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A Failed Retaining Wall

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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A leaning wall

The 40 year old retaining wall that was on the property line between the two hillside homes was leaning downhill.   Both homes had backyard in-ground swimming pools.   There were claims that the uphill home’s trees were causing the damage to the wall.   Other claims were of soil creep.   Our client wanted to know if the wall was the problem rather than tree roots or soil creep.

GEI’s assignment was two fold: conduct a survey of the common property line to locate the wall in relation to that property line (whose wall was it?), and to inspect the site to identify and determine the predominant cause and approximate age of the damage to the retaining wall.

The properties were visually inspected and photographed while a survey along the mutual property line was in progress.

The uphill home was constructed circa 1962 with later alterations and additions. County Assessor records indicated it was built in 1970, which indicated a major addition to the multi-level home down the descending slope.   The rear yard swimming pool was constructed circa 1977.

The house faced north at the end of a short cul-de-sac.   In the south end of the lot, in the lower rear yard, a pool was constructed.   Property drainage was down slope to the rear yard and pool deck.   There were no water collection devices on the pool deck and drainage was to the surrounding landscaping on the east and south in a down slope direction.   There were eight trees along the rear wood fence line, approximately 2 to 5 feet from the fence.   The trees varied in diameter and distance from the retaining wall.  The wood fence was approximately 12 to 18 inches from the back of the rear south property line retaining wall.   Soil probing with a 0.25-inch diameter metal rod indicated that the landscape soil was moderately loose to a depth of 12 to 24 inches between the trees and wood fence.   No tree roots were encountered during the soil probing.

The property line and wall conditions were surveyed.   The survey map indicated that the south property line was 61.37 feet long and the retaining wall was on the downhill property except for 20 to 30 feet where the base of the wall crossed onto the property line and ran coincident with it to the east corner of the lot.

The down slope home was constructed circa 1939, with an addition circa 1987.  The rear yard swimming pool was constructed circa 1986.   No permit record was found for the retaining wall along the northerly common property line.

The downhill property was developed by leveling a pad into a cross sloping lot, such that a west to east running retaining wall was necessary along the north property line with the rear of the northerly two neighbors’ south property lines to cut into the toe of the ascending slope.   The northerly property line retaining wall dated to circa 1939 to the 1940s and was, therefore, 60 to 70 years old.

The retaining wall was approximately 5 feet in height along the mutual property line.  The wall height was constructed approximately 1 foot above the rear grade on their property.  The wall was constructed of 4-inch thick concrete block with periodic vertical pilasters made with 6-inch concrete block units.  The wall had a decorative 8-inch high lattice block atop the solid faced block.

The wall had a limited number of visible weep holes to remove seepage water and reduce hydrostatic pressure behind the wall and had no surface back drain to remove uphill surface runoff.   The size of the retaining wall footings, steel reinforcement, and number of grouted cells in the retaining wall were unknown.   The wall was leaning over at the top into the downhill property approximately 5 inches with an approximate lean of 8 percent.  The amount, or degree, of wall lean was more or less uniform along the wall.   A depressed planter ran nearly the entire length of the wall between the wall and the swimming pool, patio, and wood steps up to an elevated wood deck.   The planter had been stripped of previous vegetation, as evidenced by remaining roots.

A retaining wall is a structure that holds back soil.  It provides support for vertical changes in surface grade.  The retained material is trying to move forward due to gravity creating lateral earth pressure behind the wall.  The pressure is usually smallest at the top of the retained material and increases as the square of the height toward the bottom of the wall.  The earth pressure will push the wall forward, or overturn it, if the wall is not designed and constructed for the imposed loading.  In addition, any water behind the wall, that is not dissipated by an adequate subsurface drainage system, imposes an additional horizontal hydrostatic pressure on the wall.   Walls designed per Code include adequate surface and subsurface drainage control to mitigate hydrostatic pressure and adequate concrete footings, concrete block thickness, and horizontal and vertical steel reinforcement to withstand the soil and water loadings imposed over the life of the structure.  The usual design of a concrete block retaining wall is 8-inch block, filled with concrete, with both vertical and horizontal rebar.

Tree roots develop and survive where there is adequate oxygen and moisture.  Most active tree roots are in the top 3 feet of soil; the majority is in the top 12 inches.  The more compacted or poorly drained the soil, the closer the roots are to the soil surface.  When roots encounter concrete foundations, or retaining walls adequately designed for soil pressures, the roots will travel parallel with the concrete structure in the softer soil.

The conclusion? The wall was on the downhill property and the predominant cause of the outward deflection of the wall was an original inadequate design and/or construction of the wall.   In addition, the inadequate relief of the hydrostatic pressure behind the retaining wall contributed to the failure.   This had been going on for a number of years and it was unlikely that the tree roots were a significant factor.