Can failed retaining walls be safely rebuilt?

Sep 1, 2011 by

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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