Basement Flooding, Sump Pumps, Water Table
1. Dig a hole and you might hit water. The quickest way to see this is to do it at the beach, just above the flat wet sand area where the waves are coming in. (If a wave comes up and swamps your hole, start a new hole somewhere else. You want to see water in the hole without having poured any in yourself.) If you pump or suck the water from the hole, the hole quickly refills with water from the surrounding water soaked soil or sand.
The level of water in the hole represents the water table, the depth below which the ground is soaked with water. The water table may vary from place to place depending on how porous the soil is and how close you are to a pond or stream. The water table also rises when it rains.
2. If the water table is above the level of your basement floor and you do nothing, your basement will get flooded. It is impossible to keep the joint where the walls meet the floor completely sealed all the way around . Also, concrete itself is somewhat porous and moisture might come right through the concrete floor.
3. By digging a pit (a sump) in your basement floor and installing a sump pump, water that would otherwise seep up into your basement will collect in the pit. The water has a tendency to drip out of the soaked soil nearby and into the air space which is the pit (provided there are holes in the sides of the pit). The level where the remaining water soaked soil begins is lower than it was before, or we can say that the water table in the vicinity of the pit has been artificially lowered.
But the water table is artificially lowered for only a short distance on each side of the pit. This distance is greater if the soil is porous (sand and gravel are more porous than clay). The white line through the blue indicates where the water table might be if there was a layer of gravel under your concrete basement floor as opposed to ordinary soil. Without porous material underneath, you might need a pit at each corner of your house to collect enough water fast enough that the soil dries out on all four sides.
4. Perimeter drain systems (also called French drains or weeping tile systems) make it possible for the water table to be lowered artificially all around the house foundation while having just one sump pump pit (or maybe two for a very large house). The drain pipe is perforated all along its length and is normally placed at the level of the foundation footings. It forms an air pocket all around the foundation into which water in the surrounding soil can drip, drying out the soil somewhat. Again, the water table is artificially lowered only a few feet to each side of the air pocket (drain pipe and any surrounding coarse gravel) but this should be enough to put the artificial water table below the basement floor and then no water will seep up into the basement. In a real life situation, the water table under the middle of the house (note the red arrow) should be a little lower than the blue area suggests since that area is sheltered from the rain.
Usually the drain pipe is surrounded by a few inches of gravel rather than having sand or soil packed around it. This retards the flow of dirt into the pipe that might clog it and also provides a larger air pocket for water to collect in and make its way to the pit.
A perimeter drain system just outside the foundation performs a little better than a system just inside the foundation. It doesn't take as long to lower the water table near the foundation. But the inside system is usually easier to install after the house has been built. Do not dig below the level of the bottoms of the footings close to the footings when installing a new perimeter drain system for an existing building since disturbing the soil that far down can cause the foundation to move. There should be at least a 1:3 slope of undisturbed earth where, for example, if you want to dig the trench bottom to be 4 inches below the bottoms of the footings, then the entirety of the trench has to be at least 12 inches away (measurements not on centers).
Use porous cloth (weed control cloth will work) to keep dirt from getting into the drain pipe but we suggest not wrapping the drain pipe with the cloth. Instead the cloth should be cut wider and laid in the trench first (or maybe on top of a small amount of gravel or sand. Then add gravel and lay the pipe and add more gravel. Finally bring the edges of the cloth around the top of the pipe. Some gravel can be put "outside" the cloth to make it easier to get the cloth in position around the pipe with gravel in between.
(updated 4/12) It is recommended that the perimeter trench not be allowed to fill completely with water, which would happen if the sump pump does not come on soon enough and the pit became nearly full. Without a continuous air space extending all the way around the foundation to the pit, the surrounding soil will remain soaked as opposed to let water drip into the drain pipes. The water table would revert to be more like diagram #3 as opposed to diagram #4. For starters we suggest having the sump pump come on before the drain pipe ends are more than half submerged.
Sometimes you have a dilemma. The water in the pit might seek a constant level just a few inches below the basement floor with the pump not running at all whereas keeping the drain pipe ends clear of water may result in constant running of the pump. You can leave it that way with the pump turn on float set high, but keep a close watch on the floor at the far side of the basement. You might even have two start floats and switches with a manually operated master switch on the wall to select the pump starting water level. During dry season the higher level start switch is selected to allow the water table to fluctuate more without starting the pump. During rainy season the lower level start switch is selected so less water accumulates in the perimeter drain pipes and desaturation of the soil at the far side of the basement is improved.
If the system was not operating or could not keep up with the water flow, and the basement flooded, then it may take a few days before the water table is artificially lowered again and the floor dries out.
