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Can failed retaining walls be safely rebuilt?

The insured’s back yard retaining walls failed. A concrete deck was cracked, one retaining wall was tipped, two retaining walls had collapsed, and a water line was broken. The client had two questions for GEI.

First, did the walls fail first, breaking the pipe or did the water line fail first, causing the walls to fail? Secondly, the client asked GEI to inspect the insured’s site to identify the slope soil conditions and determine if the walls could be safely rebuilt.

The answer to the first question was that the damages to the collapsed retaining walls were caused by water flowing unchecked from a broken irrigation pipe for a period of several days. This caused the wall foundation to be undercut and the wall subsequently to collapse. The collapse of this wall, combined with the unchecked water flow, then overloaded the next wall, causing it to collapse also.

The thirty year old house was a two-story wood-frame stuccoed structure with a slab-on-grade foundation. The home was perched on the hillside above the retaining walls. Roof drainage was to the driveway and street, except where it ponded locally. The rear slope descended from the rear pad at an approximate 1.5:1 (H:V) gradient for 40 or more feet. The slope was terraced with retaining walls to support decks and planting areas.

Twenty five years ago the owners added a retaining wall to the slope and extended an existing deck out over the slope. This is the red wooden deck that is pictured in the photographs. The work was done with a county permit at that time and a soil report. Nineteen years later they added two more walls lower down on the slope. The walls were kept to a three-foot height, so no permits were required. These were the retaining walls that failed, which then undermined the foundation of the older wall as well. In the first photograph , the laborers are working on the middle of the three walls.

Soils in the failure area were silty clay and probed soft to depths of at least two feet. The soils were expansive and were creep prone on the steep slope. Existing footings exposed by the failure were shallow. A wood stairway that descended the slope was supported on wood posts encased in shallow postholes filled with concrete. The failed walls and slope failure were shallow in nature and not part of a deep soil failure.

There was evidence of past retaining wall structure instability in the form of wall repairs and separations due to slope soil creep.

Soil creep is generally defined as an imperceptibly slow and more-or-less continuous downward and outward movement of slope soils. Creep affects both the near surface and deeper soils. Long-term creep over a number of years produces permanent deformations in structures with foundations at shallow depths on slopes, or near the top of slopes.

Expansive soils contain clay, and exhibit volume changes with changes in moisture content; i.e., such soils shrink and crack when dry and swell and expand when wet. Soil moisture moves from moister soil to drier soil in expansive soils. Past experience indicates that expansive soil movement is cyclical and ongoing with soil moisture changes from summer heat to winter rains, changes in groundwater, sprinkler, drainage and plumbing leaks, ponding, and changes in landscape water practices.

 

Based on the information reviewed, replacement retaining walls could be rebuilt on the slope, taking into account the sloping ground, possible expansive soil conditions, residual loose soil from the failure, and the creep prone soils. Replacement walls would require deeper foundations and higher design pressures and heights based on the sub-surface geotechnical engineering investigation.

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A backyard mud slide

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.

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Sliding down a slippery slope

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.