There are so many different ways to approach residential upgrades that it's difficult to discuss in general. In this specific incidence it's not clear whether you are removing the old structure and adding vertically, or if you are adding to the foundation making it larger. Either way, it has to start with a geotechnical report and use that information for directing the design.
It is a little surprising that for a $12MM+ residential modification, the helical piers was cost prohibitive. It is important to spend what is necessary for a substantial foundation, the remainder will be at risk. Here in north central Texas, where we build on deep, highly expansive clays, it is not uncommon for the foundation to be in the neighborhood of 10% of the cost of a building.
Most residential modifications include adding on to the foundation. There the challenge becomes getting the foundation to all act as one. That requires the soil report, of course, but also some sort of determination of what the existing foundation consists of. Then design a foundation that works with it. Simple, right?
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Jerry Coombs P.E., M.ASCE
President
Coombs Engineering
Plano TX
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Original Message:
Sent: 09-05-2019 12:11
From: Luis Duque
Subject: Working on Existing Foundation for High End Custom Residential Structures
I am currently finishing a project for a $12 million+ residential structure over an existing foundation. I want to open a discussion around this topic to read what others have done in the past. The current foundation requires drilled piers and grade beams due to the expansive nature of the soils, making the design a little more complicated.
At first, we were suggesting adding helical piers and underpinning helical piers to support any new loading but this turned to be cost prohibited for the customers. We then decided to do some testing to figure out the depth and reinforcement of the piers as well as the foundation walls. After the testing was done, we were able to eliminate all the underpinning helical piers as we were able to check the grade beams and reinforce them as needed.
There are many challenges with re-using foundations, especially for high-end structures. What have been some of the challenges you have faced when re-using foundations for residential, commercial, or other types of structure?
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Luis Duque EIT, A.M.ASCE
Structural Engineer
Broomfield CO
<maskemail>luis@...</maskemail>
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