Austin’s Rock – Explained.

Geotechnical Engineering

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.

 

 

3 Proven Ways to Tame Swelling Soils

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Posted by Johnny F Flores, P.E. (TX).   If you are building a bridge, a road, a parking lot, a school, an office building, an apartment complex,  or a  high-rise you need to watch out for swelling clays.  Swelling clays, also called  “expansive soils”  can cause severe and costly damage,  by exerting jacking forces that crack concrete and bend steel.

You need to take steps to tame them or design for the eventual movements they will cause.  But designing for these forces comes with significant added cost. Figuring out how much the above ground parts of your project cost are usually straightforward.  The prices of materials are predictable and the design is known in advance.  It’s the underground, hidden parts of the world which supports the foundations, the floor slabs and the pavements, the parts that never win architectural awards,  that can add to the uncertainty and surprise both in terms of cost and performance, when it comes time to build.

Here are three proven ways to tame these swelling soils.

Option Number One – Avoid Them if You Can

If you have an alternate site for your project which does not have expansive soils, you should consider it, all other things being equal.  This may mean that within a site, your should locate your building  strategically  to lessen the impact of these soils, or you should select a completely different site, if you have a choice.  Public infrastructure projects rarely if ever have the choice, but building developers and corporate site selection teams can at time exercise choice or use the added cost  associated for dealing with the expansive soils as a negotiation point with land sellers.   So with your geotechnical reports in hand – (see my earlier post), all other things being equal, you can pick the better site.

Option Number Two –  Remove and Replace 

If your site has a thin layer of swelling soil, you might be able to scrape by, literally.  Sometimes the most economical thing you can do is to scrape away the clay and build on the stable competent ground below.  After you remove the clay (and put it somewhere away from your structure), you may have to replace the removed clays with non-swelling material.    If you are lucky and work with an innovative earthwork contractor, the non-swelling material could be mined from a non-critical part of your project site.  Otherwise, you may wind up paying to have non-swelling material, crushed stone or derivative of crushed stone, hauled to your site from a quarry or pit.

Option Number Three  – Bridge Over Them

If you are risk averse or your structure requires a high level of performance – from an aesthetic or functional perspective,  you can bridge over these soils and  isolate your structure from the most destructive expression of these clays, that of swelling in slow motion against a floor slab or some other foundation element. In this case, performance trumps first construction cost and you  need a high  performance foundation system.  For this approach,  shafts are drilled  down past the swelling soils into a stable and competent supporting stratum and filled with concrete and steel.  The connecting beams and or floor slabs, supported entirely by the drilled shaft,  are built over a  crawl space or air gap, out of reach of the swelling clays.  Done right, the  clays shrink and swell harmlessly without touching parts of the building sensitive to movement.  Not all movements is eliminated, but generally, this is the option chosen by risk averse owners.  If you think you may need this approach, you also need to make sure that water will not accumulate under the slab.

Every project has  soil features and performance requirements.  The choice of a method to deal with swelling clays is best made with the advice of an experienced geotechnical engineer.

 

How to Build Over the Edwards Limestone

Geologic Hazards, Geotechnical Engineering

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.

 

Six Geologic Hazards to Avoid in Austin

Consulting, Geologic Hazards, Geotechnical Engineering
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Austin is blessed from the geotechnical engineer’s perspective with an extraordinary variety of geologic hazards and  geotechnical challenges that include caves,  expansive clays, running sands, faults and fault gouge, random groundwater levels and man-placed fill  such as found at old quarries filled with who knows what.

If you are new to Austin, (Welcome!), and you are  an architect, structural or civil engineer ,    (or you employ architects, structural or civil engineers – thank you! ) it is only natural to bring  your own  template  of design solutions or expectations from wherever you practiced your profession.

“A savvy geotech report is your first defense against a nasty surprise.”

Everything you know from the ground up will likely transfer very easily.  However, for foundations and pavements you are  best served by a qualified  local geotechnical firm ( i.e. a firm with a long history in Austin)  to help navigate the Austin’s  hidden underground world of geologic hazards and geotechnical challenges.

Here is a list of some, not all, of the  geologic and geotechnical  hazards commonly found in the Austin / Central Texas area:

  1. Caves and karst features – principally encountered in the Edwards formation, rarely in the Austin formation.
  2. Expansive soils -you will find these almost everywhere, but the most troublesome are  the  clays of the Eagle Ford,  Del Rio and Taylor formations.
  3. Running sands – these are found in deep alluvium – particularly downtown near Lady BirdLake.
  4. Faults, and fault gouge – These are found in abundance along the 5- mile wide belt of faults known as the Balcones fault zone.
  5. Man-placed fill – can be found anywhere.
  6. Random Groundwater levels -Just what it says.

