fill debris and seepage near the river. The investigation was therefore staged:

1. Utility and tank surveys were completed before drilling.

2. Trial pits characterized shallow fill and collected bulk samples.

3. Boreholes were located at the building corners, basement centre, river edge and suspected historical features. They extended through alluvial deposits into mudstone.

4. CPT soundings were performed between boreholes to identify loose sand lenses and thin compressible layers.

5. Standpipes and vibrating-wire piezometers monitored groundwater over several months.

6. SPTs were energy-checked and reported with equipment details; rotary cores characterized mudstone weathering and discontinuities.

7. Laboratory tests included classification, oedometer tests, triaxial tests, permeability, aggressive-water chemistry and contaminant analysis.

Ground model and solution

The resulting model comprised variable made ground, soft organic alluvium, loose to medium dense sand and gravel lenses, and weathered mudstone. Groundwater was found to rise seasonally and locally under artesian pressure in a sand layer. The building was supported on piles socketed into competent mudstone, while the basement used a secant-pile wall with staged excavation and internal propping. A water-management plan incorporated cut-off measures, controlled pumping and monitoring of adjacent buildings.

The investigation also identified contaminated fill requiring segregation and regulated disposal. CPT data revealed a loose sand lens not intersected by the first boreholes; this lens was included in settlement and liquefaction screening. Construction monitoring confirmed the groundwater model and allowed pumping rates to be adjusted.

Outcome and learning points

The staged approach reduced the risk of selecting shallow foundations on variable alluvium, identified environmental liabilities early and provided parameters for both permanent and temporary works. The principal lesson is that investigation must be designed around plausible failure and construction mechanisms. Borehole count alone is a poor measure of investigation quality; integration, monitoring and explicit uncertainty are more important.

9. Reporting, Governance and Professional Practice

A geotechnical report should distinguish factual data, interpretation, assumptions, design parameters and recommendations. It should include investigation methods, locations, depths, ground and groundwater logs, laboratory procedures, quality-control information, interpreted profiles, parameter ranges, limitations and residual risks.

Work must comply with applicable national standards, occupational safety law, environmental regulation, drilling permits and professional codes. Fieldwork requires risk assessments for unstable ground, confined spaces, contaminated soil, buried services, traffic, drilling plant and groundwater. Data integrity is an ethical obligation: anomalous results must not be selectively omitted, and correlations must not be presented as direct measurements.

Digital databases, geographic information systems, three-dimensional geological modelling, remote sensing, automated CPT interpretation and machine-learning tools can improve efficiency. However, artificial intelligence may reproduce biased training data, obscure uncertainty or generate geologically implausible interpolation. AI outputs should be traceable, independently checked against primary data and treated as decision support rather than professional judgment.

10. Best-Practice Workflow

A defensible investigation generally follows these principles:

• define decisions, hazards and performance requirements first;
• review geology, history, services and environmental constraints;
• use multiple complementary investigation methods;
• target locations and depths to potential failure mechanisms;
• specify sample quality and preserve samples properly;
• calibrate in situ and geophysical methods with recovered material;
• monitor groundwater long enough to capture relevant variation;
• apply correlations only within their empirical limits;
• separate observations, interpretations and assumptions;
• quantify variability and report confidence, not false precision;
• update the ground model during design and construction; and
• maintain observational methods, trigger levels and contingency actions for unexpected conditions.

Chapter Summary

Ground investigation is a risk-based process for developing a geological and geotechnical ground model. It begins with objectives, desk study and reconnaissance, then uses appropriately located boreholes, trial pits, sampling, geophysics, in situ tests and laboratory testing. SPT, CPT, pressuremeter and vane tests provide different types of information and must be interpreted with attention to equipment, soil type, drainage, disturbance and empirical limitations.

Laboratory tests should reproduce the relevant engineering mechanism and use samples of suitable quality. Groundwater requires dedicated monitoring because a drilling water strike is not necessarily an equilibrium level. Spatial variability and epistemic uncertainty must be distinguished, described statistically where appropriate and incorporated into characteristic and design values. A high-quality investigation integrates all evidence, documents limitations, complies with safety and environmental requirements and continues through construction using observational feedback.

Key Terms

Desk study: Review of existing geological, historical, environmental, topographic and engineering information before field investigation.

Ground model: An interpreted representation of geological units, engineering behaviour, groundwater and hazards.

Geotechnical unit: A body of ground grouped according to sufficiently similar engineering behaviour.

Made ground: Anthropogenic deposits placed or significantly altered by human activity.

Disturbed sample: A sample in which natural soil structure or stress condition has been substantially altered.

Undisturbed sample: A sample obtained with minimal alteration of natural fabric, water content and stress history.

SPT: A dynamic penetration test using a driven split-spoon sampler to obtain penetration resistance and a sample.

N60: SPT resistance normalized approximately to 60 percent reference hammer energy with stated corrections.

