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Those Unappealing Bulges

Article written by Jack Carr, P.E., R.S., LEED AP, CRITERIUM ENGINEERS
 
What a winter we have had.  And it is not over yet. Winter effects linger well after the parking bans are lifted from Boston to Portland. A hard winter produces many by-products that condominium boards and their buildings and grounds committees need to address come spring.

Of course there are the broken parking lot paving surfaces due to frost heaving and the damaged gutters from the heavy icicles, but what I would like to discuss is the effects of a heavy winter on your ground related structures, such as retaining walls. But before we focus exclusively on site retaining walls let us first consider winter damage to building foundations which act in many ways like a retaining wall.

In addition to providing the vertical load support of the condominium building, a foundation wall also provide horizontal support to keep both soil and water from entering the building whether the space is a simple crawlspace or a beautifully finished family room. These walls often experience additional horizontal forces both during and just after a heavy winter.

The cold season means more oil deliveries. Depending on the location of the oil tank fill pipe and the nature of the foundation walls, a simple oil delivery can damage a wall. A full oil truck carries a lot of weight on its rear wheels. If your condo building has a driveway near the foundation wall and the truck needs to park near the wall to fill the tanks, the point load of those wheels can place an enormous side load on your foundation wall causing cracking or worse. This is particularly true in older foundations made with stone, brick, or concrete block as they lack the horizontal strength found in reinforced concrete walls.

If your condo building has these conditions, a periodic inspection of the walls is warranted to search for new hairline cracks, mortar dislodgement, or other signs of wall movement. Keep in mind vertical cracks are much less problematic than horizontal or diagonal cracking.  Vertical cracks are more an issue for promoting water infiltration than structural problems. Horizontal cracking or inward wall movement is more serious as these conditions can lead to serious damage to the building.

Similarly, horizontal forces on your foundation wall can also be significant from the winter snow melt, particularly, if the building is on a hill side. When the spring thaw comes and snow begins to disappear its water is often given up to the ground where it travels both downward and horizontally depending on the slope of the land and soil conditions.

Here in New England most hills seem to be built on rock or ledge deposits. When ground water percolates downward, the ledge surface can act as a water chute channeling water down a hillside at great pressure due to the height of the hill. When this water comes in contact with a vertical surface, such as the exterior of a foundation wall, a considerable horizontal force is being applied, often referred to as hydraulic head. With enough force, even the sturdiest wall can fail.

These conditions can have the same effect on landscaping or site related retaining walls. These walls can be made with a variety of materials ranging from railroad ties to sophisticated cantilevered counterfort concrete walls. They can be used for simple terracing earth grading or constructing a level surface such as a driveway to pass through a steep slope.

Landscape walls move over time. They are often constructed by landscapers or homeowners without attention to proper construction details. Bulging of these walls is common, as are tipping and sliding. The higher the landscaping wall, the more concern you should have about movement since a complete failure could have other consequences. The use of a plumb line to measure for leaning over time is a typical due diligence practice. A horizontal lean or bulge of more than 1 ½  inches is too much. Cracks of more that ¼ inch should be evaluated professionally.

As we discussed, water pressure build up behind a wall is one of the most important elements to be concerned, as it is often more powerful than the earth itself. Therefore, drainage behind and through the wall is extremely important. Walls are usually coated with waterproofing or filter fabric material. Perforated PVC piping is installed with weep holes to allow trapped water to escape. Backfill soils should be porous such as crushed stone two feet thick with filter fabric covering to prevent other soils from filling in the voids between the stone. Never use soils containing clay.

With spring we are all looking to shed some winter weight and work on those unappealing bulges. Let us not forget our condo ground walls, as they too could use some tender care.      

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