The Short Answer
A structural engineer needs a geotechnical report because a building's foundation transfers all of its loads into the ground, and the ground's behavior can only be known through investigation — not assumption. The report provides the allowable bearing pressure, expected settlement, foundation type recommendations, lateral earth pressures, and seismic parameters that the structural engineer must have to design footings, piers, piles, and retaining walls safely and in compliance with code. Without it, the engineer would either design blindly or apply overly conservative assumptions that a building department may not accept.
What the Structural Engineer Uses From the Report
Allowable Bearing Pressure
This sets how much load each footing can impose on the soil, which directly determines footing sizes. A reliable, site-specific value lets the engineer size foundations efficiently and safely rather than guessing.
Settlement Estimates
The report estimates how much the structure will settle and whether that settlement will be uniform or differential. Differential settlement can crack finishes and distress the structure, so the engineer designs to keep it within tolerable limits.
Foundation Type Recommendations
On good soils, shallow spread footings may suffice. On weak or compressible soils, the report may recommend ground improvement or deep foundations. The structural engineer designs to the recommended system.
Lateral Earth Pressures and Seismic Parameters
For retaining walls and below-grade walls, the report provides the soil pressures the structure must resist. In seismic regions, it provides site classification and design parameters that feed the structural seismic analysis.
Why It Can't Be Skipped
Soil is not visible or uniform, and its properties vary across a site and with depth. A footing sized for strong soil placed over a soft or expansive layer can settle, tilt, or fail. Because the structural engineer takes professional responsibility for the safety of the foundation, they must base that design on real subsurface data. Building departments recognize this and frequently require a geotechnical report before approving foundation design. The report is the bridge between what is underground and what gets built.
When It Applies
A structural engineer needs a geotechnical report on new buildings, additions with new foundations, retaining walls, and any project on difficult soils. On interior-only work or projects that add no new load to the ground, a report may not be needed. When we take on a structural project, we advise early whether a geotechnical report is required so it can be scheduled without delaying the design.
Frequently Asked Questions
Can the structural engineer just be conservative instead?
Sometimes, but conservative assumptions can produce oversized, costly foundations and may still be rejected by a building department that requires site-specific data. A report is usually the better path.
Who arranges the geotechnical report?
The owner typically commissions it, often on the structural engineer's recommendation. Coordinating early keeps the project on schedule.
What if a report already exists?
The structural engineer reviews it to confirm it covers the current design and site. If it does, additional investigation may not be needed.