#ProTipTuesday: How and Why About Bowing Basement Walls
D.J. Clayton, PE is part of the Engineering & Training Team at ECP. He will be sharing engineering topics on our blog so check back frequently for updates from Don.
The house was likely built by a reputable contractor, built to building codes and inspected… So why is the basement wall cracked and bowing?
To answer this question we must learn about what is going on outside of the basement wall. We must learn how the soil moisture content affects the force applied against the basement wall.
Basement walls are designed and constructed to resist the force of damp, or slightly moist, soil against the outside of the wall. Let us say the height of soil against a basement wall is eight feet. The force of the soil pushing against the basement wall is 1,150 pounds along each foot of wall length. Another way to think about this is the force pressing inward on the basement wall is more than half ton per lineal foot. Building codes require design and construction of basement walls to resist this normal amount of force against the basement wall.
So why is the wall bowing?
The answer is excessive hydraulic pressure. Hydraulic pressure is a fancy way to say that there is water pressure against the outside of the basement wall. If water has worked its way into the soil and has saturated the soil behind the basement wall, the force against the wall increases from 1,150 to 2,880 pounds per foot of wall due to this water intrusion.
It is easy to understand that increasing the force on the basement wall from half ton to nearly 1-1/2 tons per foot is the cause of wall bowing. The typical basement wall is unable to resist a force of 2,880 lb/ft without bowing. In nearly every case the distress can be traced back to the existence of saturated soil on the outside of the basement wall. It is not cost effective to build a basement wall that is thick enough, and strong enough to resist 1-1/2 tons per foot along the wall.
What to do?
To solve this problem there must be an investigation into how to correct and prevent excessive moisture behind the basement wall. Afterwards supplement support can be added to the wall to stabilize, and possibly straighten the basement wall. There are two issues here. First is the cause of the bowing wall problem, and second is the stabilization/restoration of the wall.
Let’s look into correcting problems that allowed excessive moisture to saturate the soil outside the basement wall.
There are some things that the homeowner can do to reduce soil saturation near the perimeter:
- Install roof guttering and discharge the roof water away from the perimeter of the house.
- Make sure that planter beds are HIGHER than surrounding soil and that borders are not trapping water near the basement wall.
- Check for good drainage after rain to ensure no water is ponding on the surface near the basement wall.
- Check for leaking faucets and any A/C drains discharging near the foundation and not piped to the sewer.There are several other causes that should be investigated. These are beyond what a homeowner can do. A trained professional is needed to determine if there are any hidden sources of excess water. It is necessary to do a site investigation, soil testing and for the investigator to prepare written recommendations. Some hidden sources of moisture infiltration could be:
- Subterranean plumbing leaks.
- Underground water migration or spring water.
- High water table.
Finding and correcting all areas of possible water intrusion is neither simple nor inexpensive. Without the professional assistance of a drainage expert or registered engineer, it is impossible to find all of the drainage and/or water intrusion problems. Professional assistance will provide the homeowner with a plan to remediate all water intrusion problems.
Do not let a salesman convince you that simply installing some sort of wall anchorage will solve the problem. Without controlling the excessive water, the problem will persist and a simple wall anchor installation will not be successful.
It is most important to determine the cause of the bowing basement wall and to eliminate it BEFORE any wall restoration or stabilization work begins. The bowing wall is a symptom of the problem. The plan for stabilization/repair of the wall could require several different companies and trades, depending upon the findings and recommendations of the drainage expert. For example, but not inclusive;
- If the water is due to seasonal fluctuations of the water table; or subterranean water migration, a “French drain” might be needed. A “French drain” is designed to intercept and direct away any underground water before it can reach the basement wall.
- It is possible that the builder improperly graded the lot. For the homeowner to obtain positive natural drainage, the yard might need additional grading to direct surface water away from the house.
- There could be a plumbing leak or sewer leak causing the unwanted water to collect near the basement wall. Sometimes water has been found to migrate toward the house along improperly compacted trenches were dug for the installation of a water line, gas line or sewer main or underground electric service.
