Geotechnical Investigation

A geotechnical investigation (also called a soils investigation, subsurface investigation, or geotech study) is the systematic process of determining the subsurface conditions at a construction site. It is one of the most important — and most cost-effective — investments in any construction project. The cost of a typical geotechnical investigation for a commercial building is 0.1% to 0.5% of the total construction cost, but the consequences of building on inadequately characterized soil can be catastrophic — foundation failures, excessive settlement, slope failures, and water intrusion problems that may cost many times the original construction budget to repair.

This lesson covers when and why geotechnical investigations are needed, how they are conducted, what tests are performed, and how to read and interpret the resulting geotechnical report.

Building codes and good practice require geotechnical investigation for:

• All commercial and institutional buildings
• Multi-family residential buildings
• Any structure more than three stories
• Structures on slopes or near slopes
• Sites with known or suspected problematic soils (expansive clay, fill, organic soils)
• Sites in seismic zones (liquefaction evaluation)
• Structures with heavy or unusual loading (warehouses, industrial facilities)
• Earth-retaining structures (retaining walls, basement walls)
• Sites where grading will change the drainage or loading pattern significantly

For single-family residential construction, local requirements vary. Some jurisdictions require a geotechnical report for all new construction; others allow presumptive bearing values for simple structures on known-good soils.

A typical geotechnical investigation proceeds in three phases:

Phase 1: Desktop Study and Site Reconnaissance

Before drilling a single borehole, the geotechnical engineer reviews existing information:

• Geological maps: Published maps showing the geological formations, soil types, and bedrock depth in the area.
• USDA soil surveys: Detailed maps of surface soil types published by the Natural Resources Conservation Service (NRCS). Available online through the Web Soil Survey.
• Topographic maps: Reveal slopes, drainage patterns, and potential landslide areas.
• Aerial photographs: Historical photos can show previous land use (was the site a landfill? A pond that was filled? A demolished building?).
• Previous geotechnical reports: Reports from adjacent properties or earlier projects on the same site.
• FEMA flood maps: Identify flood zones and high water table areas.

A site reconnaissance (walk-through) identifies:

• Visible soil types and rock outcrops
• Surface drainage patterns
• Signs of slope instability (scarps, tilted trees, bulging soil)
• Evidence of fill placement
• Existing structures and their condition
• Vegetation patterns (hydrophilic plants indicate high water table)
• Access for drilling equipment

Phase 2: Subsurface Exploration

This is the core of the geotechnical investigation — physically sampling the soil and rock beneath the site. The primary method is drilling boreholes (borings).

Boring Methods:

• Hollow-stem auger: The most common method for soil exploration. A rotating auger advances the borehole while a hollow center allows sampling tools to be lowered through the auger. Suitable for most soil types.
• Solid-stem auger: A continuous-flight auger that brings disturbed soil to the surface. Less expensive but does not allow undisturbed sampling.
• Rotary wash boring: Uses a rotating bit and circulating fluid (water or mud) to advance the borehole. Used in difficult soils and rock. The drilling fluid stabilizes the borehole walls.
• Hand auger: A manual boring tool suitable for shallow depths (up to 15–20 feet) in soft soils. Used for small projects or preliminary investigations.

The Standard Penetration Test (SPT):

The SPT (ASTM D1586) is the most widely used in-situ soil test in North America. At regular depth intervals (typically every 2.5 or 5 feet), a split-spoon sampler — a hollow steel tube 1.375 inches in inside diameter and 2 inches in outside diameter — is driven into the soil at the bottom of the borehole by a 140-pound hammer falling 30 inches. The number of blows required to drive the sampler each of three 6-inch increments is recorded. The SPT N-value (blow count) is the sum of the blows for the second and third increments (the first increment is considered a seating drive).

The N-value is a rough measure of soil strength and density:

N-value	Granular Soil Condition	Cohesive Soil Condition
0–4	Very loose	Very soft
5–10	Loose	Soft
11–30	Medium dense	Medium stiff
31–50	Dense	Stiff
> 50	Very dense	Very stiff / Hard

The SPT also recovers a disturbed sample — the soil captured in the split-spoon is preserved in glass jars for visual classification and laboratory testing.

