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Re: Crawlspace Sump Pump Installation | Sump Pit

From: myers.steve@sbcglobal.net
Category: Drainage/Waterproofing
Remote Name: 69.227.160.118
Date: 13 Dec 2005
Time: 09:36 PM

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Question: I live on a raised foundation with an 18" (approx) crawlspace. The crawlspace is uneven dirt. I live at the bottom of a large hill and when I get heavy rains (in Southern California), the water table rises and I end up with about a foot of water under the house. The only temporary solution that I have come up with is putting a submersible sump pump in a laundry basket and pumping it out my front door (since the crawl space access is in the house). I am looking for a more permanent solution. Here is what I have come up with so far: buy a submersible, or pedestal sump pump and install it in some sort of basin...not much of a start because I keep stumbling over certain details as follows:

• Once installed I will only have access to the sump pump (if it's in the crawl space) during dry seasons
• Since I'm dealing with rising ground water I assume that my sump pit should not have an impermeable bottom
• Am I deluding myself that I will even be able to pump out as much water as I need to?
• How do I keep silt and debris from entering and clogging the sump pump?
• Does the sump pit / basin need to be perforated? I've heard about covering the basin with some sort of filter fabric but that seems like it would just clog.
• Does my sump pit need a lid?

Thanks for your help.

Builders Websource® Answer: To properly answer your question requires some background discussion of soils conditions and engineering principles. To permanently treat crawlspace drainage and moisture problems requires careful site analysis, proper engineering, and quality planning and workmanship. You may need to hire a civil engineer or experienced drainage contractor to survey the property and make specific drainage recommendations. in some cases, though less common, a geotechnical engineer may perform test borings at certain spots on your property. The soil samples are tested in a lab  to determine the type and permeability of the soil and whether there are unusual conditions that may be contributing to high groundwater on your property. While geotechnical reports are now mandatory on most new construction projects, they are frequently impractical to develop  for an existing home depending on site access to drilling rigs.

Depending on the age of your home and the development in which you live, it's possible that a geotechnical report already exists. Check with your city building department. Or, if any of your immediate neighbors have a newer home, check with them or their architect if they have a copy of the geotechnical report. If you find such a report, often you can glean important information that can give you important clues. Generally a geotechnical report will identify any unusual conditions (such as subsurface aquifers, high water table, etc.) and make specific drainage, grading, and seismic recommendations. While this information may not apply directly to your lot, it will give you a sense of the geotechnical conditions in your general area. For example, sandy soils drain quickly and retain less water. Clayey soils such as is common in many parts of California (particularly around the San Francisco Bay Area), absorb and hold extreme amounts of moisture, causing ground swelling and foundation damage during the wet season.

Although you say the water table rises during heavy rains, you may also be experiencing sub-surface drainage problems due improper exterior grading and drainage management. After extended rainfall, the exterior ground becomes saturated due to rain and runoff from nearby hills and downspouts. Hydrostatic pressure causes water to find its natural equilibrium following a path of least resistance. During heavy rains, the first few feet of soil can become saturated. If the saturation level drops below the bottom of your footing, pressure will force the water up on the interior side of the footing, resulting in high crawlspace humidity or worse, standing water in your crawlspace. Since most crawlspace floors are plain dirt, the water readily percolates to the surface. This moisture can wreak havoc with your foundation, crawlspace, sub floor, and infiltrate your home, causing high indoor humidity. Furthermore, since most crawlspace elevations are at least 12" below the exterior grade, the crawlspace acts like a reservoir surrounded by concrete on all sides. The concrete traps the water until such time that it evaporates or recedes to normal levels.