- Water could be seeping from a nearby culvert or a city storm water drain pipe.
Determining the cause of the soil saturation and taking steps to prevent it must be completed before, or with, repair work on the basement wall. It cannot be emphasized enough that for a successful, long term repair it is important that the homeowner understand that correcting the soil saturation problem is the first and most important task. The second part of the project is to repair the bowing basement wall.
What are the repair options?
There are two options for structural repair of the basement wall. Let’s look at these choices:
First. Use helical tieback anchors to support and stabilize the basement wall. In other words the wall is anchored in the existing bowed configuration. The anchors add supplement support to the wall, which will stabilize and prevent any further bowing. This is the less expensive way to support and stabilize a bowing wall. The installation of helical earth anchors provides the added lateral support to counteract any future wall overload due to soil saturation.
If a salesman tells you that he can “possibly restore a bowing wall”, and tells you “not to worry” about drainage corrections. If he claims that his anchors are, “All that are necessary to hold the wall”, you need to consider that this person might be looking for a commission and proposing a viable solution for your problem. Mathematics proves that this “possible” restoration of a bowing wall is absolutely untrue.
Drainage systems can fail, plumbing can leak in the future and geological/climatic conditions could increase the hydraulic pressure to a higher level in the future than it is today. It is imperative to do the job right and rely on trained professionals. The good news is that if the job is done properly, it is fixed once and for all time; no future concerns, movements or problems.
Second. The second repair method is to restore the wall back to a “like new” vertical condition using the wall anchors. This method is more expensive because it requires the excavating soil adjacent to the basement wall. It is agreed that this is somewhat drastic compared to the first option, but there can be additional benefits than just returning the wall to the original configuration. After exposing the back side of the basement wall there are other beneficial options available to the homeowner.
The earth anchors can restore a wall only when there is no soil against the wall. Simply put, no soil excavation – no wall restoration. Do not let anyone tell you that they can straighten a wall without removing the soil from outside the wall.
- Once exposed the exterior side of the basement wall can benefit from having waterproofing installed once the wall is exposed. This is a huge benefit because it will provide a lower humidity and dry basement without seepage.
- After restoring the basement wall, select gravel backfill wrapped in geotechnical cloth can be installed in place of saturated soil. Having a gravel buffer against the wall will insure that any water migrating toward the basement wall in the future will fall through the gravel to a drain pipe below the footing. This collected water will be removed by piping before any water gets near the wall.Alternatively there are proprietary prefabricated drainage systems available from drainage companies that intercept water and direct it away from the house much like the gravel backfill. These systems are preferred by many because the excavated soil can be returned to the trench and not trucked off site. These systems make it impossible for ground water reach the wall.
Of course, the big bonus to the second method is that the basement wall is restored to like new. The ugly bow in the wall will be gone and the cause of the soil saturation was corrected. The basement wall can be waterproofed and sealed; and an external drainage system put in place to prevent future water pressure from impacting the basement wall. An additional benefit is that the wall penetrations required for soil anchor installations can be sealed from the outside of the wall, which is more secure and are less likely to seep or leak.
ECP is a manufacturer of Torque Anchor™ brand of helical earth anchors. These products are manufactured in the USA. ECP is also a supplier of drainage products. ECP has independent certified installers that are trained in anchor installation, waterproofing and external drainages systems.
It is very important to choose a qualified and certified installer. Better yet invest in the knowledge and opinions of a licensed engineer. Remember, the engineer gives independent analysis and advice. He or she is working only for you. The engineer’s training and expertise could save you more than his fee by recommending only what is necessary and most economical methods, and no more. He can insure that all companies bidding for the work are providing equivalent products and service. He can also inspect and insure that the job is done right the first time.
If you have a bowing basement wall, cheap and fast is definitely NOT the way to go. Eventually you will have to do it right, so why not do it right the first time?