Sliding down a slippery slope

Nov 1, 2006 by

The case of the month relates to a geotechnical issue.   An adjuster called us with a case where a slipping hillside brought a wall of muddy debris into the backyard of the downhill neighbor.   The uphill neighbor represented by our client had an in-ground swimming pool.   The accusation was that the pool owner was negligent in maintaining his pool, which had caused the slipping hillside.

Specifically, the complaint was that during heavy rains, the pool owner allowed his pool to overfill and then overflow.  This theoretical overflowing supersaturated the downhill soils, which then slid down into the rear neighbor’s back yard.

Our expert inspected the site.   The insured’s property was located approximately 30 vertical feet above the claimant’s property.   The slope failure occurred in a cut slope adjacent to the downhill neighbor’s carport.   The debris from the slide rested on the toe of the slope covering the retaining wall.  The area was partially covered by plastic at the time of the inspection but enough of the slope was exposed to view the size of the failure and scarp (top) area.   The failure area was approximately 15 feet wide along the slope and the scarp of the slide was just at the property line.   Adjacent slope areas were landscaped with a thick growth of ivy.   The right half of the slide area was only one or two feet deep.   The root zone had pulled away and moved downslope with the ground cover.   The left half of the slide was deeper at the top being approximately four to five feet deep at the scarp.   The scarp area had the appearance of also being eroded as though seepage may have come out of the top of the slide in the scarp area.   There was no sign of the slope failure or erosion on the uphill side of the property line chain link fence.

The primary source of the water that saturated the slope was rainfall from the heavy storms that preceded the slope failure and possibly an underground spring, or other seepage, in the hillside that was increased over normal levels by infiltration of rainfall over a larger uphill area.   Our expert also noted one other contributing factor was that at the bottom of the retaining wall on the claimant’s property, a block planter had been built in front of the retaining wall.   Planter soil covered the open head joints in the bottom of the wall that were put there for relief of the hydrostatic pressure that would build up during rainy periods.

Our expert inspected the path that any pool overflow would take toward the downhill property if the pool were to overflow.   He found that any overflow of the pool would cross the concrete perimeter deck and enter perimeter drains that were routed into the gutter of the street.  Therefore, our expert concluded that overflowing of the pool from heavy rains, even if it did occur, did not contribute to the slope saturation.

Slope failures on steep soil slopes are common occurrences during periods of heavy rains especially when other rains have preceded the heavy rains.   In this case, the soil materials that made up the downhill slope were normally dense and quite stable when dry and laterally supported.   Weathering processes of gravity, heat and cold, rain and vegetative root growth all conspire to weaken the near surface soil materials on slopes over time.   Under the correct conditions of earlier rainfall and a particularly heavy storm event, the level of saturation in the slope can be enough to cause the slope to give way and slide downslope.

This common type of slope failure is routinely remedially repaired by compacting soil, usually mixed with 3% to 6% cement, in horizontal layers (lifts) in a key and on benches cut into the slope as the slope repair proceeds up from bottom to top.   Sub-drainage should be added through horizontal pipe sub-drains and/or “chimney” drains to remove any underground seepage that may develop behind the repair in the future.   A geotechnical engineer should be consulted for the precise repair specifications and to observe and test the work for compliance during grading.

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