Posted by Johnny F. Flores P.E. (TX).
According to legend, during the construction of McNeil High School in Round Rock, a suburb north of Austin Texas, a bulldozer punched through the roof and collapsed down into a cave.
During the widening of RM 620 in Round Rock, Texas, a trench excavation for the storm water line exposed a cave the size of a small house just a few feet underneath the ground surface.
Inner Space Caverns in Georgetown, Texas was discovered during the geotechnical exploration phase for Interstate 35 in Georgetown, Texas.
If you are planning on building a bridge or highway embankment or a building over the Edwards formation, you should be on the alert for caves and karst features.
Environmental regulations notwithstanding, large parts of Austin and Central Texas are built over the caves of various size, bedrock riddled with voids, or even clay filled voids, or very porous rock. Collectively, these features are called karst features because they are all related to the mechanism of the rock minerals dissolving in water over time.
To reduce your risk of a nasty surprise, before you acquire the property if possible, or at the earliest stage of design as possible, you should get a thorough geotechnical study. The geotechnical study should be conducted by qualified geotechnical engineers and their focus should be on characterizing the engineering properties of the bedrock and developing design and construction recommendations for your project. You should share the project details with your geotechnical consultant so that he or she understands the location, the loads, and the constraints of your project. The geotechnical study could include desktop studies, reviews of existing information, and site reconnaissance to understand the site physiography. With this information in hand, your consultant can develop a drilling and sampling program, a laboratory testing schedule to develop an understanding of the lithology and engineering properties of the soil and rock that make up the underground.
Depending upon the size and scope of the project, a geophysical survey, for example combining Ground Penetrating Radar and Electrical Resistivity soundings, could be strategically used to survey large areas very quickly and economically compared to drilling, and identify areas that may be more likely to contain karst features, such as voids and old collapse zones. With this information in hand, properly interpreted, a more focused drilling program can be directed to truth these apparent anomalous areas where necessary. The take away is that geophysical methods are useful for scanning, and borings are useful for design.
Foundations in Karst Areas
Drilled shafts, designed for side friction, are commonly used for high concentrated loads such as bridge piers or building columns. These shafts can bypass clay layers and voids, and develop resistance based on friction between the bedrock and the hardened concrete, without reliance on the tip of the shaft to develop end bearing capacity. You need to have a qualified engineer or technician log the drilled shaft excavation to document the proper amount of limestone has been encountered.
The above approach has advantages over the design and construction of shallow foundations over known voids. With shallow foundations, a high degree of certainty is required to support loads over the roof of the cave. Assuming this risk is easier if the voids are very deep, and much more difficult if they are shallow. What is meant by deep or shallow? The answer – it depends.
It may not be practical to support your structure on drilled shafts. If your structure covers a large area, for example a water tank, or an embankment, then the underlying network of voids, fissures and clay filled seams can be improved by compaction grouting. This subject is so broad it cannot be covered in this post. But, it is sufficient to say that this technique involves injecting grout, under pressure, into an arranged group of holes to a particular depth, to fill in voids and compact soft layers. This work is usually carried out by specialty contractors. In a more broad-brushed fashion, large caves can be filled with lean concrete or controlled low strength material or grout, in much the same way a dentist fills a cavity.
