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Understanding what lies beneath the surface of the earth is a critical piece of planning in the fields of construction, environmental due diligence, and geotechnical engineering.
Conducting soil borings is one of the most reliable methods for gaining this insight. Soil borings provide essential data about subsurface conditions, helping engineers, architects, and developers make informed decisions about site suitability, foundation design, and environmental impact.
What Are Soil Borings?
Soil borings are a geotechnical investigation technique used to collect soil samples from below the ground surface. These samples are analyzed to determine the physical and mechanical properties of the soil, such as composition, density, moisture content, strength, and bearing capacity. The information gathered is crucial for designing safe and effective foundations for buildings, roads, bridges, and other infrastructure.
Soil borings are also used in environmental assessments to detect contamination, monitor groundwater, and evaluate the suitability of land for agricultural or industrial use.
Why Are Soil Borings Important?
The subsurface conditions of a site can vary significantly, even within a small area. Without proper investigation, construction projects risk encountering unexpected soil behaviors such as settlement, liquefaction, or slope instability. These issues can lead to structural failure, costly delays, or environmental hazards.
Soil borings help to:
- Assess soil stability and load-bearing capacity
- Identify groundwater levels and flow
- Detect the presence of contaminants
- Inform the design of foundations and earthworks
- Ensure compliance with building codes and environmental regulations
Types of Soil Boring Methods
Several methods are used to conduct soil borings, each suited to different site conditions and project requirements. The most common include:
Auger Boring
This method uses a helical screw blade (auger) to drill into the ground. It is typically used for shallow borings in soft to medium soils. There are two main types:
- Hand augers are used for depths up to 10-15 feet
- Power augers are used for deeper or more resistant soils
Auger boring is relatively quick and cost-effective but may not be suitable for very hard or saturated soils.
Rotary Drilling
Rotary drilling uses a rotating drill bit to cut through soil and rock. It is often used for deeper borings and can penetrate a wide range of soil types. The drill bit is lubricated and cooled with drilling fluid (mud or water), which also helps to bring cuttings to the surface.
This method allows for continuous sampling and is ideal for complex geotechnical investigations.
Percussion Drilling
Also known as cable tool drilling, this method involves repeatedly dropping a heavy bit to break up soil and rock. The loosened material is then removed from the borehole. While slower than rotary drilling, percussion drilling is effective in hard or boulder-laden soils.
Wash Boring
Wash boring uses a jet of water to loosen soil, which is then removed by a suction pump. It is commonly used in soft soils and for preliminary investigations. However, it does not provide undisturbed samples, limiting its usefulness for detailed analysis.
The Soil Boring Process: Step-by-Step
Conducting a soil boring involves several key steps, typically carried out by a geotechnical engineering firm:
1. Site Preparation
Before drilling begins, the site is surveyed and cleared. The boring locations are selected based on the proposed construction layout and known geological conditions. Permits may be required, especially in urban or environmentally sensitive areas.
2. Drilling and Sampling
The drilling rig is set up at the designated location. As the borehole is advanced, soil samples are collected at regular intervals – usually every 2 to 5 feet or at changes in soil strata.
There are two main types of samples:
- Disturbed samples, which are suitable for classification and basic testing
- Undisturbed samples, which preserve the soil structure and are used for strength and compressibility tests
Sampling tools include split-spoon samplers, Shelby tubes, and piston samplers, depending on the soil type and required data.
3. Standard Penetration Test (SPT)
One of the most common in-situ tests performed during soil borings is the Standard Penetration Test (SPT). It involves driving a split-spoon sampler into the soil using a standardized hammer and recording the number of blows required to penetrate a set depth. The resulting "N-value" provides an indication of soil density and strength.
4. Groundwater Monitoring
As the borehole is drilled, groundwater levels are observed and recorded. In some cases, temporary or permanent monitoring wells are installed to track fluctuations over time.
5. Logging and Documentation
Throughout the boring process, a detailed borehole log is maintained. This log includes:
- Depth and type of each soil layer
- Sample locations and types
- SPT results
- Groundwater observations
- Any anomalies or obstructions encountered
This documentation forms the basis for the geotechnical report.
6. Laboratory Testing
Collected samples are sent to a geotechnical laboratory for further analysis. Common tests include:
- Grain size distribution
- Atterberg limits (plasticity)
- Moisture content
- Shear strength
- Consolidation and compaction characteristics
These tests provide the data needed to model soil behavior under load and design appropriate foundations.
Interpreting the Results
The final deliverable of a soil boring investigation is a geotechnical report. This report synthesizes field and lab data to provide:
- Soil profiles and stratigraphy
- Engineering properties of each soil layer
- Recommendations for foundation design
- Assessment of potential risks (e.g., settlement, liquefaction)
- Construction considerations (e.g., dewatering, excavation support)
This report is a critical tool for engineers and architects, guiding decisions that affect safety, cost, and long-term performance.
GPRS offers nationwide, precision utility locating services to help ensure the success of your soil boring projects.
Utilizing state-of-the-art subsurface investigation technology such as GPR scanning and EM locating, our SIM-certified Project Managers provide you with complete and accurate data bout the built world beneath your project site, so you can excavate without the risk of costly and potentially dangerous subsurface damage.
All this data is at your fingertips 24/7 thanks to SiteMap® (Patent Pending), GPRS’ project & facility management application that provides accurate existing conditions documentation to protect your assets and people.
From soil boring clearances to skyscrapers, GPRS Intelligently Visualizes The Built World® to keep your projects on time, on budget, and safe.
What can we help you visualize?
Frequently Asked Questions
What do I get when I hire GPRS to conduct a utility locate?
Our Project Managers flag and paint our findings directly on the surface. This method of communication is the most accurate form of marking when excavation is expected to commence within a few days of service.
GPRS also uses a global positioning system (GPS) to collect data points of findings. We use this data to generate a plan, KMZ file, satellite overlay, or CAD file to permanently preserve results for future use. GPRS does not provide land surveying services. If you need land surveying services, please contact a professional land surveyor.
Please contact us to discuss the pricing and marking options your project may require.
Can GPRS Project Managers distinguish between different underground utilities that they locate?
In most situations, we can identify the utility in question without any problems, although it is not always possible to determine what type of utility is present. When this happens, we attempt to trace the utility to a valve, meter, control box, or other signifying markers to determine the type of utility buried.
