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.

The toppling back yard walls

The homeowner lived on a hillside.  The house was at street level and the backyard descended steeply down the slope for several dozen yards. The homeowner hired a contractor to build a retaining wall and then a timber deck extending from the rear of the house to the top of the retaining wall.  Several years later, the homeowner had a contractor put in additional retaining walls to effectively terrace the remaining back yard with decks and planting areas.  A year later an irrigation pipe broke and the water caused several of the walls and stairs to collapse.  Our client wished to know whether the walls failed first or whether the pipe failed first.

We noted the following at our site inspection.  The rear garden area had five walls, the first at the highest level that was still standing, but its foundation was compromised.  The next two were being demolished because of their failure. The lowest two walls remained intact.

Inspection of the walls showed that the upper wall, which acted to support the rear yard and the house, had a compromised foundation, which was now exposed.  The soil under the wall was clayey with small (4 inch) round rocks included.  Below this wall a second wall had been built, and below that a third, and then a fourth and fifth wall.  The insured stated that the contractor, when building the walls, had dirt-fill imported to the site to backfill against the retaining walls.  The same contractor that built the walls was at the site in the middle of the demolition process, so he was available for questions.   The fill material was laid behind the wall and then compacted using a small machine tamper.  From our inspection of the soil, it was apparent that the fill material was not compacted in more than two layers for a total depth of fill of 4 feet.  Normal practice is to tamp the fill at every 12 inches of depth to achieve suitable fill for construction purposes and to provide a stable foundation.  Failure to compact the fill results in voids being left that subsequently fill with water and fine silty material, both of which are unsuitable for construction.

Engineered retaining walls are properly constructed of poured concrete, using reinforcing bars laid in a mesh pattern, which is tied in to a similarly reinforced foundation slab.  On a steep slope, such as that which existed at the site, it is also normal to provide a “heel” to the wall that penetrates below the level of the foundation slab for stabilization.  No “heel” was provided, nor was a foundation slab provided on these retaining walls.  The walls constructed at the site were of masonry block construction, not poured concrete.  The masonry block was reinforced with vertical bars tied in to a reinforced strip foundation.  This type of construction is typical of garden walling, which has no lateral loads imposed upon it.   It is unsuitable for use as a retaining wall containing fill material with superimposed loads from a house and deck, constructed on a steep slope.

The properly constructed retaining wall also has included “weep holes” which are drainage outlets for the release of water that accumulates behind the wall.  These provisions were not seen in the walls being demolished.  Provisions should also have been made for properly draining water (that accumulates behind the wall) into those “weep holes” by providing porous soil (gravel) behind the wall surface.  This provision was not made.

Concrete staircases that provided access to the lower levels of the rear garden of the residence collapsed from a lack of support when the walls collapsed, as did the rear deck. The reinforcement provided to the house slab, on which the upper level deck was resting, was lacking in great part from the proper reinforcement provisions and would only provide minimal crack prevention to the slab surface.

Our conclusion was that the damage to the two retaining walls, associated stairways, and the timber deck that was formerly supported on the collapsed wall was caused by water flowing from a broken irrigation pipe unchecked for a period of possibly several days. The upper wall foundation was undercut which caused the wall subsequently to collapse. The collapse of the higher-level wall, combined with the unchecked water flow, then overloaded the lower wall, causing it to fail also.

If they had been properly constructed, they would not have failed.