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.