CPT: A quasi-static cone penetration test measuring cone resistance, sleeve friction and possibly pore pressure.

Friction ratio: Sleeve friction expressed as a percentage of cone resistance in CPT interpretation.

Pressuremeter: An in situ device that expands in a borehole to measure pressure–deformation behaviour.

Field vane test: A test for estimating undrained shear strength of soft cohesive soil by rotating a vane.

Piezometer: An instrument used to measure pore-water pressure or groundwater head.

Small-strain stiffness: Soil stiffness measured at very small strains, often related to shear-wave velocity.

Aleatory variability: Natural spatial or temporal variation in ground properties.

Epistemic uncertainty: Uncertainty caused by incomplete knowledge, measurement limitations or model inadequacy.

Characteristic value: A selected representative parameter for a defined design situation and failure mechanism.

Geostatistics: Statistical analysis that incorporates spatial dependence between observations.

Observational method: A design and construction strategy involving monitoring, predefined criteria and planned responses.

Review Questions

Short Answer Questions

1. What are the principal purposes of a desk study?

2. Distinguish a disturbed sample from an undisturbed sample.

3. State two limitations of the SPT.

4. Why is groundwater monitoring often required after drilling?

5. Define epistemic uncertainty.

Long Answer Questions

1. Explain how a geological model is transformed into a geotechnical ground model.

2. Compare SPT, CPT, pressuremeter and field vane testing in terms of measurements, applications and limitations.

3. Discuss the factors governing borehole location and investigation depth for a basement development.

4. Explain how sample quality affects laboratory strength and compressibility results.

5. Describe a professional workflow for quantifying and communicating spatial variability in a site investigation report.

Analytical / Application Questions

1. A site contains variable fill over soft alluvium and dense sand, with a proposed basement beside an existing building. Design a staged investigation and justify the methods, locations, depths and monitoring requirements.

2. CPT soundings indicate a continuous weak layer, while widely spaced boreholes do not intersect it. Explain how the ground model and design decisions should be revised, including the treatment of uncertainty.

3. A project team proposes using the mean undrained shear strength from three laboratory tests as the design value for a slope. Critically evaluate this proposal and describe the additional information needed to select an appropriate design parameter.

Multiple Choice Questions with Answers

Question 1

Which activity should normally occur first in a risk-based ground investigation?

A. Conducting all laboratory tests available

B. Defining the engineering decisions and potential ground hazards

C. Selecting a uniform borehole grid

D. Calculating foundation settlement

Correct Answer: B. Defining the engineering decisions and potential ground hazards

Question 2

Which sample is generally most appropriate for determining fabric-dependent compressibility of soft clay?

A. A carefully obtained high-quality undisturbed sample

B. A dry bulk sample

C. A washed gravel sample

D. A highly remoulded sample

Correct Answer: A. A carefully obtained high-quality undisturbed sample

Question 3

What does CPT primarily provide compared with conventional intermittent sampling?

A. A continuous penetration profile

B. A direct measure of contaminant concentration

C. A complete undisturbed core

D. A direct measurement of long-term settlement

Correct Answer: A. A continuous penetration profile

Question 4

In the expression G_max = ρV_s², V_s represents:

A. P-wave velocity

B. Shear-wave velocity

C. Groundwater velocity

D. Drilling speed

Correct Answer: B. Shear-wave velocity

Question 5

The field vane test is principally intended to estimate:

A. Drained friction angle of clean sand

B. Undrained strength of soft cohesive soil

C. Rock joint spacing

D. Hydraulic conductivity of gravel

Correct Answer: B. Undrained strength of soft cohesive soil

Question 6

If the liquid limit is 58% and the plastic limit is 24%, the plasticity index is:

A. 24%

B. 34%

C. 58%

D. 82%

Correct Answer: B. 34%

Question 7

Which statement about a water strike during drilling is most accurate?

A. It always equals the stabilized groundwater table

B. It proves that the aquifer is unconfined

C. It may not represent an equilibrium groundwater level

D. It eliminates the need for piezometers

Correct Answer: C. It may not represent an equilibrium groundwater level

Question 8

Which uncertainty is most directly reduced by obtaining additional well-targeted boreholes?

A. Natural spatial variability itself

B. Epistemic uncertainty about the ground model

C. The density of mineral grains

D. The geological age of the deposit

Correct Answer: B. Epistemic uncertainty about the ground model

Question 9

For the Mohr–Coulomb effective-stress equation, which parameter is paired with effective normal stress?

A. Total unit weight only

B. Effective friction angle

C. Water content only

D. SPT hammer energy

Correct Answer: B. Effective friction angle

Question 10

Why should geophysical anomalies normally be calibrated with intrusive investigation?

A. Geophysical methods never measure physical properties

B. An anomaly can have more than one plausible geological cause

C. Intrusive methods are always cheaper

D. Calibration removes all natural variability

Correct Answer: B. An anomaly can have more than one plausible geological cause