While sump pumps are often a necessary component of a complete drainage management solution, they are prone to mechanical failure, power outages, clogging, and frequent cycling. If the pump is in a relatively inaccessible area, maintenance will be awkward, leading to future failure. Where possible, we recommend starting with the exterior first. It's always best to intercept the water before it enters the foundation area. While this is not always practical, it's best to slope all landscaping and hardscaping away from the foundation with at least a 2% grade. Consider installing a curtain drain along the portion of the house near the hill coupled with a complete perimeter French drain system. The French drain will intercept water along the footing before it has a chance to enter the crawlspace. The French drain should be its own system (not tied to any other downspouts or yard drainage). If possible, use gravity to allow the French drain to flow out to the street or some kind of exterior sump pit (depending on the elevation of your lot). Furthermore, a comprehensive drainage program includes a separate, closed drainage system to catch surface runoff as well as from downspouts. The goal is to keep all water away from the foundation. The combination of the dual drainage system (surface drainage plus subsurface perimeter French drain) normally cures most foundation drainage problems.

On occasion, even the above precautions fail to solve all crawlspace moisture problems, although this is sometimes due to improper upfront engineering and/or poor installation and workmanship. Or, if you live in an area with a truly high water table (such as at or below sea level) or in an area with a natural aquifer underground, external drainage will do little to control moisture under the home. Still, In other cases, it may simply be impractical or too costly to tunnel through mature landscaping and other fixed obstacles on the exterior such as decks, driveways and walkways. In these cases, a crawlspace sump pit may be your best hope. In any case, you should always divert your downspouts well downstream of your foundation. Flexible corrugated extension pipe is inexpensive and often does the trick. It's surprising how many homes dump all the rainwater from their roofs right alongside the foundation. Where is that water going? It's going under your house, creating a muddy mess.

Depending on crawlspace access, the crawlspace dewatering approach can be somewhat invasive. Generally, a suitable access point is required. An inconspicuous location such as a closet floor is usually your best bet. Assuming your lot is generally level and you've selected a suitable location (with direct access), the process involves excavating a ground pit approximately 2' x 2'  x 2'. Line the pit with a permeable geotextile filter fabric and backfill with washed pea gravel. In the middle of the sump, you'll install a sturdy plastic sump pit approximately 18" in diameter by 24" deep. A proven system is available from Basement Systems called a SUPERSUMP®. This is a turnkey unit that includes a round plastic sump pit called a SuperLiner, a 1/3 HP 2650 gallon/hour Zoeller® sump pump, an optional sump pedestal, an airtight lid and an overflow alarm. The sump pit has a solid bottom and is perforated along the sides. No perforations are required on the bottom of the sump pit as hydrostatic pressure will force the water up through the drain rock along the sides of the sump pit. For the ultimate protection, the company also offers a TripleSafe™ sump pump solution that includes both main and back-up pumps (Zoeller M-53 1/3 HP + M-98 1/2 HP) plus a tertiary battery-operated pump in the event of a total power loss. The pumps are staggered to turn on in sequence depending on need. Keep in mind that the most likely time for a power failure is precisely when you need power the most: during a heavy storm. The TripleSafe system gives 24x7 peace of mind. However, all sump systems require periodic maintenance (at least annually) to inspect for debris, clogs, and clear operation.

The sump pump has a float valve with built-in hysteresis. As the water level rises in the sump pit, the pump automatically activates. As the pit drains and the water level drops a few inches, the pump shuts off automatically to protect the pump from overheating. You'll need to find a suitable discharge point for the 2" plastic drain line. Be sure to divert this well away and downstream of your foundation, otherwise you'll just be recycling the water endlessly. Do not discharge your sump into your sewer or septic system. This is illegal and could allow sewage to backflow into your sump pit, and crawlspace. Of course, you'll need to supply a GFCI-protected power source for the pump. Since pumps have high turn-on inrush current and can use considerable energy during normal operation, a dedicated 15A or 20A circuit is generally required. This will require a new home run to your electrical subpanel. Be sure to follow all codes and regulations and do not compromise safety when it comes to electrical installations, particularly those subject to moisture and water.

For additional information, download and read our comprehensive technote on RESIDENTIAL DRAINAGE AND BASEMENT WATERPROOFING. Whether you do the work yourself or hire a professional contractor, the information in this technote provides valuable insight into proper drainage considerations.

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