What is Hydrostatic Pressure? (and How to Fix or Prevent It)

When soil water pressure increases, so does the potential for leakage into your basement.

What is Hydrostatic Pressure? (and How to Fix or Prevent It)

Hydrostatic pressure” is the technical term for water pressure in the soil around your foundation, and it’s a major cause of basement water problems. The foundation concrete is a porous material. This means that near-microscopic holes and passages in its structure allow water flow through. Although water flows through concrete much slower than water passing through, say, a towel, it can happen when the soil outside the foundation contains high hydrostatic pressure—lots of water content. Hydrostatic pressure is a major cause of basement water problems, potentially causing leaks, or worse, cracking and bowing of the foundation, but it can be fixed, and even prevented.

When you touch a concrete wall, it’s hard to believe that water can pass through it. It seems solid, but water pressure finds a way, pushing slowly and steadily into the pores, which can cause seepage through even the toughest concrete.

Hydrostatic pressure from groundwater ebbs and flows with the seasons. Rainy months put more groundwater in the soil than dry months, which may have close to zero hydrostatic pressure. But how do the wet months manage to exert so much pressure?

If you’ve ever carried a five-gallon bucket full of water (about 40 lbs.), you know it’s heavy stuff. One cubic foot of water weighs about 60 lbs. Multiply this by the surface area of your foundation (average: 2,000 square feet), and saturated soil can exert up to 120,000 lbs. of pressure against the concrete. This is how water gets through; it’s literally pushing its way in.

How to relieve hydrostatic pressure

Because concrete is porous, even the most well-constructed foundation can’t provide a perfect barrier to hydrostatic pressure. It needs help. The most common solution is an interior waterproofing system, which adds a drain tile (a hidden drainage system that channels water away from the foundation). We frequently use this system with excellent results. They prevent wall and floor seepage, and can also stop water that permeates decayed concrete and masonry walls. An interior system channels drain tile water to a sub-floor sump pump, which automatically channels it to a safe location outside the structure. Because they don’t utilize outside drains, which can clog with silt and roots, interior systems are very reliable.

Basement water gets in one of two ways
• Through the walls: water enters through foundation cracks, pipe penetration leaks, the sill or box beam, around basement windows, or it seeps directly through the wall.
• Around or through the floor: water enters through the floor slab, or it pushes through the joint where the wall meets the floor, which is called the cove joint.

Getting the right answers about basement water problems

Understanding your situation—and the recommendations provided by basement and foundation contractors—is the first step in finding a solution. Beware of waterproof paint products that claim to seal out moisture. These coatings wear out over time, and they completely fail to address the real problem, which is hydrostatic pressure in the soil. Some foundation contractors offer this type of low-cost solution, but we don’t because it’s a waste of money.

Exterior waterproofing solutions

A well-designed drainage and landscape plan can go a long way toward preventing basement leakage. Many problems can stem from the fact that water is constantly draining toward the foundation rather than away from it. Here’s the best-case scenario for exterior waterproofing:
• The landscape around the home, including garden beds, pitches water at least 15 feet away from the foundation.
• No soil has been added within two inches of the foundation sill plate (top).
• Grounds that slope toward the foundation have a subsurface drainage system. If this is not the case, they should be regraded away from the foundation.
• Gutter downspouts extend at least 15 feet away from the foundation.
• Window wells do not accumulate water.
• There are no foundation cracks or leaks.

Interior waterproofing solutions

Even on a property that’s well-drained and graded, there’s still the issue of subsurface water—underground moisture that has soaked into the soil rather than draining away. This water fluctuates seasonally with changes in rainfall, and during wetter months, the resulting hydrostatic pressure can push through basement floor slabs.

The most common solution for this symptom is to install a drainage system, sometimes called a French drain. It has four main parts: the drainage pipe, gravel (drainage stone), a layer of concrete over the gravel, and a discharge system, which includes a sump basin with a pump in it. The pump discharges water to a safe location away from the structure.

The beauty of these systems is that they remove the water before it can rise above the level of the basement floor, so there is never any hydrostatic pressure against the concrete. If you’re seeing any seepage at all in your basement, the concrete is being gradually weakened, and it will eventually fail. Waterproofing systems stop 100 percent of this dangerous water pressure.

We install these systems along the interior edges of the foundation, and this is a very common retrofit for older homes, many of which have no built-in drainage systems. Eventually, most every home reaches an age where this retrofit becomes necessary to preserve the integrity of the foundation.

Newer homes often have built-in drainage. Building codes in some cities are requiring drainage systems as part of new construction. Some builders are constructing homes with drainage systems for liability reasons. Whatever the reason, it’s a good idea. These systems usually fall into one of two categories:
1. Shallow depth method: a rectangular-shaped channel is installed 2-4 inches under the basement floor along the perimeter of the foundation. Water runs down the wall to a gap at the floor and drains into the channel and on to a collection basin, where a sealed sump pump evacuates the water away from the home.
2. Deep channel drainage system: the most common method. Concrete is removed where the floor and wall meet. Soil is removed down to and below the lowest point on the foundation, where a perforated pipe is laid into a bed of gravel. The pipe is pitched to drain toward a sealed sump pump basin, where the water is pumped away. The floor is re-cemented, sealing the trench.

With either method, you’ll want a sump pump that has battery back-up. Those dark and stormy nights that dump water into the soil are also notorious for knocking out power. It’s also important that sump basins and other discharge pipe joints are sealed; this prevents moisture from escaping into the basement interior.

An additional safeguard against basement moisture is a vapor barrier, which is attached to the foundation walls. It allows seepage into the drain system while blocking radon gas, hydrocarbon gases, soil vapors, and moisture away from the interior.

Does your basement have seepage? Give us a call. We’ll help you identify the problem and find a solution that will keep your home sturdy and beautiful for generations to come.

AAA Basement and Foundation Repair, Wichita, Kansas
(316) 733-5400