Posted by Johnny F. Flores, P.E (TX)

“It’s all rock out there. Or is it?”

I spend a fair amount of time studying  rock.  Through my professional practice, I (more accurately, talented and dedicated crews that I oversee) drill cores, measure the cores, describe the core, trim the cores, test the cores.  Then having  done all that, I and teams of engineers that I lead, render opinions and provide advice on what types and sizes, foundations would, for a particular project, work best.   In this post, I describe some of the typical characteristics of the rock formations in Austin – from an engineering perspective and the types of foundations that have proven successful.

Austin’s bedrock can be generalized into the following four basic categories.

1. Hard Rock

2. Mixed Rock

3. Soft Rock

4. Clay

Hard Rock

The hard rocks in Austin are geologically known as the  Edwards  (Members 2,3 and 4 south of the Colorado River), Buda formation and the Walnut (Bull Creek,  Cedar Park and Whitestone members).   When we core, we often find a thin veneer of residual soil that looks nothing like the underlying rock, underlain by a mix of rock fragments and soil that is termed completely weathered limestone, which is further underlain by the harder and less weathered  “parent” bedrock.  It is the harder less weathered bedrock that is the subject here.

The typical  compressive strength of cores from these formations is reported by Report No. 86 to range from about 800 psi to 6000 psi.  In my experience – I have seen some this range exceeded, but the typical  range is useful for discussion.  To determine these strengths in the laboratory, we first core with double tube core barrels using diamond bits to get high quality samples.  Its often difficult to get these samples because the rock may be highly fractured making coring difficult and slow.  But once  obtained, we test the samples in a calibrated compression testing machine, while obtaining stress measurements.

Foundations in the less weathered bedrock can be shallow spread footings, mat foundations or deep drilled shafts. Bearing capacities for shallow foundations typically used in design fall in the range from 5000 psf to 10,000 psf depending on the rock quality.  However, as discussed in an earlier post, if their are karst features involved, drilled shafts designed for side friction can be used to work around, or work through the karst issues.  In the case of drilled shafts, the side friction used for design depends on the design method selected, but can typically range from 3000 psf to 6500 psf.

Mixed Rock

When I drive Loop 360, I can’t help but notice alternating layers of hard and soft limestone exposed in the roadside cuts.  You can see it too, its not difficult to discern. These alternating  layers of the Glen Rose Formation  and also the Georgetown Formation  are examples of  “mixed rock”.

Typical compressive strengths range from 700 psi  to 3500 psi.

Foundations in the less weathered bedrock can be shallow spread footings, mat foundations or deep drilled shafts. Bearing capacities for shallow foundations typically used in design fall in the range from 3000 psf to 5000 psf.  In the case of drilled shafts, the side friction used for design depends on the design method selected, but can range from 1500 psf to 2500 psf.

Soft Rock

Much of central Austin is built over the Austin limestone formation – one of the “soft rocks” in this group.  The others include the Walnut formation (Keys Valley Marl and Bee Cave members) and the Comanche Peak Formation.

Unconfined compressive strengths range from 350 psi to 3500 psi.

Foundations in the less weathered bedrock can be shallow spread footings, mat foundations or deep drilled shafts. Bearing capacities for shallow foundations typically used in design fall in the range from 3000 psf to 5,000 psf if the underlying rock quality is good.  In the case of drilled shafts, the side friction used for design depends on the design method selected, but can range from 2000 psf to 6500 psf.

Clay

The clay formations in Austin share many characteristics. They are relatively weak. The all have shrink – swell potential. They all exhibit some degree of stability – if you are careful where you bottom your foundations.  These formations – and I am  generalizing here because volumes could be written on each one of these formation – include the Taylor formation, the Eagle Ford, the Del Rio formation.

Unconfined compressive strengths range from 14 to 350 psi.

For lightly to heavily  loaded structures, drilled shafts are probably the most appropriate if sensitivity to movement is a concern.  Typical design values for  skin friction can range from 300 psf to 2000 psf depending upon circumstances.

Because these clays may swell- they may produce uplift forces on the deep foundations that tend  to pull up on the drilled shaft.  The magnitude of this force depends upon how much of the swelling pressure is transmitted to the surface of the drilled shaft – which is a fairly complex analysis.  You should obtain the services of a qualified and experienced geotechnical engineer to make this analysis for you.

The values discussed above, although notional, are based on information that is believed to be reliable.  Nevertheless, the values presented above are not recommendations for design. All site conditions and projects are unique and require the services of a qualified geotechnical engineer to set forth an appropriate site investigation and interpretation for the given project.