One of the most common questions we get from clients is “how can we build our foundations in this waterlogged area?” In this article we’ll cover how to properly build foundations in an excavation that is below the water table. In general you’ll want to:
- 1. Contact a geotechnical engineer to determine a suitable bearing surface and provide site specific recommendations
- 2. Excavate to that level while pumping water constantly
- 3. Build the site back up with washed drainage gravel completely wrapped in a geotextile filter fabric
- 4. Install a sump pit in the drainage gravel and resume pumping
- 5. Continue with foundation construction. Ensure the sump pit is accessible once the house is completed as it is important that it is always working
This process may not be applicable to every situation. It is important to ensure a geotechnical engineer has reviewed your site prior to building foundations or basements below groundwater.
Continue reading for a more in depth explanation on building foundations below the water table. Pumping out the water is a part of the solution, but it won’t be enough to successfully complete the installation of the foundations. You’ll need to do a bit of extra work, but it’s not very difficult to do properly.
Why Is The Hole Filling With Water?
It’s likely that your project is in an area with a high groundwater table. You may have noticed a river or wetland nearby – when you see things like this you can assume that there may be a high groundwater table in the area.
Groundwater tables are typically highest in the spring when snow is melting, there is increased rainfall, and runoff from any nearby mountains is causing rivers to rise. All of these factors contribute to the elevation of the groundwater throughout your entire city.
Can I Still Build My Basement With A High Water Table?
Yes, you can but we recommend talking to a local geotechnical engineer to get site specific advice as there are a lot of variables that will dictate the best way to proceed.
We’ll give you a general solution so you have an idea of what may be suggested to you. It’s important to note that in areas with high water tables its likely that soils are too soft for what your structural engineer has assumed in their design, so make sure they’re aware of the situation so they can talk with the geotechnical engineer.
How To Build A Basement Foundation In A High Water Table
Your first step to getting your site ready for foundation construction below the water table is to grade the site to a low point, dig a hole, and install a temporary sump pump to keep the site drained. The hole should be approximately 1 to 1.5 feet deep. The diameter of the hole isn’t overly important, just make sure there is enough room for the pump to not be contacting the sides of the hole.
We will provide a general solution that will work in most situations but not all. A local geotechnical engineer who has visited your site should provide final recommendations as soft soils can can excessive settlement and damage to foundations. It is possible that less excavation is required, saving you material and labor costs.
This general solution is commonly called a “gravel raft” and can be constructed as follows:
- Excavate an additional 450 mm (18″) below the designed footing elevations
- Line the entire excavation, including the excavation walls, with a non-woven geotextile fabric
- Fabric should be overlapped 300 mm (12″) at all seams.
- You can use landscape staples to hold fabric in place on the excavation walls or base
- Place a sump basin wherever the sump pit will be in the home, on the base of the excavation
- You may need to fill the basin with gravel to keep it in place. Remove the gravel when complete.
- Place weeping tile (drainage tile) along the perimeter of the excavation.
- Port the weeping tile to the sump basin. You don’t need to cut a hole in it, just guide the water to the basin with the drainage tile.
- Backfill with 40 mm (1.5″) washed drainage gravel to the original foundation elevation
- Make sure that the fabric completely separates the drainage gravel backfill from the native soils
- The gravel will be backfilled around the sump basin
- While backfilling, use a vibratory flat plate tamper to gently compact the gravel in place. Not much effort is needed, but the gravel should be compacted to avoid settlement.
- Wrap the top of the gravel with the non woven geotextile, again making sure that the gravel is completely separate from the native materials
- Put the temporary sump pump into the sump basin and continue pumping water
- You are now ready to build your footings
Download our free gravel raft example drawings below.
Free Download: PDF Gravel Raft Example Drawings
Recommended Products For Gravel Raft Construction
Geotextile
Geotextiles come in many lengths, sizes, and most importantly – thicknesses, most of them interchangeable. The 6 oz thickness is sufficient for a gravel raft.
Sump Basin – Jackal SF22A-DR
A sump basin works simply by keeping the gravel away from your sump pump but allowing water to pass through it. Not having something to separate gravel from your sump pump will result in your sump pump sucking up gravel and getting damaged or plugged.
Sump Pump – Zoeller Mighty Mate
The Zoeller Mighty Mate is a bit more expensive that other sump pumps, but is absolutely worth the money. Typically lasting up to 15 years in harsh conditions, this little guy will keep your basement and foundation free of water for a long time. It’s always a good idea to have a backup sump pump in case of a malfunction too!
Conclusion
We hope that this article has helped you in dealing with high groundwater levels within your foundation excavation. If you need help with your new home build, feel free to contact us
