Case of the Month: A Failed Retaining Wall

Mar 3, 2015 by

The Half Moon Bay home had a leaning fence and the integrity of the retaining wall on the property line was questionable.

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GEI was assigned to investigate the cause and determine what was due to recent events, long-term conditions, construction defects or deferred maintenance.

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The damaged area was on the side pathway of the residence and was downhill from the entrance. Surface soil was loose, pebble-like, and free flowing, varying in size up to ½ inch. The damaged fencing on the property line was clearly off plumb. Inquires of recent utility work or failures were not acknowledged nor apparent. The main house was in good order and nothing out of the ordinary was noted. Our investigation focused on the exterior yard areas. We saw stonewall cracks, relating to prior soil movement. A wider inspection of the surrounding neighborhood confirmed that soil erosion and settlement issues were common in the area. Further investigation suggested that this particular soil had failed. The pathway was leaning away from the home, toward the neighbor’s property line. The retaining wall with the fencing atop it also failed along with the adjacent soil. The characteristic of the failure suggested that it has been slowly occurring over the past year.

A sump-pump was not operating although the owner stated that it was fixed two years ago when he purchased the property. The sump-pump operated in conjunction with a French drainage system rather than only a sump pump, relying on pumping out seepage at the low point of the property. The property uphill most likely had a similar dewatering system based on the outlet noted at the street.

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The soil failure was due to the non-operating sump-pump/French drain system. The failed soil overburdened the wooden retaining wall system, resulting in the fence leaning toward the neighbors. The supporting posts and surrounding soil had failed and needed to be replaced.

The side concrete foot path was also tipping downhill. Due to the soil movements, the French drainage system more likely would be clogged and compromised. The sump-pump was not verified if non-operating or just not turned on.

When soil is subjected to inundation of water, say from annual rainfalls, and if the water is not properly removed or dissipated via a dewatering system, or naturally via percolation in the latter case, certain types of soil can “boil” and either reconsolidate or flow. In this process, settlement or erosion of the soil can occur resulting in loss of support to structures above. This ranges from minor cracks to the total collapse of the structure. In homeowner’s current situation, the dewatering system was rendered inoperable due to a non-functioning sump-pump. This destroyed the integrity of the underlying soil resulted in soil failure from erosion and settlement. Adjacent neighborhood areas showed similar settlement of structures due to soil failure and that was the nature of the type of soil in the area. An engineered dewatering system was effectively not in place to prevent the resultant failure.

The sidewalk, retaining wall, and fence damage occurred within the past two years, and was most likely was a result of the seasonal rains earlier in the year.

Necessary repairs will encompass the length of the failed section of the concrete pathway with a four foot width, from the yard entrance to the base of the property line, where the sump-pump is located. The soil needs to be reconditioned, prepared, and additional makeup soil brought in. The sump-pump and French drainage system needs to be checked, repaired, and/or replaced. The wooden fence retaining wall and post system also needs to be replaced as they have failed.

The cause of all this damage was the occurrence of underlying soil movement of erosion and settlement due to a non-operating sump-pump.

Expert of the Month: Jeffrey Lee, P.E.

Mr. Lee is a Registered Civil Engineer in the State of California. He holds a Bachelor of Science Degree in Civil Engineering from the University of California, Davis. He has more than twenty years experience in design, project management, and construction. His employers have included Bechtel, the United States Navy, B & T Construction and Barrett Consulting. As a GEI expert he handles investigation and analysis involving issues relating to commercial and residential structures, property damage, nuclear and hydroelectric plants, hazardous waste remediation/rehabilitation including asbestos, petrochemical, and underground storage tank matters, flood control structures, waste management, and contract specification disputes. Mr. Lee is fluent in English and Chinese.

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