To answer the question, you first need to know what you get with a soils report and what you do not get. Soils reports are based on samples. To obtain samples, a hollow tube, several inches in diameter is pressed into the ground. The tube is then pulled out of the ground, the contents of the tube are removed and sealed in plastic, to keep the water in and contaminants out. Another tube is pressed in and a sample is taken from deeper soils. The process is repeated until the tubes can no longer be forced into the ground or until a predetermined depth is reached. Once the borings are complete, the samples are taken to a lab to be analyzed.
Soil Sample Process
Back at the lab, samples are taken from the samples collected in the field. (That is correct, you do not actually test all of the material returned to the lab.) Samples are taken from the soils collected at different depths. The soils are analyzed for basic physical properties. The samples are dried and passed through screens to see particle size, the samples are also dried to see how much water they contained, and they are also allowed to soak up water to see how much they can swell up. One test measures the pressure the samples can generate as they absorb water and swell. The samples are also tested for hardness, to see how much water they must absorb to become plastic and how much water they need to absorb to become a viscous liquid. The last two tests are referred to as Atterberg Tests.
Because it is not possible to test all of the soils, samples are used. An implicit assumption with the process is that the samples accurately represent all of the soils. Of course, this is not always true. As a result, when the physical properties of the actual soils under a home vary from the samples taken in the borings, there can be performance problems. This is when foundation repairs can become necessary.
Generally, two borings are done. The process takes one to three weeks, beginning to end and costs from $2,000 up. The price increases with the number of borings, the number of samples tested, and for additional types of tests. The information is typed up in the report.
If you are going to repair your home using drilled piers, the information in a soils report can be very helpful. Drilled piers are often used in new construction, and engineers typically use the soils reports to determine the depth, diameter, reinforcing, and other design characteristics of drilled piers.
For contemporary foundation repairs, the standard technologies use techniques that take advantage of modern knowledge of soils and structures. For example, consider helical piers and steel pilings. Helical piers are steel shafts that pass through round metal plates. The plates are cut and bent so that they act like the threads on a screw. Helical piers are literally screwed into the ground. The deeper a pier goes, the more force is required to turn the shaft. Millions of piers have been installed. Data has been collected. Tables have been prepared that show how much weight a specific helical pier can support given the force, referred to as torque, required to screw it into the ground. For pressed steel pilings, the pilings are pushed into the ground. From experience, the installation pressure required to support a given load is known. When installing steel pilings, you just need to drive them until the required pressure is achieved. Typically, the goal is to drive a piling into the ground with a force capable of supporting twice the load that will be rested on a pier.
Soils reports typically do not provide information that is useful when designing a plan of repair for a foundation. At the same time, the information in soils reports is not useful when installing piers or pilings. Of course, if you plan to use drilled piers for any purpose other than a minimal repair, soils reports are needed.
The average residential foundation repair in Texas costs about $8,000. Getting a soils report would add 25%, or more, to the cost of the average repair, without providing information that could be used to improve a repair. As a result, few contractors use or recommend soils reports.
As a final thought, remember that a soils report was done on your lot or subdivision before your home was designed and built. Despite the best efforts of your builder and your builder’s engineers, foundation repairs are now needed. Soils reports are enormously useful and are required for new construction. Because they use sampling techniques, there is a margin of error. If the first report for your home, used when it was built, did not enable you to avoid foundation repairs, another report will not necessarily help.