Wall-Wise: What is a Geotechnical Report and What Data Should it Contain?

Installing a retaining wall without adequate knowledge of the site’s soil can lead to structural failures, increased maintenance costs, and potentially serious safety hazards.

A geotechnical report is your most effective tool for ensuring a sound, site-specific design and construction of your retaining wall.

What is a geotechnical report?

A geotechnical report (or geotech report) is prepared by a geotechnical engineer and provides detailed information about your site’s soil and subsurface conditions. This report may also be called a “geotechnical subsurface investigation report” or “soil investigation report.” A retaining wall engineer/designer will use the recommendations in the geotech report to help create the wall design and installation plans.

What should a complete geotech report include?

A comprehensive geotech report should include information on the subsurface soil layers and provide information on soil composition, density, moisture content, shear strength, groundwater levels, and bearing capacity of those soil layers. It should also provide recommended properties for on-site soils, that may or may not be suitable for use in a retaining wall, and import soils. The geotech report should also identify potential future issues such as soil erosion, slope and global stability concerns, and seismic activity, awareness of which can facilitate proactive measures to mitigate risks.

Why is a geotech report important?

You want your wall to do the job for as long as you need, right? The more your wall is designed and installed specifically to its site conditions, the more likely it is to perform at a high level for you for many years to come. Think of it this way: If you have to spend your day outdoors and aren’t entirely sure how to dress, wouldn’t you check the weather before choosing your gear? Wall designs that fail to consider site conditions and future considerations can lead to many problems.

A geotech report will contribute to a safer, more effective, and longer-lasting wall and ensure your wall design and installation are optimized to its specific site. Your wall won’t be over-engineered for the intended loads and conditions. Over-engineering can result in a project using larger blocks, longer reinforcement lengths, and more imported backfill material than necessary. It may also prompt unnecessarily deep embedment depths, overly complicated drainage systems, and needless structural complexity. This would likely result in more materials and higher costs than necessary if a geotechnical subsurface investigation had been performed.

What should I expect to spend on a geotech report? Is it worth it?

Like many facets of construction, costs can fluctuate significantly based on your location, the project size, scope and complexity, availability of professional expertise, and other considerations. Depending on these factors, a professionally prepared geotech report could cost as little as $1,000 or more than $30,000. For more accurate figures, request bids from reputable geotechnical engineers in your area.

Don’t let the dollar figures scare you. Good retaining walls aren’t inexpensive, and they often safeguard life and property valued at much more than you can spend on a wall. Smart money invested in project planning can pay serious dividends down the road. A poorly planned project can require numerous costly adjustments throughout its construction and can even result in repair or replacement well before it should. If there’s one thing to take from this article, it’s that a thoughtfully engineered wall project with a little more investment in its planning stages can deliver more value over the course of the wall’s lifespan.