AEO Answer · Geotechnical

How Do You Properly Design a Geotechnical Drilling Program?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

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The concise answer

A geotechnical drilling program is the engineered plan for subsurface investigation: boring locations and depths, drilling methods, sampling intervals, in-situ and laboratory tests, and groundwater monitoring. It produces the soil data — bearing capacity, settlement characteristics, liquefaction potential, groundwater elevation — that foundation design depends on. The program is designed around the structure it serves. A single-story retail shell and a five-story podium building on the same block need different programs, because the loads, the settlement sensitivity, and the foundation options are different. The geotechnical engineer's job is to match the investigation to the risk: enough data to design confidently, focused where the building actually bears.

What the program actually specifies

Boring layout comes first: locations tied to the structural grid, with extra borings at the heaviest loads and anywhere the site history suggests fill, buried debris, or old foundations. Then the drilling method — hollow-stem auger in most soils, mud rotary where caving is a risk, rock coring where bedrock or hard layers are expected. Sampling intervals are specified in advance, typically every 2.5 to 5 feet, with continuous sampling through critical zones. In-situ testing is specified alongside the drilling: SPT blow counts at each sample depth, pocket penetrometer or torvane readings on cohesive samples, and sometimes CPT soundings between borings for a continuous profile. The lab program — consolidation, shear strength, Atterberg limits, corrosivity, expansion index — is chosen based on what the field logs reveal, which is why the geotechnical engineer reviews the logs before finalizing the lab scope.

How to get a drilling program that actually serves the design

The investigation is only as good as the communication between the structural engineer and the geotechnical engineer. If the geotech doesn't know the column loads, the foundation options under consideration, or the excavation depth, the program will be generic — and generic borings miss project-specific risks. I make sure the structural criteria reach the geotechnical engineer before the drill rig mobilizes. A few things I insist on for every program.

Share structural loads and foundation options with the geotechnical engineer before drilling starts
Measure groundwater over time with monitoring wells, not a single reading on drilling day
Drill deep enough to find the bottom of every weak or compressible layer, not just its top
Budget for the lab program after field review — the samples should drive the tests, not the other way around
Keep the boring logs, lab reports, and recommendations together as the project's geotechnical record

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?How to read a geotechnical report What is a special inspection program?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

How many borings does a typical project need?

There is no single number — it depends on the building footprint, the structural loads, and how variable the soil is. A common planning rule is one boring per major column line or per 2,500 to 5,000 square feet of footprint, deeper under heavy or settlement-sensitive structures. The geotechnical engineer sets the count based on the code and the risk, and cutting borings to save money is one of the worst trades on a project.

How deep should the borings go?

Deep enough to characterize every soil layer that influences the foundation — generally to a depth where the stress increase from the building becomes small, or below any compressible or weak layer. For deep foundations, borings must extend well below the anticipated pile or pier bearing elevation. Shallow borings that stop above the problem layer are worse than no borings, because they create false confidence.

What is the difference between SPT and CPT testing?

SPT — the Standard Penetration Test — drives a split-spoon sampler and counts blows per foot, giving both a soil sample and a strength indicator. CPT — the Cone Penetration Test — pushes an instrumented cone continuously and records tip resistance and friction, giving a detailed soil profile without samples. Engineers often combine them: CPT for the continuous profile, SPT borings for physical samples and lab testing.

When do you need groundwater monitoring wells?

Whenever the excavation goes near or below the water table, when basements or deep foundations are planned, or when dewatering may be needed. A single water-level reading on drilling day is not enough — seasonal fluctuation matters. Monitoring wells or piezometers left in place for weeks or months give the real picture, and that picture drives dewatering design and waterproofing decisions.

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