Planning & Scope of Geotechnical Investigation: A Complete Guide

Planning & Scope of Geotechnical Investigation — A Practical Guide

A geotechnical investigation reduces uncertainty about subsurface conditions and delivers the input parameters engineers need to design safe, economical foundations and earthworks. This post explains how to plan the investigation, what tests to include, how to interpret results and how to deliver clear recommendations.

Why geotechnical investigation matters

All structures interact with the ground. A good geotechnical investigation reduces risk, prevents costly redesigns and construction delays, optimizes foundation type, and often saves money overall by avoiding over-conservative designs. In short: investigation pays by turning unknowns into quantified design inputs.

(See general overview of site-investigation benefits and steps.)

Primary objectives

Identify subsurface layers, their thickness and lateral variation.
Measure engineering properties: strength, compressibility, density, permeability.
Assess groundwater conditions and seasonal fluctuations.
Provide design parameters for foundations, retaining walls, slopes and earthworks.
Highlight geohazards (compressible layers, high groundwater, contamination, collapsible soils, rockhead variability).

Scope & initial planning

Planning the scope is the first and most critical step. The geotechnical scope is not one-size-fits-all — it depends on:

Project risk (importance of structure, consequences of failure).
Foundation type being considered (shallow footings, piles, raft, deep foundations).
Site complexity (heterogeneous deposits, existing fills, groundwater, contamination risk).
Access constraints and environmental limits (urban sites, restricted access, noise limits).
Budget and program constraints.

Start with a risk-based scope: spend more time where failure consequences are high. For a medium-risk building, combine boreholes, SPT/CPT, some laboratory tests and at least one groundwater observation. For high-risk infrastructure, add redundant borings, CPTs and specialized testing like seismic CPT or piezocones. This approach mirrors good industry practice for scoping investigations.

Desk study & site reconnaissance

Desk study (what to collect)

Before boots hit the ground, collect:

Geological maps and previous geotechnical reports for nearby projects.
Topographic maps, old site plans, and utilities drawings.
Historical land use (old ponds, reclaimed land, previous landfills or quarrying).
Hydrogeology data and regional groundwater tables.
Satellite imagery and aerial photos to identify visible features.

Site reconnaissance

Walk the site to note ground conditions, evidence of settlement, exposed strata, vegetation patterns, existing structures and access. Recon helps decide safe borehole locations and identify obstacles for rigs.

Field investigation methods (detailed)

Field methods produce the raw observations and in-situ data. Common methods include:

Boreholes with sampling

Purpose: obtain soil/rock samples and install monitoring wells or instruments.

Typical depths: depend on expected foundation level and underlying geology — often 5–30 m for buildings but can be much deeper for major works.

Equipment and sample types: auger, rotary, sonic rigs; samples can be disturbed (grab samples) or relatively undisturbed (Shelby tube, piston sampler) for lab testing.

Standard Penetration Test (SPT)

What it is: a sampler driven into soil using a 63.5 kg hammer falling 760 mm; blows per 300 mm give the N-value. SPT provides empirical data for relative density and strength in cohesionless soils and an index for many correlations.

Use: common for foundation design, liquefaction assessment and for correlating to engineering properties. SPT is most useful when coupled with samples and site logs.

Cone Penetration Test (CPT / CPTu)

What it is: a cone probe is hydraulically pushed into the ground at a constant rate. It provides continuous records of tip resistance and sleeve friction; with pore-pressure sensors (CPTu) you also record pore pressure. CPT gives high-resolution stratigraphy and continuous strength indices.

Advantages: continuous profile, fast, minimal spoil, great for stratigraphy and correlations to shear strength and settlement.

Trial pits / test pits

What they are: shallow excavations (1–4 m typical) that let you visually inspect and sample near-surface layers and groundwater conditions. Useful for shallow foundations, identifying fill layers, and for geotechnical photography. Note safety rules: pits deeper than about 1.2 m often require shoring if entered by personnel.

