Hydro-static pressure is a term commonly used to describe the amount of force exhibited by the water under the floor slab. Why would there be water under the floor slab? Because the soil around the house was disturbed to pour the foundation, therefore water will want to seek its lowest point/level which is now the area near the floor slab. Water around the base of the foundation is not uncommon, as a matter of fact, it is anticipated. If the excessive Hydro-static pressure is left un-resolved seepage may occur up through the cracks in the floor. However, the more common point of egress would be the seam between the floor slab and the foundation walls known as the cove joint.
Since the early 50’s it had become code to install drainage tile down next to the footer. Originally these tiles were individual 12″ long 4″ round pieces of baked clay laid end to end. To allow the water to seep into the tile a small gap was left between the pieces. However, since there is no filter material covering these gaps it also allowed for dirt, tree roots and other fines to enter the tile as well. A second problem with clay tiles is that they are almost never glazed like the lines used for sewer installs. Now clay being an organic material naturally breaks down over time and collapses.
In order to rectify this problem most new tiles are made of either polyethylene plastic or ridged PVC. However, if a filter sock is not wrapped around the piping the possibility of a clog will still exist. A cloged, broken or collapsed tile will result in a system failure. Since the system is next to the footer and out of sight a problem is not detected until seepage occurs.
Just as important to a properly functioning weeping tile system is the final destination of the water collected by the tile. 90 % or more of all drainage tile systems carry the water to a sump pump basin.
As the water collects in the basin a sump pump ejects the water safely away. Where it eventually winds up is based on your village codes. The majority of sump pumps discharge the water back to splash. This is just another way of saying back out to grade. More and more cities are beginning to require the discharge be connected directly to the storm drain. This may require a new run of 4″ piping from the house to city’s storm. Unfortunately, if required this is an additional charge beyond the initial tile install.
When choosing the correct sump pump the 2 most important things to consider are the volume of water being pump as well as the style of basin the pump will be located in. However there are a couple other things to consider when looking for your ideal sump pump. One is the head at which the water will be pumped. The head refers to the distance from the base of the pump to the top height of the discharge line just before the line angles towards its final destination. Secondly is the depth of the pit in reference to the bottom of the inlet from the drain tile. In order to prevent the tile system from getting back charged with water you would like the pump to activate before the water rises above the bottom of the inlet tile.
Submersible storm water sump pumps come with one of three different types of float mechanism and a couple different horse power.
The most fail safe float design is what is referred to as a mechanical float. This is the style of float which looks like a small ball/bulb that rides up and down on a small shaft attached to the pump.
Another type would be a tethered switch which is a small bulb on the end of a cable which will rise with the water level. When the bulb turns almost upside down the electrodes in the unit make contact and activates the pump. However being as this float is a free moving extension off the pump it is prone to getting jammed against the side of the basin and not turning on the pump.
The last style of float is a pressure diaphragm. This is a small canister attached to the base of the pump. The premise is that as water builds up in the basin water pressure will push up on the rubber diaphragm causing the two contacts to meet turning on the unit. There are 2 draw backs to this type of float. First the pit will have to be cleaned on a regular basis, if dirt and debris cover the bottom of the float it effects the amount of pressure needed to activate the pump, secondly, this pump will require an average water depth of 14″ in the pit before the pump starts to evacuate.
The horse power needed is based on the total footage of the tile connected to the sump again as well as the volume of water anticipated.
In most smaller homes a good 1/3 hp pump will discharge approx. 36 gallons per minute or about 2400 gallons per minute at a 10 foot head.
An average 1/2 hp pump will kick out around 60 gallons per minute or 3600 gallons per hour at a 10 foot head.
Of course there are other options available for more drastic volumes of water.
While hydro-static water pressure is inevitable with a proper drain tile and sump pump system it does not have to be a source of damaging water seepage.
Written by Walter Slowinski