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  • 1.  Working on Existing Foundation for High End Custom Residential Structures

    Posted 09-05-2019 12:14 PM
    Edited by Tirza Austin 09-05-2019 12:13 PM
    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
    luis@...
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  • 2.  RE: Working on Existing Foundation for High End Custom Residential Structures

    Posted 09-06-2019 07:46 AM
    Edited by Tirza Austin 09-06-2019 10:35 AM
    In 2017 we strengthened foundation of a 17 story residential building in Dar es Salaam, Tanzania, comprising 800 thk Raft by drilling 10m deep 75mm dia. micro-piles of 300kN capacity each into Sandy Clay soil strata. Micro-piles were chosen because basement with its limited by height could be accessed by the drilling equipment.
    Depending on column loads on Grade beams, cost of drilling micro-piles, this option may be worth evaluation. 
    Regards,
    Pendharkar 
    ------Original Message------

    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?

    ------------------------------
    Luis Duque EIT, A.M.ASCE
    Structural Engineer
    Broomfield CO
    luis@...
    ------------------------------


  • 3.  RE: Working on Existing Foundation for High End Custom Residential Structures

    Posted 09-09-2019 10:41 AM
    It is very common in my area for existing residential foundations built on expansive soils to be reinforced and supported with high compression strength cylinders, 8000psi 6" dia x 12" height.  They are spaced approximately 3'-5' on center under existing load bearing grade beams and driven to bearing.

    This method is very popular because it uses inexpensive equipment to install and doesn't require much demo on existing slab. 


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    Nicholas Carrillo P.E., M.ASCE
    Principal Engineer
    Bay City TX
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  • 4.  RE: Working on Existing Foundation for High End Custom Residential Structures

    Posted 09-13-2019 08:36 AM
    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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  • 5.  RE: Working on Existing Foundation for High End Custom Residential Structures

    Posted 09-15-2019 02:40 AM
    ASCE has online course (members can access through benefit list) pertaining to reinforcing existing foundations, Might be helpful in this case....

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    Ammar Fakhoury P.E., M.ASCE
    Riyadh
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