A backyard mud slide

Apr 2, 2010 by

The insured turned in a claim for damages to his home from a mudslide or earth movement.   GEI was assigned to visually inspect the insured’s site to identify and photograph the damages and determine causation.

The house was located at the eastern end of a small ridge in hilly terrain with slopes descending to the north, east, and south.  The house had a large concrete and brick patio along the south side, which ended at a concrete walk at the top of the slope.  Drainage of this patio area was over the slope.

The insured owned the 50 plus year-old home for three years.   He had not done any landscaping or exterior painting since purchasing the property.

He first noticed the slope failure problem a few months earlier when his power and cable TV stopped working.   He investigated and found that due to mudslides, trees had fallen onto the overhead wires crossing the small canyon south of his home.

Back at his house, the south-facing slope appeared to be a natural slope with numerous large trees present.   There was a thin wedge of fill along the top of the slope.  A slope failure in the back yard occurred along the top of the slope at the westerly end of the patio on a steep, 40 feet high slope.  The top of the slope dropped 3 feet along a length of 20 feet and widened to 40 feet lower on the slope.

Slope failures are common occurrences during periods of heavy and intense rains, especially when other rains have preceded the heavy rains as had been occurring locally.  The surficial soil materials that underlie the slope were normally loose, as they were derived from the underlying weathering parent material at depth on the slope.  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 the earlier rainfall and a particularly heavy intense storm event, the level of water saturation in the soil brings the weight of the soil in the slope up to a critical point. When the weight of the soil exceeds the capacity of the slope to hold it in place, the slope soil materials give way and slide.  Drainage from up slope areas will frequently contribute even more water to the point where failure occurs.

The slope failure on the south rear descending slope, which undermined a previously tilted concrete walkway and caused trees to fall and other brush/plantings to move down slope was caused when the slope soils became wet and saturated from heavy rains, became heavier, lost strength and slid undermining the walk and causing trees and brush to rotate and fall.

The source of the water causing the wetting of the slope soils was incident rainfall and runoff from the patio and upper slope running into the slope failure area.

Independent of the current slope failures, there were other damages as well.  These consisted of settlement stucco cracks, uneven deck surfaces, retaining and planter wall cracks, and tilted concrete walkway separations.  These were old and were due to long-term differential soil movement and tree roots.  They were not due to the recent slope failure.