Geotechnical Investigation and Soil Testing Guide: Boring Logs, Bearing Capacity, and Foundation Recommendations

Every building sits on the ground, and the ground is not always what you think it is. A geotechnical investigation tells you what is actually under your site so you can design the right foundation, avoid costly surprises, and keep the building standing for the long haul.

If you are a general contractor, builder, or developer, understanding geotechnical reports is a core skill. You do not need to be a soils engineer, but you need to know enough to read the report, understand the recommendations, and make informed decisions about your project.

This guide covers what happens during a geotechnical investigation, how to read the results, and what those results mean for your foundation design.

What Is a Geotechnical Investigation?

A geotechnical investigation is a systematic study of the soil and rock conditions at a construction site. It answers three basic questions:

1. What type of soil is down there? (Sand, clay, silt, rock, fill, organic material)
2. How strong is it? (Can it support the building without failing or settling too much?)
3. Are there any problems? (High water table, expansive clay, soft layers, contamination)

The investigation produces a geotechnical report with boring logs, lab test results, and engineering recommendations for foundation type, depth, and design parameters.

When Do You Need a Geotechnical Investigation?

Almost Always

• Commercial buildings (most jurisdictions require it)
• Multi-story residential
• Any structure on sloped terrain
• Sites with known problem soils (expansive clay, collapsible soils, peat)
• Projects near water or in flood zones
• Retaining walls over 4 feet
• Any project where the foundation will be deeper than a standard spread footing

Sometimes

• Single-family residential on flat lots in areas with known good soils
• Additions to existing buildings (depends on size and local requirements)
• Detached accessory structures

Always a Good Idea

Even when not required by code, a geotechnical investigation is cheap insurance. A $3,000 soil study can prevent a $100,000 foundation problem. If you are building anything of significant value, get the investigation done.

The Investigation Process

Phase 1: Desktop Study

Before the drill rig shows up, the geotechnical engineer does background research:

• Geological maps of the area to understand the regional soil types
• Previous investigations on nearby sites (many geotech firms maintain databases)
• Topographic maps to assess slope, drainage, and potential issues
• Aerial photos to look for old fill areas, former ponds, or other red flags
• USDA soil survey maps for general soil classification

This desktop work helps the engineer plan the boring locations, depths, and sampling methods.

Phase 2: Field Investigation (Drilling and Sampling)

This is where the drill rig comes in. A crew drills test borings at specified locations across the site. The number and depth of borings depends on the project:

Residential (single building): 2 to 4 borings, typically 15 to 25 feet deep

Commercial (single building): 4 to 8 borings, 25 to 50 feet deep (or deeper for heavy loads or multi-story)

Large developments: Multiple borings per building pad plus borings along utility routes and roadways

Drilling Methods

Hollow-stem auger. The most common method for standard soil investigations. An auger with a hollow center drills into the ground, and samples are collected through the hollow stem. Works well in most soil types above the water table.

Mud rotary. Used when drilling below the water table or in unstable soils where the borehole would collapse. Drilling fluid (mud) keeps the hole open during drilling.

Rock coring. When you hit bedrock, switch to a diamond-tipped core barrel that cuts a cylindrical rock core sample. The core gives you information about rock type, quality, and fracture patterns.

The Standard Penetration Test (SPT)

The SPT is the most common in-situ test performed during a geotechnical boring. Here is how it works:

1. A split-spoon sampler (a hollow tube, 2 inches outer diameter) is placed at the bottom of the borehole
2. A 140-pound hammer is dropped from a height of 30 inches onto the drill rod
3. The number of hammer blows required to drive the sampler each of three 6-inch increments is recorded
4. The blow count for the last two increments (12 inches total) is the N-value

What N-values mean:

SPT samples are bagged, labeled with the boring number and depth, and sent to the lab for further testing.

Other Field Tests

Cone Penetration Test (CPT). A cone-tipped probe is pushed into the ground at a constant rate. Sensors measure tip resistance and side friction continuously, giving a detailed soil profile without the need for sample recovery. CPT is faster than SPT but does not provide physical soil samples.

Vane shear test. A four-bladed vane is pushed into soft clay and rotated to measure the undrained shear strength. Used primarily in soft clays where SPT values would be very low and unreliable.

Hand auger borings. Shallow borings (up to 10 to 15 feet) done by hand. Used for preliminary investigations or sites where a drill rig cannot access.

