Soil Testing 101: What Geotech Reports Actually Tell You

Every custom home sits on dirt the architect never met. A geotech report is the document that tells your structural engineer what to engineer for. Skip it, fake it, or hire the cheapest geotech and you're betting your foundation on optimism.

Here's how to read what comes back.

What a geotech report actually tests

A typical residential geotech costs $1,500–$5,000 and includes drilling 3–6 borings on your lot to depths of 15–30 feet, collecting soil samples, lab-testing them, and reporting findings. The four numbers that matter most:

• Bearing capacity — how much weight per square foot the soil can hold without settling. Expressed in PSF (pounds per square foot). Typical residential range: 1,500–4,000 PSF.
• Plasticity Index (PI) — how much the soil expands when wet and shrinks when dry. PI < 15 is low-risk. PI > 25 is expansive (problematic). North Texas clay regularly tests PI 35–50.
• Groundwater depth — how deep before you hit water. Matters for basements and below-grade work.
• Soil type classification — USCS or AASHTO codes that tell you whether you have clay, sand, silt, rock, or fill.

What it means for your foundation

Different soil profiles dictate different foundations. A few common patterns:

• Low-PI sandy soil with good bearing: Standard slab-on-grade or basement, minimal engineering. Cheapest, easiest.
• High-PI expansive clay (North Texas, much of the South): Engineered post-tension slab, or pier-and-grade-beam to stable strata below the active zone. Add $30K–$150K.
• High water table (< 6 ft from grade): Basement becomes a long-term moisture battle. Often spec'd against, or with major waterproofing investment.
• Fill soil from prior development: May require deep piers or full removal and recompaction. Expensive surprise.
• Rocky subgrade: Expensive to excavate but excellent bearing. Pier-and-beam with shallow piers often wins.

The cheapest geotech is the most expensive report you'll buy. Pay for a thorough one, and pay the structural engineer to read it carefully.

Red flags inside a geotech report

When the report lands on your designer's desk, ask them to flag any of these:

• PI above 25 across all borings (significant expansion risk)
• Groundwater within 10 feet of grade (basement / foundation moisture risk)
• Variable bearing capacity across the lot (foundation needs site-specific engineering, not a one-size design)
• Fill or organic material in the upper layers (must be removed or stabilized)
• Slope-stability concerns on hillside lots (retaining walls, deep piers)
• Subsurface utilities or voids that need to be addressed

Who orders it and when

The geotech gets ordered as soon as you have a lot under contract and before the structural engineer draws anything. The sequence is:

1. Lot under contract (or owned)
2. Geotech drills borings (1–2 weeks)
3. Lab analysis & report delivery (2–3 weeks)
4. Structural engineer designs the foundation around the report findings
5. Foundation gets bid and built

A geotech ordered AFTER the foundation is designed is useless. The whole point is that the design responds to the soil — not the other way around.

The plain-English takeaway

Get the geotech early. Read it with your structural engineer in the room. Don't let anyone build a foundation that doesn't respond to what the report says. And budget for the foundation type the soil actually requires — not the cheaper one you wish would work.

— Daniel Caro, Construction Manager. Twenty years running jobsites — foundation, framing, mechanicals, and the unglamorous details that decide a great home. Get the free Ultimate Home Building Checklist for the field-tested list we walk every Angel home through.