The Short Answer
A soil boring is a vertical hole drilled into the ground to sample and characterize the soil and rock beneath a site. As the drill advances, samples are recovered at intervals and the layers are logged, while in-situ tests measure soil density and strength and groundwater is observed. The samples go to a laboratory for further testing. Borings are the primary way geotechnical engineers learn what is underground so they can recommend safe foundations. Without them, foundation design rests on assumptions rather than facts.
How a Soil Boring Works
Drilling and Advancing the Hole
A drill rig advances a borehole to a planned depth determined by the structure and expected loads. Deeper structures and heavier loads require deeper borings to reach and characterize the soils that will actually carry the building.
Sampling
At intervals, the drilling stops and a sampler is driven or pushed into the soil to recover a specimen. Some samplers also provide a field measurement of soil resistance, which correlates with density and strength. Samples are sealed and sent to the lab.
Logging
A field technician or engineer records a boring log — a depth-by-depth description of each soil and rock layer, sample recovery, and groundwater observations. This log becomes part of the geotechnical report and is the factual record of subsurface conditions at that location.
What a Soil Boring Reveals
Borings identify the sequence and thickness of soil layers, the presence of problem materials such as fill, organic soils, or expansive clays, the depth to groundwater, and — through sampling and lab testing — the strength and compressibility of each layer. Together this information supports the analysis of bearing capacity, settlement, and, where relevant, seismic behavior.
When Do You Need Soil Borings?
Soil borings are typically performed for new buildings, additions with new foundations, retaining walls, and pavements — anywhere the ground must carry new load and the design must be based on real data. The number and depth of borings depend on the footprint, structure type, and jurisdiction. On some small or well-documented sites, alternative methods such as test pits may suffice, but borings remain the standard for most structural foundations.
Frequently Asked Questions
How deep do borings go?
Depth is set by the structure and its loads, extending far enough to characterize the soils that will carry the foundation. Heavier and deeper foundations require deeper borings.
Are borings the same as test pits?
No. Test pits are shallow excavations that expose soils directly, useful for near-surface conditions. Borings reach greater depth and are standard for foundation design.
Will borings damage my site?
Boreholes are small and are backfilled after drilling. Access and restoration are planned as part of the investigation.