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Not Tumbling Down

Our Hudson Heights cooperative recently received an Environmental Control Board (ECB) violation for our retaining wall, which has multiple cracks, some with vegetation sprouting out, and is crumbling in spots. In addition, several posts holding a chain-link fence on top of the wall are loose. The wall, made of stone with a stucco finish, is roughly 15 feet high and about 3 feet thick. A parking lot adjacent to the wall has had drainage problems over the years, especially after heavy rains, which we suspect may have contributed to the wall’s deterioration. But water sometimes seeps through cracks in the wall even when it hasn’t been raining. What is the recommended course of action given the wall’s condition, especially now that we’re facing an ECB violation?

Retaining walls are designed to hold back soil that would otherwise shift along a natural slope or incline if the walls were not in place. Over time, these structures can deteriorate from repeated seasonal changes and a lack of maintenance, compromising their stability and posing a safety risk.

Although the DOB has issued your co-op an ECB violation, they don’t expect a failed retaining wall to be repaired overnight. They do want to see, however, that the building owner is taking steps to properly address the situation. They will require that all necessary emergency safety and protection measures be put in place, such as temporary bracing to prevent collapse. An evaluation report from an engineer, explaining the cause of any failure, and recommending a repair/replacement plan, will usually buy the owner more time to pursue the proper repairs.

If the chain-link fence on the top of your wall is loose, be warned: the New York City Building Code requires fall protection for any wall with more than a 30-inch drop between the top of the wall and the ground. Although technically not part of the retaining wall, rails and fences installed on top must be maintained not only for obvious safety reasons, but also for the wall’s long-term stability. Railings and fence posts must be properly embedded to provide optimal structural support and to prevent water from entering the wall. Even properly installed penetrations loosen over time, allowing water to enter penetrate, so they should be regularly checked for stability and a proper seal.

If the damage is not very severe, you may try installing permanent buttresses. Buttresses are typically rectangular projections at the front of the wall designed to brace it against lateral loads. The size and spacing of buttresses are determined by a structural analysis. They are generally built to match the original construction as much as possible, but can be modified to accommodate an owner’s aesthetic preferences.

Drainage

A properly designed retaining wall should have weep vents spaced roughly four feet on center, so water can freely drain from behind the wall. These vents should be lined with PVC or another durable material, not stone or masonry, which can wear away. Poor drainage will cause water to build up in the soil behind the wall, exerting more pressure on it. The extra weight from a heavy rain, for example, could more than double the load on a wall with inadequate drainage.

You write that your wall suffers from water seepage even when it hasn’t been raining; this could be an indication of a broken underground pipe behind the wall. To try to pinpoint the source of the leak, the water can be tested for chlorine – a positive test points to a fault in a domestic water system. Adding a nontoxic dye to the hot water tank or boiler can also help trace the leak source.

Repair and Replacement

If the deterioration of a retaining wall is limited to a few locations and its overall stability has not been compromised – as seems to be the case for your cooperative – it may be possible to repair the damage by patching or replacing the masonry. However, if it has extensive damage, or has severely shifted, bulged, or partially collapsed, then replacement of that section – or possibly the entire wall – may be required.

Replacement of a retaining wall is a major undertaking. It is usually done by removing and replacing small sections at a time in a carefully sequenced manner to prevent its collapse. Temporary shoring may be needed, such as embedding soldier piles and lagging behind the wall to hold the soil in place. The adjacent areas – in your co-op’s case, the parking lot – will need to be kept clear during the repair program.
Segmental Retaining Walls

Another option involves using pre-cast concrete segments (typically with replicated masonry finishes) stacked in a staggered fashion and held together by pins, without mortared joints. A geo-textile fabric placed behind the wall stabilizes the soil, thereby reducing lateral loads. Because segmental retaining walls are not held together by mortar, they have a built-in flexibility. This allows them to accommodate small displacements without cracking, and they do not require footings to be built below frost depth. Costs for segmental retaining walls can be competitive with comparable masonry or concrete walls.

Before it gets to that point, however, you should understand exactly what role you can play to avoid major problems requiring costly fixes in the first place.

