Failed retaining walls

Sep 23, 2010 by

The insured had a slope failure behind his home.   Two retaining walls had collapsed and a water line was broken.   The client wished to know whether the walls failed first or whether the pipe failed first.

Our expert visited the insured’s site with the insured, his contractor, and our client. The property was a single level family dwelling, built on a steep slope with a rear timber deck.   The rear garden area had five retaining walls; the first at the highest level was still standing, but its foundation was compromised.   The next two were being demolished because of their failure.   The lowest two walls were intact.

Inspection of the walls showed that the uppermost wall, which supported the rear yard and the house, had a compromised foundation and was now exposed.

The concrete staircases that provided access to the lower levels of the rear garden of the residence also collapsed from a lack of support when the retaining walls collapsed, as did the rear deck.

The soil under the wall was clayey with small (4-inch) round rocks included.  Below this wall, the second wall had been built and below that a third and then a fourth and fifth wall.   The insured said that the contractor, when building the retaining walls, had dirt brought in as backfill.   The contractor agreed that this was the case.   The insured stated that the fill material was placed 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 of retaining walls.

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.  On these walls, no “heel” was provided, nor was a foundation slab provided.  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 and is unsuitable for use as a retaining wall containing fill material with superimposed loads from a house and deck, constructed on a steep slope.

A properly constructed retaining wall also has “weep holes”, which are drainage outlets for the release of water that accumulates behind the wall. These 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 also was not observed.

The reinforcement provided to the slab, on which the upper level deck was resting, was lacking in great part from proper reinforcement provisions and would only provide minimal crack prevention to the slab surface.

The damages to the two retaining walls, associated stairways, and the timber deck were caused by water flowing from a broken irrigation pipe unchecked for a period of several days.   This caused the wall foundation to be undercut and 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.

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