Solid Hill Country Bedrock... or not?

Post date: Nov 20, 2014 8:38:22 PM

As a Structural, I hear it all the time.  "My building sits on solid rock, why do you have so much reinforcement in the foundation?"  It seems that the common perception is that foundations are over rated and over designed.   There are many reasons this perception is wrong.  Today, however, I'm going to focus on one.  Solid Hill Country Bedrock.  What does bedrock look like?  In our area, the bedrock is a jumbled mass of highly fractured and weathered limestone.  In addition, it has somewhere between 6 and 20 inches of highly expansive clay over it.  Even the areas that have rock outcroppings also have hollows filled with expansive clay just waiting to expand and crack your foundation.  

This happens to be a pile of bedrock.  See how it's little pieces and they are all dirty?  That's the result of weathering.

Here's the largest piece in the pile I could find.  It's only about 14" across in the biggest dimension.

So, I think it is clear that solid bedrock isn't solid.  And all the spaces between the rocks are filled with expansive clays.  That's why I have reinforcing in my foundations.