Why Some Piers Settle!

Take a look at the picture folks!!  Excessive pier settlement (7+ inches) was a direct result of an undersized underream (design - 76 inch diameter; constructed - 55 inch diameter) and an abundance of loose material (clods) in the bottom of the pier at the time the concrete was placed. (The pier was not cleaned of loose debris, hence the large voids in the concrete.)  The design was based on 4,000 psf, total load.  Actual load at the base of the pier was around 7,000 to 8,000 psf, dead load only. 

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Shrinkage Cracks and "Wrinkled" Grass

The ripples visible in the grass are associated with partial closure of shrinkage cracks in the subgrade at the Indian Creek golf course.  This photograph was taken the Saturday after a tropical storm (Herminie?) went through the Dallas/Fort Worth area.  We had a prolonged dry spell, followed by 7 inches of rain over about 48 hours.  Because the golf course had been watering, the grass grew across the shrinkage cracks.  When the shrinkage cracks partially closed because of the sudden rainfall, and because the the course was too wet to mow, the "wrinkles" in the grass could be observed.  (I checked, and a shrinkage crack was below each "wrinkle" in the grass.) First time I have seen this this clear in 25 years. 

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Suspended Floor and In-Ground Plumbing

One of the solutions for a building in expansive soils is to use a suspended floor supported on drilled piers.  A void is constructed between the floor and underlying soil to allow the soil to move.  Why then, do plumbers think it is ok to put the plumbing in the ground, extending through the suspended floor with no allowance for differential movement of the plumbing?  This is nuts!!

The Role of the Expert

The consultant’s role, in the writer’s opinion, is to evaluate each case and develop probable or likely opinions as to the causes of the observed behavior or legal claim.  If this opinion supports his or her clients claim, so be it.  If not, the consultant must be honest and forthright with the client.  To be less so, is simply selling the profession to the highest bidder. 

Factor of Safety Factor

A question recently arose regarding the meaning of Factor of Safety (F.S.). Specifically, the question was, is the F.S. for the benefit of the design engineer, building owner or contractor. For example, if the structural engineer states that a design has a factor of safety of 1.5, does that mean that the owner can overload the structure without concern of failure? Or, is the F.S. added into the design calculations to account for construction variances, loading unknowns, and the inherent variation in material properties? 

In historical soil and foundation literature, the design factor of safety is dependent upon the reliability of the design loads and information regarding soil strength. In other words, a higher factor of safety is recommended in design where the loads may be hard to fully define and or the soil strength may not be fully known. The literature clearly discusses the design aspect, not the “as constructed” condition.