Undisturbed Sampling:

For detailed laboratory testing (consolidation, triaxial shear strength), undisturbed samples are needed — samples that preserve the soil's natural structure, moisture, and density. The most common method is the Shelby tube (thin-wall tube sampler, ASTM D1587) — a thin-walled steel tube 3 inches in diameter that is hydraulically pushed into clay at the bottom of the borehole. The tube is sealed with wax and transported to the laboratory.

Boring Layout and Depth:

The number, location, and depth of borings depend on the project:

• Number: Typically one boring per 2,500 to 10,000 sq ft of building footprint, plus borings at critical locations (heavily loaded columns, retaining walls).
• Depth: Generally extend to a depth of at least 1.5 times the width of the foundation below the foundation level, or until a competent bearing layer (strong soil or rock) is reached. For multi-story buildings, borings may extend 50 to 100+ feet.
• Layout: Borings should cover the entire building footprint and any areas of grading, retaining walls, or utilities.

Phase 3: Laboratory Testing

Samples recovered from the borings are tested in the geotechnical laboratory. Common tests include:

Test	ASTM Standard	Purpose
Moisture content	D2216	Determine natural water content
Grain size analysis (sieve)	D6913	Classify soil by particle size
Hydrometer analysis	D7928	Classify fine-grained soil
Atterberg limits	D4318	Determine plasticity (LL, PL, PI)
Proctor compaction	D698 / D1557	Determine optimum moisture and maximum density
Unconfined compression	D2166	Measure cohesive soil strength
Triaxial shear	D2850 / D4767	Measure shear strength under controlled conditions
Consolidation (oedometer)	D2435	Measure compressibility and consolidation rate
Direct shear	D3080	Measure shear strength along a plane
CBR (California Bearing Ratio)	D1883	Evaluate soil for pavement design
Swell/collapse potential	D4546	Identify expansive or collapsible soils

A geotechnical report is a formal engineering document that presents the findings of the investigation and provides recommendations for foundation design and earthwork. A typical report contains:

1. Project Description: Location, proposed construction, loading conditions.

2. Site Description: Topography, vegetation, existing structures, surface drainage.

3. Regional Geology: Geological setting, known geological hazards.

4. Subsurface Exploration: Description of boring methods, locations, depths. Includes a boring location plan (site map showing where borings were drilled).

5. Boring Logs: The most detailed part of the report. Each boring has a log that shows:

• Depth of each soil layer
• USCS classification for each layer
• Soil description (color, moisture, consistency, inclusions)
• SPT N-values at each test depth
• Groundwater level (measured during and after drilling)
• Sample types and depths

6. Laboratory Test Results: Tables and graphs of all test results.

7. Engineering Analysis and Recommendations: This is the most important section for the construction team. It typically includes:

• Recommended foundation type (shallow spread footings, mat foundation, deep foundation)
• Allowable bearing capacity
• Estimated settlement (magnitude and time frame)
• Groundwater and dewatering recommendations
• Excavation recommendations (slopes, shoring)
• Earthwork recommendations (fill material, compaction requirements)
• Pavement section design
• Seismic design parameters (if applicable)
• Lateral earth pressure coefficients for retaining walls

8. Limitations: All geotechnical reports include a limitations section stating that the findings are based on conditions at the boring locations and may not represent conditions between borings.

A boring log is read from top to bottom (surface to depth). Key information to extract:

1. Soil layers and transitions: Where does the soil change from one type to another? Is there a clear bearing layer (dense sand, stiff clay, or rock) at a reasonable depth?
2. Consistency and density: Are the N-values increasing with depth (normal and favorable) or are there soft/loose zones at depth (problematic)?
3. Groundwater level: How deep is the water table? Will excavations encounter water? Will dewatering be needed?
4. Fill vs. natural soil: Is there fill at the surface? How deep does it extend? Fill must be treated differently from natural soil.
5. Problematic conditions: Are there organic layers, soft clay, loose sand, or other concerning conditions?