Ongoing Maintenance

Like the exterior wall of a building, a retaining wall must be monitored as part of a regular maintenance plan. Building staff should check for deteriorated mortar joints, cracks, bulges, shifting, crumbling, and changes in adjacent areas. Drainage vents must be kept clear, and the staff needs to take particular note of any water pooling at the base of the wall, which could weaken the foundation. Fences and railings should be checked for stability and penetrations properly caulked and/or sealed.

Most New York City property owners and managers are familiar with Local Law 11/98 (now called the Façade Inspection and Safety Program, or FISP), which requires buildings taller than six stories to have their façades inspected every five years. Less well known is Local Law 37/08, which requires retaining walls 10 feet or taller that front a public right of way to be inspected every five years to ensure they are properly maintained.

Similar to FISP, owners must file an inspection report with the Department of Buildings (DOB) stating the retaining wall condition as safe, unsafe, or safe with repair and/or engineering monitoring. Property owners with unsafe walls, such as your cooperative, are issued an ECB violation and face fines of up to $1,200. Once a year, the DOB offers a no-penalty inspection program for retaining walls. (See box at left.)

Constant Exposure

Retaining walls are constantly exposed to soil and moisture and the same freeze/thaw cycle that causes masonry on a façade to crack, spall, loosen, and eventually fall. Cracks allow more water to penetrate, creating more cracks and causing the wall to bulge, shift, and potentially collapse. Vegetation is a particularly vexing problem. Plant growth not only draws moisture to the wall, which can corrode supporting steel, but also displaces masonry and concrete. In addition, when the plant dies and decomposes, it creates a void and allows more water to enter.

Construction

Retaining walls are typically made of concrete, masonry, or sometimes wood. The taller the wall, the more load it must support, and therefore the thicker the wall and footing must be. For every foot in height, about 30 to 50 pounds of pressure per square foot is exerted on the wall. A 15-foot wall, such as the one at your cooperative, has anywhere from 450 to 750 pounds of lateral soil pressure per square foot at the base of the wall – and even more with saturated soil. The potential for instability and collapse is therefore greater with a taller retaining wall that has not been properly maintained.

A common problem with these walls, especially older ones, is that many were constructed as makeshift structures and were not designed by a professional engineer. For example, walls built by patching together dissimilar materials – such as concrete and stone – are not as strong or cohesive as those constructed from a uniform material; they tend to crack where the different materials are joined.

Poor construction can cause damage in other ways. Makeshift walls typically do not have control joints to allow the concrete or masonry to expand and shrink. Without these joints, cracks typically form approximately 15 feet apart – where the joints should have been.

Soil

Critical to the stability of a retaining wall is the soil in which it is embedded. To ensure the soil has adequate bearing pressure, a wall’s footings must be embedded below the minimum frost depth required by law, which in New York City is four feet. Building the footing at this depth mitigates the risk of damage caused by frost heave, which occurs when water in the soil freezes and expands. That expansion causes the wall to shift. Before a new wall is built, the soil must be tested to determine its bearing capacity.

Load Change

These walls can shift and even collapse over time because of the additional loads imposed when use of the adjacent space changes. For example, if the area behind a wall is converted from a grassy area to a parking lot or a building, additional lateral loads will be imposed on the wall, which it may not have been designed to support. Whenever a change in use is planned for an area adjacent to a retaining wall, an engineer should evaluate the potential new loads to determine their effect.

Although retaining walls are often out of sight, they shouldn’t be out of mind for building owners and managers. Addressing small problems as they crop up will prevent major problems – and expenses – down the line.

 

==HABITAT SIDEBAR==

DOB’s No-Penalty Retaining Wall Inspection Program

Every spring, from mid-April to the end of May, New York City property owners can request a free inspection of their retaining walls without receiving a violation if it is found to be in poor condition. Under New York City’s No-Penalty Inspection Program, DOB inspectors will examine the wall’s structural condition for safety. If repairs are needed, the DOB will defer issuing violations to allow property owners time to take corrective action. If dangerous conditions are found, it will take immediate action to ensure these are corrected.

• The program was created in 2005 after the collapse of a retaining wall above the Henry Hudson Parkway.

• Over the past five years, more than 400 inspections have been performed.

• For more information, please visit the DOB’s website at:
http://www.nyc.gov/html/dob/downloads/pdf/retaining_walls.pdf.

 

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