Soil Reports: What do they really say?

By Christopher E. Carter, P.E.

Soil investigations are a necessity before building upon the earth. You need to know the soil properties you’re dealing with, so that your structure will perform as designed and intended. Look at the soils in our own country. In the north, there are silty soils vulnerable to frost heave. In the east and southeast, there are expansive soils, trash pits and sinkholes. In the west and southwest, there are expansive and collapsible soils. And anywhere in between, there can be poorly compacted fill.

One of the challenges in deciding how to build on these soils, is often in understanding the soils report itself. To demonstrate, here is an example of one sentence I randomly pulled out of a past report from our area: “The frequency and severity of damage caused by swelling of the steeply dipping bedrock are higher than for sites where bedrock is found with similar swelling characteristics in a more flat-lying orientation to the east.” Most Denver-metro geotechnical (soils) engineers after reading this would say, “Duh, everyone knows that!” But many industry folk, especially those new to Colorado, would ask, “Is that good, or bad?”

Here are some other common phrases I’ve selected from local soil reports, given with my interpretation, to better illustrate the challenge at hand:

“On low and moderate swell sites, slab heave of 1 to 3 inches is considered normal and we believe in the majority of instances, movements of this magnitude constitute reasonable slab performance.”

Interpretation: Don’t attach anything to, or support anything on, these slabs.

“Additional foundation movements could occur if water from any source infiltrates the foundation soils.”

Interpretation: Don’t build here.

“To reduce the potential for perched groundwater to impact foundation bearing soils and enter the basement of the residence, installation of an exterior perimeter drainage system is recommended.”

Interpretation: This foundation is going to settle and leak even with a drain.

“We judge future pier heave of 0 to 4 inches is possible with about 0 to 3 inches of movement more likely.”

Interpretation: Maybe I should consult the weatherman.

“The following procedures will help improve slab performance, minimize movement and prevent damage if some risk of movement can be tolerated:”

Interpretation: How much risk is the homebuyer going to tolerate?

The key to (really) understanding a soils report is to sit down with your geotech and review the findings. This will allow you to ask probing questions, and sift for information more often found between the lines.