Test pits. Backhoe-excavated pits that allow direct visual inspection of the soil layers. Limited to about 15 feet deep and not practical in all soil types, but they give the best visual representation of the subsurface.

Reading a Boring Log

The boring log is the core document of any geotechnical investigation. Each boring gets its own log that shows:

Header Information

• Boring number and location
• Ground surface elevation
• Date drilled
• Drilling method
• Groundwater level (if encountered)

Soil Profile Column

This is the main body of the log. It shows each soil layer encountered, including:

• Depth range (e.g., 0 to 3 feet, 3 to 12 feet)
• Soil description using the Unified Soil Classification System (USCS). Example: “Brown silty CLAY, medium stiff, moist (CL)”
• SPT N-values at each test interval (usually every 2.5 or 5 feet)
• Sample type and recovery (split spoon, Shelby tube, rock core)
• Groundwater observations

Soil Classification (USCS)

The Unified Soil Classification System uses two-letter codes:

As a contractor, the codes that should get your attention are CH (high-plasticity clay, likely expansive), OH and PT (organic soils, very compressible), and any notation of “fill” (may be uncontrolled and unsuitable).

Laboratory Testing

Soil samples from the borings go to a laboratory for testing. Common tests include:

Moisture Content

The percentage of water in the soil sample by weight. High moisture content in clay can indicate soft, weak conditions. Very low moisture in expansive clay means it has room to swell when it gets wet.

Atterberg Limits

These define the boundaries between soil behavior states:

• Liquid Limit (LL). The moisture content at which the soil behaves like a liquid
• Plastic Limit (PL). The moisture content at which the soil becomes brittle
• Plasticity Index (PI). LL minus PL. High PI values (over 35) suggest expansive clay

If the report shows a high PI, pay attention. Expansive clay is one of the most common and expensive soil problems in construction.

Grain Size Analysis (Sieve Analysis)

Soil is passed through a series of sieves to determine the distribution of particle sizes. This tells you whether you are dealing with gravel, sand, silt, or clay (or a mix). The results are plotted on a grain size distribution curve.

Unconfined Compression Test

A cylindrical soil sample (usually clay) is compressed until it fails. The maximum stress it can handle is the unconfined compressive strength, which is used to estimate bearing capacity.

Consolidation Test

A clay sample is loaded in stages and the compression over time is measured. This test predicts how much a clay layer will settle under the building load and how long that settlement will take.

Proctor Compaction Test

Determines the maximum dry density and optimum moisture content for a given soil. This is the standard that field compaction is measured against. When a spec says “compact to 95% Modified Proctor,” it refers to 95% of the maximum dry density from this test.

Swell Potential Test

Measures how much an expansive clay sample swells when given access to water. The result is expressed as a percentage of swell. Values above 3% are considered moderate to high risk.

Foundation Recommendations

The most important section of the geotechnical report is the foundation recommendations. This is where the engineer takes all the data and tells you what kind of foundation to build.

Spread Footings

The most common foundation type for standard conditions. The report will specify:

• Allowable bearing pressure (e.g., 2,000 psf, 3,000 psf)
• Minimum footing depth (must be below frost line and below any weak surface soils)
• Minimum footing width
• Estimated settlement (total and differential)

If the allowable bearing pressure is decent (2,000+ psf for residential) and the expected settlement is within acceptable limits (typically less than 1 inch total, less than 1/2 inch differential), spread footings work fine.

Drilled Piers (Caissons)

When the surface soils are too weak or expansive, the report may recommend drilled piers that extend down to a more competent bearing layer. The report specifies:

• Pier diameter and depth
• End bearing capacity (the load carried by the bottom of the pier)
• Skin friction (the load carried by friction along the sides of the pier)
• Void space requirements (in expansive soils, the pier must be isolated from the swelling clay with a void form around the upper portion)

Drilled piers are more expensive than spread footings but are often the right solution for expansive clay, fill over native soil, or sites where competent bearing soil is deep.

Driven Piles

For very deep soft soils (common in coastal areas and river valleys), the report may recommend driven piles. These are steel, concrete, or timber members driven to refusal (when they will not go any deeper) or to a specified bearing layer. The report provides:

• Pile type and size
• Estimated driving depth
• Design load per pile
• Pile spacing and group effects

Mat (Raft) Foundations

When the building load is heavy and the soil is moderately weak, a mat foundation (one continuous slab under the entire building) distributes the load over a larger area. The report specifies the required thickness, reinforcement, and expected settlement.