Vane shear test (VST), Plate load and Pressuremeter

Vane tests measure undrained shear strength in soft clays. Plate load tests provide direct bearing capacity and settlement under controlled loads for shallow footings. Pressuremeter testing measures stiffness and in situ lateral stress for more advanced design cases.

Geophysical surveys

Where access or budget limits invasive testing, geophysics (refraction seismic, MASW, electrical resistivity) can map large areas and identify anomalies. Use these as supplements, not replacements, unless validated for the site conditions.

Sampling & laboratory testing

Lab tests convert samples into engineering parameters. Typical laboratory program:

Index/Hygroscopic tests: grain size distribution, Atterberg limits, natural moisture content, specific gravity.
Strength tests: consolidated undrained (CU) triaxial, direct shear.
Compressibility: oedometer (consolidation) tests for settlement predictions.
Permeability and CBR: where drainage or pavement design is relevant.
Chemical/contamination tests: where fills or industrial past uses exist.

Notes on sampling: obtain undisturbed samples for strength/compressibility tests whenever possible. Label, seal and transport samples promptly — moisture loss changes results.

Data interpretation & deliverables

Deliverables should be written clearly for designers and clients; common items include:

Borehole/CPT logs: annotated logs with stratigraphy, sample depths and in-situ test results.
Groundwater observations: static water level and seasonal notes.
Laboratory results: tables and interpreted parameters (e.g., friction angle φ, undrained shear strength cu, compression index Cv).
Design recommendations: allowable bearing pressures, recommended foundation types (shallow, piles, raft), estimated settlements and pile embedment depths.
Limitations & further work: list any areas of uncertainty and propose additional tests if needed.

Clarity is critical: show key values used in design and how they were derived (graphs, correlations and conservative assumptions where appropriate).

Advice by project size (practical scope)

Small projects (single buildings, low-rise)

2–4 boreholes to foundation depth + SPT/CPT where possible.
Basic lab program: grain size, Atterberg, oedometer if soft compressible layers are present.
One trial pit if near-surface fill is suspected.

Medium projects (commercial buildings, moderate infrastructure)

Grid of boreholes and 2–4 CPT soundings for lateral coverage.
Detailed lab tests based on expected foundation type; plate load test if shallow foundations planned.
Groundwater monitoring wells if dewatering or deep excavations are expected.

Large/high-risk projects (bridges, towers, heavy industry)

Extensive borehole/CPT network, redundancy in key locations, advanced testing (pressuremeter, seismic CPT, dynamic probes).
Long-term monitoring (inclinometers, piezometers) for deep excavations and slopes.
Specialist tests for liquefaction, contamination or unusual geology.

Scoping examples and lecture series on planning investigations illustrate that the program should be proportional to consequence of failure. :contentReference[oaicite:5]{index=5}

Health, safety & environment

Fieldwork must follow local safety codes: excavations, confined-space entry, machine operations, and lifting all carry risk. Also control spoil, manage groundwater and respect environmental permits. Ensure competent supervision and toolbox talks before drilling starts.

Budgeting & typical timeline

Costs vary widely by access, depth and tests required. As a rough guide:

Simple site (a few shallow boreholes + lab tests): days to 2 weeks, modest cost.
Medium program (CPTs, multiple boreholes): 1–3 weeks on site + 2–4 weeks for lab/analysis.
Large program (extensive testing and monitoring): many weeks on site and months for analysis and monitoring.

Allow time for sampling, lab turnaround, data QC and report writing. Unexpected ground conditions often extend schedules — build contingency.

Quality control & common pitfalls

Poor sample handling: leads to wrong strength/compressibility values.
Insufficient lateral coverage: sites are rarely uniform — spaced investigation points are critical.
Ignoring groundwater: groundwater drives settlement and design choices; monitor it.
Using inappropriate correlations blindly: empirical correlations (e.g., SPT→φ) are useful but must be validated with local soil behavior and lab tests.
Poor reporting: unclear assumptions and missing data cause rework and extra cost.

