Soil Borings and Classification: What Happens Before the First Foundation Is Poured
Every structure starts below the surface. Before a foundation is designed, a slab is specified, or a roadway alignment is finalized, engineers need reliable data about the subsurface conditions at the project site. That data comes from soil borings and soil classification, two interconnected processes that form the backbone of sound geotechnical engineering.
At NOVA Engineering, our geotechnical team performs soil borings and classification across residential, commercial, municipal, and infrastructure projects throughout the Southeast. Here is a closer look at how the process works, what data is collected, and why it matters to builders, developers, and owners.
What Is a Soil Boring?
A soil boring is a drilled hole into the ground where subsurface samples and field data are collected. It is the primary method engineers use to determine what lies beneath a site before construction begins. The depth, spacing, and number of borings depend on the size and complexity of the project. A residential foundation may require borings to 10 or 15 feet. A commercial or multi-story project may call for borings extending 100 feet or more.
Our field crews use auger rigs, rotary drills, and push rigs depending on the site conditions and the type of data required. The goal is to build an accurate profile of the soil layers, groundwater conditions, and material properties at the specific location where construction will take place.
How Soil Samples Are Collected
During a boring, our engineers and field technicians collect two types of soil samples at measured intervals throughout the depth of the hole.
Disturbed Samples
These are loose samples retrieved using a split spoon sampler. They provide material for visual identification and laboratory classification. While the natural structure of the soil is altered during collection, disturbed samples give us the grain size and composition data needed for classification.
Undisturbed Samples
For projects where soil strength and deformation characteristics are critical, we collect undisturbed samples using thin-walled Shelby tubes. These samples preserve the soil's natural structure and are used for laboratory strength testing, consolidation analysis, and other performance-related evaluations. Undisturbed sampling is particularly important in cohesive soils like clays, where structural integrity directly affects foundation design.
The Standard Penetration Test
One of the most valuable pieces of field data collected during a soil boring is the Standard Penetration Test, or SPT. This test measures the resistance of the soil to penetration and provides a direct indicator of density and strength.
During the SPT, a 140-pound hammer is dropped 30 inches to drive a split spoon sampler into the soil at the bottom of the borehole. The number of blows required to advance the sampler 12 inches is recorded as the N-value, or blow count.
A low N-value indicates soft or loose material. A high N-value indicates dense or hard soil. These values are used throughout foundation design and earthwork planning to assess bearing capacity, settlement potential, and the suitability of native soils for structural support.
Field Logging
As each boring is advanced, our engineers create a detailed field log documenting what they observe at every depth. This log includes the soil type based on visual and manual identification, the color of the material, moisture content, the depth and thickness of each layer, and the depth at which groundwater is encountered.
Field logs provide the initial framework that our geotechnical engineers use to develop recommendations. Combined with laboratory data, they create a comprehensive picture of subsurface conditions that guides every downstream design decision.
How Soil Classification Works
Once samples return to the laboratory, they are tested and categorized using the Unified Soil Classification System, or USCS. This system provides a standardized language for describing and comparing soils, which is essential when multiple engineers, designers, and contractors are working from the same data.
Grain Size Analysis
The first step in classification is determining the percentage of gravel, sand, silt, and clay in each sample. Coarse-grained soils are analyzed using sieve tests, where material is passed through a series of progressively finer screens. Fine-grained soils require hydrometer testing, which measures particle settlement rates in a liquid suspension. Together, these tests produce a complete grain size distribution that defines the composition of the soil.
Atterberg Limits
For fine-grained soils, grain size alone is not enough. Engineers also measure the Atterberg limits to understand how the soil behaves at different moisture contents. These limits include the Liquid Limit, which is the moisture content at which the soil transitions from a plastic to a liquid state, the Plastic Limit, which is the moisture content at which the soil stops behaving plastically, and the Plasticity Index, which is the range between the two. These values are critical for distinguishing between silts and clays and for predicting how the soil will perform under load and in the presence of water.
USCS Designations
Based on the combined results of grain size analysis and Atterberg limits, each sample is assigned a USCS classification. Common designations include GW for well-graded gravel, SP for poorly graded sand, CL for low plasticity clay, and CH for high plasticity clay. These standardized labels allow engineers, architects, and contractors to quickly understand the materials present at a site and the implications for design and construction.
Additional Testing
Depending on the project requirements, our laboratory and field programs may also evaluate shear strength, bearing capacity, compaction characteristics, and permeability. These properties inform decisions about everything from retaining wall design and slope stability to pavement section thickness and stormwater management.
Why This Data Matters for Your Project
Soil borings and classification are not academic exercises. They directly inform the decisions that determine whether a structure performs as intended over its design life. The data collected through this process drives foundation type selection, whether a project calls for a shallow spread footing, a mat slab, or deep foundations like driven piles or drilled shafts. It informs drainage planning and waterproofing strategies by identifying permeable and impermeable layers. It predicts settlement and cracking risk, which is critical for both structural and architectural finishes. And it determines trench stability for underground utilities, including sewer, water, and stormwater lines.
Skipping or underinvesting in this phase of a project creates risk that compounds as construction progresses. Foundation issues, drainage failures, and unexpected soil conditions are among the most expensive problems to correct after the fact. Accurate geotechnical data reduces that risk and gives design teams the information they need to make confident, cost-effective decisions from the start.
Our Approach at NOVA Engineering
With 29 years of experience, more than 600 personnel, and over 20 offices across the Southeast, our geotechnical team brings proven expertise to projects of every scale. Our licensed Professional Engineers and Professional Geologists manage the entire process, from scoping the field program through delivering final recommendations.
We understand that builders and developers need accurate data delivered on a timeline that keeps their project moving forward. Our field crews are equipped to mobilize quickly, and our laboratories are staffed to turn results around efficiently without compromising quality.
Whether you are developing a single residential lot or planning a large-scale commercial project, our geotechnical services provide the subsurface data your design team needs to build with confidence.
Contact NOVA Engineering
If you have an upcoming project that requires soil borings, geotechnical evaluation, or construction materials testing, we are here to help. Contact our team to discuss your project scope and timeline, and let us put our expertise to work for you.
