Foundation Design in permafrost soil condition
Have you ever design any foundation / grade slab in northern hemisphere of the globe, like in arctic zone, where most of the time in a year soil is covered with snow? If not, then you can read this page and get a preliminary idea of how to handle this situation.I hope you know the definition of soil permafrost condition. Due to sub-zero temperature of the weather for maximum time of a year, ground water inside the soil gets frozen. As a result, soil heaves out from its normal position because of increase in volume. So you can understand that a foundation slab can experience a huge uplift / upward force (approximately 50 mm upward heaving of soil) due to this abnormal condition of soil. However, don't worry, there are some ways to handle these situations.
To calculate the frost / snow load on foundation, you need to follow the below mentioned steps:
Step-1: Soil / Geotechnical data:
Collect the following data from the soil consultant:
- Soil frost depth
- Adfreeze force pressure
- Frost heaving pressure
Adfreeze force pressure: This is a friction force acting along the depth of foundation mat and along the height of pedestals.
Frost heaving pressure: This is an upward force acting on the bottom surface of the foundation mat.
Step-2: Check 3D model for foundation space and any interference:
Before starting the foundation design, please look into the following:
- Any underground pipes, cable trays or small foundation in that area and any insulation provided.
- Any heated building close to that foundatio
Case - 1: When bottom of foundation below frost Zone:
Please look into the above sketch. In this case there is no frost heaving force acting on the foundation. Only adfreeze force (friction / drag Force) will act on pedestal / mat surfaces in upward direction. You can eliminate this force by providing impregnated board (bond braker) around the foundation and by back filling foundation with non-frost susceptable material. At finish grade level, slope shall be maintained in such a way that no water accummulation will happen around the foundation.
Case - 2: When foundation is in frost Zone:
Please look into the above sketch. In this case, you need to provide rigid insulation (say HI-40 styrofoam) at the top of mat. In this way, you can reduce the frost depth and eliminate any frost heaving forces (Upward direction forces) at bottom of mat foundation. Only adfreeze force (friction / drag Force) will act on pedestal surface in upward direction. You can eliminate this force also by providing impregnated board (bond braker) around the foundation and by back filling foundation with non-frost susceptable material. At finish grade level, slope shall be maintained in such a way that no water accummulation will happen around the foundation.
Case - 3: When foundation is resting on pile and also in frost Zone:
Please look into the above sketch. In this case, you need to calculate the above two types frost loads on foundation.
Adfreeze Force : Adfreze force will occur on pile surface and as well as on mat & pedestal surfaces. This is a friction dragging force, acting in upward direction.
Adfreeze Force = Adfezze pressure x (pile surface area + mat surface area + pedestal surface area)
Frost Heaving Force : Frost heaving force will occur on bottom surface of mat. This force will act in upward direction
Frost Heaving Force = Frost Heaving pressure x Plan area of pile cap (mat).
You can reduce this force by providing void form (which absorbs the heaving pressure from soil). Example: Geospan and Ethafoam are some types of void form available in the market.
Total upward load on foundation (tension on pile) = Adfreeze force + Frost Heaving Force
Piles need to be embeded below frost zone to take care of this tension.
Please note that void form is not the rigid insulation. We provide void-form below the mat foundation to absorb the soil heaving pressure but we place insulation at the top of mat to minimze any effect of frosting below the foundation mat.
Case - 4: Underground services in frost Zone:
Please click here for the sketches.
Please note that frost shall not be added with any transient load (wind or seismic). For factored load design, please consider live load factor for frost load.
I hope this page will be very helpful to you to understand the basic foundation design in frost condion i.e. in snow covered area.
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Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the explanations are taken from different text books and international design codes, which are referenced in the contents. Any similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.