We are suggesting a pit size of at least 18 inches square and 18 inches deeper than the bottom edges of the perimeter drain pipes within, or providing about 3 cubic feet or 20 gallons of space below the drain pipe openings and not filled with stones or gravel. This should keep the pump from starting and stopping too frequently, pumping out only a small quantity of water each time. If the pit is too small and also the water level was permitted to rise above the drain pipe bottoms, the pump may empty out the pit out and stop before the perimeter drain pipes have had enough time to empty themselves into the pit. Then the pit will start to refill quickly after the pump shuts off. In addition, if the check valve in the outlet pipe does not seal perfectly, some water in that pipe will come back down into the pit. A larger pit means that a greater amount of new water will have accumulated before the pump starts again. To take advantage of a pit this deep, the sump pump must have adjustable on and off water levels.
Water from gutters, washing machines, surface rain runoff, etc. should not be drained into a perimeter drain system or a sump pump pit. This will overload the system. In addition, water softener brine and washing machine suds can cause corrosion and shorten the life of the sump pump. Water is supposed to come from the perimeter drain pipes into the pit to be pumped out, not out the other way to soak into the ground all around the foundation. (A system of perforated underground pipes used to accept water to soak into the ground is called a leach field. It may or may not be constructed in the same manner as a French drain depending on what liquid is meant to be poured into it.)
Sump Pump Operation Dynamics
Sometimes the homeowner finds that the pump runs all year 'round when setting the turn on level higher will let the pump stay off until rainy season. This is because the water table is just slightly above the turn on level. But setting the level too high can result in very short on off pump cycles during rainy season.
If the drain pipe ends start to become submerged then water will accumulate (we could say "queue up") in the drain pipes although the pit water level is not high enough to start the pump. When the water level gets high enough the following events typically occur:
1. Pump starts.
2. Pump runs while water queued up in drain pipes pours into pit.
3. Pit is almost emptied and pump stops.
4. More queued up water pours from drain pipes into pit.
5. Just new water trickles from drain pipes into pit.
Step 4 occurs when the pump empties out the pit faster than the queued up water can pour out of the drain pipes. A shorter step 4 above means fewer and longer pump on-off cycles. The more submerged the drain pipe ends ger prior to pump starting and/or the smaller the pit is, the greater the chance of a long step 4. (If the pump starts before the water level reaches the drain pipe ends then steps 2 and 4 do not apply.)
Hopefully you will have laid your perimeter drain pipe with a better pitch then shown (exaggerated) above. For most basements it is difficult to have much in the way of pitch because the vertical distance between the bottoms of the footings and the bottom of the floor slab is not very great. Usually you won't be able to have the normal 1/4 inch drop per foot of run. The system will work with almost no pitch provided that there are no significant low points along the way.
The entire gravel filled trench acts as a channel to bring water to the sump pump pit. So long as the water level is below level C, leaving some air space under the floor, there should not be any flooding above floor level although this is not guaranteed. The pipe, with empty space inside, speeds up the flow of water to the pit. If the drain pipes as seen in the pit are less than half submerged and are not really badly pitched, the trench should not be saturated even at the far corner of the basement.
When the water level is above any high point in the pipe (shown as B and by red arrow), there should be a good flow from that point towards the pit. Once the level gets below B, the water will still drain to the pit although much more slowly, provided that there is gravel rather than plain dirt or sand under the drain pipe . The water then enters the pipe through perforations in the pipe near the pit, and then pour into the pit.
Level A represents that of the perforations in the drain pipe near the pit. Water below this level (shown as dark blue) will tend to stay behind . It may find its way to the pit slowly if there are additional small holes in the sides of the pit and the soil is porous. More likely, this water would remain in place until it can seep back into the soil below after dry weather returns and the water table goes down.
What is a French Drain?
There are numerous articles on the Internet about French drains. To summarize, it is a sloped almost horizontal channel in the ground intended to carry away water by gravity flow. Typically it is filled with gravel to keep the soil above from collapsing into it (or to make it more attractive looking and keep people from falling into it if it is at the surface). Usually it has a perforated drain pipe or loosely joined short pipe sections (drain tiles, weeping tiles) embedded in the gravel to speed the flow of water. It gets its name because a Massachusetts farmer and judge named Henry French wrote a book about drainage systems including this type of system.
Water does not rise up into a French drain, but rather the French drain must be installed below the level where there is water we wish to get rid of.
At least one Internet reference described a French drain as a surface channel (which could be a gently dished "walkway") and which was often seen outside and around the foundations of older buildings in France. Coincidentally this is the degenerate form of a gravel filled drainage trench although the example shown was not filled with gravel. The purpose is the same, to carry away (here, surface) water; this channel must also be sloped and lead "somewhere" so that the water can be gotten rid of.
Definition. Sump -- A "low point" where water or liquid is supposed to collect by gravity flow and then be pumped out. It could also be the oil pan of an internal combustion engine, or the reservoir under a fountain for recirculated water.
Belly -- An unwanted low point in a not quite straight drain pipe where water and also solid material can collect.
Last updated April 12, 2012
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