Tried and true geotechnical solutions are available  to effectively mitigate these challenges for your project’s foundations and pavements. These solutions include specialized approaches to foundation design and construction.  Each of these will be discussed in a  series of subsequent post to this site.

For your project, a savvy geotechnical study is your first defense against a nasty surprise.

 

Take the Guesswork out of Geotech!

Consulting

Posted by Johnny Flores, P.E (TX). I recently received a proposal request from an out of state organization.  The proposal request contained a site layout plan, a description of the scope of work, suggested boring locations, and suggested boring depths of 50 feet.  It was put together very well – even had a cover letter.  I was impressed.   However, the number and suggested depth of the borings really had no relation to the site geology or physiography or the likely structural loads, since this was a single story building.  A quick check of geologic maps and nearby geotechnical reports confirmed my suspicion.  The project site was located in an area with expansive clay underlain by limestone.  I contacted the organization and explained the above points, suggested they confer with their structural engineer and if the loads were indeed relatively light, they should increase the number of borings and decrease the boring depth.

I am telling this true story,  because it illustrates the importance of using available information, such as geologic maps, nearby geotechnical reports, combined with information about the proposed structural loads and site physiography to develop a savvy field investigation plan.   Geologic maps and their accompanying booklets are written by geologists for geologists.  However, good geotechnical engineers use these regularly in planning investigations and identifying formations.  Excellent geotechnical engineers obsess over them.  With regard to loads, I already knew that similar single story structures in the area had been designed with drilled shafts bottomed in the limestone at depths of about 15 to 20 feet, so more likely than not, a 30 ft deep boring would suffice.

In addition, a quick check from Google Earth Pro indicated the site was flat and level with no trees.  But, it had been previously improved with buildings and parking which were destined to be razed for the new project.   It was obvious from Google Earth Pro images the existing parking lots were showing signs of distress (consistent with expansive soils).  These observations presented additional uncertainty for the site subsurface soil regime and underscored the need for additional borings and perhaps test pits.

The above checks were completed in a matter of minutes at no upfront cost to the client and really helped eliminate the guesswork for the field investigation.

 

5 things you need before you launch your next building project

Consulting

By Johnny F Flores P.E. (TX).  Developers and Owners  from across all  sectors engage geotechnical engineers on a regular basis.  For a successful project, there are 5 things that developers and owners  need to provide the geotechnical consultant to deliver a responsive proposal and report.  Without these 5 items, the geotechnical consultant is forced to make assumptions which may cost money and time.

1)A Site Layout Plan.  This sounds simple enough, but for some fast track projects, a site plan could be an ever changing thing.  The site plan should clearly show  property lines and the limits of the proposed improvement to scale.  And, at such a scale that can be easily read and used by the reader.

2) A Grading Plan.  The grading plan shows vertical relationship of the  existing ground surface to the proposed finished lines and grades.  This plan should include all planned improvements, including finish floor elevations for the structures, and also cross sections for planned civil structures such as ponds, berms, embankments.

 3) Structural Loads.  In the early stages of project development, the structural loads may not be clearly known.  Often times the structural engineer for the project may not have been retained.   At the very least, for a preliminary study, order of magnitude structural loads should be provided.  Subsequently, before final design, structural loads should  be provided to the geotechnical engineer prior to the finalization of the geotechnical report.

4) Schedule. A realistic schedule for the design and construction phase of the project is necessary.  Geotechnical consulting most often includes field investigation by drilling, which, similar to construction, may be limited by site constraints and the weather.  Working around the site constraints may take days or weeks.  Most long delays are due to a refusal of some property owners to grant right of entry, right to clear trees, or right to operate during normal business hours.

5) Budget.  People have been taught in an negotiation, not to be the first to reveal the price they are willing to pay.   Negotiation theory aside,  establishing a realistic  budget will assist the geotechnical consultant in scoping the services to fit.  Here is one big caution.  A geotechnical budget established  for a high rise   project  in Atlanta, may not work in Austin because of differing site conditions.

Johnny F Flores, P.E (TX) is a practicing geotechnical engineer in Austin, Texas.

 

 

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