Quick planning checklist

Define design objectives & acceptable risks.
Collect desktop data & previous reports.
Visit site and mark access/safe locations for rigs.
Select appropriate mix of boreholes, CPT and in-situ tests.
Plan sampling, lab testing and data turnaround.
Budget contingency and schedule buffer for surprises.
Specify report deliverables and limits of interpretation.

Conclusion & further reading

Effective geotechnical investigation starts with clear objectives and a risk-based scope. Combine desk study, targeted fieldwork (boreholes + CPT where possible), a focused lab program and clear reporting to give designers the parameters they need. When in doubt, increase redundancy in key locations rather than relying on a few widely spaced tests.

Detailed Lab Test Matrix

Below is a suggested laboratory test matrix showing each test, sample type, purpose and typical use cases.

Test Category	Specific Test	Sample Type / Source	Purpose / Why It’s Done	Typical Use Cases
Index Tests	Grain Size Distribution (Sieve + Hydrometer)	Disturbed soil sample	To classify soil, understand particle size distribution	All granular soils; helps estimate permeability & compaction potential
Index Tests	Atterberg Limits (Liquid Limit, Plastic Limit)	Disturbed fine-grained soil	To determine plasticity and behavior under moisture change	Clayey soils, silts — for settlement and shrink-swell analysis
Index Tests	Natural Moisture Content	Disturbed sample	To know current moisture content relative to plastic limits	For all soils — input for compaction, density, and strength calculations
Index Tests	Specific Gravity	Clean, representative soil	To help with volume / weight relationships and void ratio	Used in consolidation & compressibility calculations
Strength Tests	Consolidated Undrained (CU) Triaxial Test	Undisturbed (piston / Shelby) sample	To measure shear strength parameters (c, φ) under controlled stress	For foundation design, slope stability, earth pressures
Strength Tests	Direct Shear Test	Either remolded or undisturbed sample	To find shear strength under known normal stress	Retaining wall design, interfaces, shallow footings
Compressibility / Settlement	Oedometer (Consolidation) Test	Undisturbed soil slice	To measure compressibility parameters (compression index, recompression index)	Predicting settlement for footings, embankments, or soft compressible soils
Permeability / Flow	Constant Head Permeability Test	Granular soil sample	To determine hydraulic conductivity (k)	Drainage design, seepage, groundwater flow analysis
Permeability / Flow	Falling Head Permeability Test	Fine-grained sample	To measure lower permeability values typical of clays	Seepage through clay, cutoff walls, consolidation-time estimation
California Bearing Ratio (CBR)	CBR Test	Remolded or undisturbed sample	To measure strength for pavement design	Road design, flexible pavement, subgrade characterization
Chemical / Contamination	pH, Salinity, Chloride, Sulfate Tests	Soil / Water sample	To check chemical aggressiveness, contamination, corrosivity	Sites with industrial past, landfill, fill, aggressive environment
Chemical / Contamination	Organic Content / Loss on Ignition	Soil sample	To find organic content in fills or natural soil	Landfill risk, compressibility, stabilization needs
Consolidation Behavior	Consolidation Test with Swell / Collapse	Unsaturated or saturated sample	To assess swell potential or collapse potential	Expansive soils, collapsible soils, reclaimed ground
Mineralogical / Clay Characterization	X-Ray Diffraction (XRD)	Powdered sample	To identify minerals, clay type	Sites with expansive minerals or unusual soil chemistry

Notes: Prefer undisturbed samples for strength & consolidation tests. Specify standards (IS/ASTM) in contracts to avoid ambiguity.

References & sources

Practical guidance and industry examples for planning/scope of geotechnical investigations.
Comprehensive overview of geotechnical site investigation methodology and why investigation matters.
USGS & technical notes on Cone Penetration Testing (CPT) — continuous in-situ profiling and interpretation details.
Trial pit descriptions and safety/practical guidance for shallow inspections. }
Technical descriptions and CPT equipment/interpretation resources (technical pages & handbooks).