What the TREC Standards of Practice requires of TREC inspectors
The first thing to understand is that the TREC Standards of Practice only requires the inspector to render a written opinion as to the performance of the foundation taking into consideration the age of the foundation and ordinary wear and tear. TREC inspectors are not required to render an opinion as to the need or desirability of repair. Nor are they required to determine if the foundation is performing as intended.
From the Texas Board of Professional Engineers
The Texas Board of Professional Engineers takes the position that if a TREC inspector renders an opinion concerning repair, he or she would be in violation of the Texas Engineering Practice Act unless they are licensed Professional Engineers.
If there are no structural safety or integrity issues, foundation repair should be an option only, not a requirement. This comes from the Texas Section ASCE Guidelines for the Evaluation and Repair of Residential Foundations.
The Texas Board of Professional Engineers has made it clear that engineers should recommend repairs only if such repairs are cost-efficient. Foundation repair is rarely cost-efficient.
Why Engineers Are Reluctant To Recommend Repair
When you underpin a slab-on-ground foundation it will almost always be changed from a ground-supported foundation to an elevated foundation. If the repair contractor lifts the foundation, which is almost always the case, the act of lifting means the foundation is not fully supported by the ground: it is changed from a ground-supported structure to an elevated structure, a condition for which it was not designed.
If the slab is not lifted it will probably become elevated the next time we experience a drought, which is most summers.
Either way, the result of underpinning is to take a foundation that was designed to be fully ground-supported and force it to carry loads as an elevated structure. This seems to work out in many cases, but no engineer should recommend underpinning unless he can show by calculations or testing that the foundation can function as an elevated structure. This is usually impossible due to a lack of data.
When should underpinning be recommended?