Soil Treatment

In some cases, the report may recommend improving the soil rather than going to a deep foundation:

• Over-excavation and replacement. Remove poor soil and replace with engineered fill compacted to specification.
• Lime or cement treatment. Mixing lime or cement into expansive clay reduces swell potential and increases strength.
• Dynamic compaction. Dropping heavy weights from a crane to densify loose fill or granular soils.
• Aggregate piers (stone columns). Drilled holes filled with compacted gravel that improve bearing capacity and reduce settlement.

Groundwater Considerations

The boring log will note the groundwater level if encountered during drilling. Groundwater affects construction in several ways:

• Excavation. Digging below the water table requires dewatering (pumps, well points, or sumps).
• Foundation waterproofing. Basements or below-grade spaces in high water table areas need waterproofing and possibly a drainage system.
• Buoyancy. In extreme cases, lightweight structures in high water table areas can actually float. The structural engineer accounts for this.
• Soil behavior. Saturated sand can liquefy during seismic events. Saturated clay is weaker than dry clay.

If the report notes seasonal water table fluctuations, plan your excavation timing accordingly.

What Contractors Need From the Geotech Report

When you receive a geotechnical report, focus on these sections:

1. Site description and subsurface conditions. Understand what is under the site in plain language.
2. Groundwater. Know the water table depth and whether it affects excavation.
3. Foundation recommendations. This drives your foundation plan and cost.
4. Earthwork recommendations. Requirements for fill placement, compaction, and subgrade preparation.
5. Pavement recommendations. If the project includes parking or roadways, the report specifies the pavement section.
6. Special considerations. Expansive soils, corrosive soils, seismic hazards, or environmental concerns.

If anything in the report is unclear, call the geotechnical engineer. They expect questions and would rather clarify now than deal with a field problem later.

Common Soil Problems and What They Mean for Your Project

Expansive Clay

Swells when wet, shrinks when dry. Causes foundation heave, cracking, and structural damage. Mitigation includes drilled piers, structural slabs, moisture barriers, and careful drainage design. This is the most common soil problem in the central and southern United States.

Collapsible Soils

Loose, dry soils that suddenly collapse and compress when saturated. Common in arid regions with wind-deposited (loess) soils. Mitigation includes over-excavation and compaction, pre-saturation, or deep foundations.

Organic Soils and Peat

Extremely compressible and unsuitable for direct support. Must be removed or bypassed with deep foundations. Settlement on organic soils can continue for years.

Fill

Previously placed soil that may or may not be suitable for building support. Controlled (engineered) fill placed in lifts and compacted to specification is usually fine. Uncontrolled fill (old dump material, construction debris, topsoil) is not acceptable without treatment.

High Water Table

Requires dewatering during construction, waterproofing for below-grade spaces, and may limit foundation options. Can also cause buoyancy concerns for lightweight structures.

Corrosive Soils

Soils with high sulfate content or low pH can attack concrete and steel. The report will specify sulfate-resistant cement requirements and protective coatings for metal in contact with the soil.

Tracking Geotechnical Work in Your Project Timeline

The geotechnical investigation happens early, often before design is finalized. But the results affect everything downstream: foundation design, excavation planning, material orders, and construction schedule.

Using Projul’s project management tools, you can track the investigation timeline, store the report with your project documents, and make sure the foundation subcontractor has the information they need before they show up on site.

When you are managing multiple projects at different stages, having all your geotechnical data, inspection results, and foundation costs in one system keeps things from falling through the cracks. Check out Projul’s pricing or request a demo to see how it works for site development and foundation work.

Working With Your Geotechnical Engineer

A good relationship with your geotech engineer saves time and money. Here are some tips:

• Involve them early. Get the investigation started as soon as you have site access. Waiting until design is done means potential redesign if the soils are bad.
• Share the building plans. The engineer needs to know the building footprint, number of stories, and approximate column loads to plan boring locations and depths.
• Ask about phased investigations. For large sites, a preliminary investigation can guide the design, followed by detailed borings once the building location is finalized.
• Keep them in the loop during construction. If you encounter conditions that do not match the report (unexpected fill, soft pockets, water where there should not be any), call the engineer immediately. They can provide field guidance and updated recommendations.
• Budget for construction observation. Many projects require a geotechnical technician on site during excavation, fill placement, and foundation construction. This is not the engineer being nosy. It is quality control that protects everyone.

The geotechnical report is not just a piece of paper to satisfy the building department. It is a guide to building on the ground you actually have, not the ground you wish you had. Read it, understand it, and follow it.