Fire Wall

The Thomas Fire of 2017 was a massive wildfire that burned approximately 282,000 acres, becoming the largest wildfire in modern California history. The insured’s home in Ventura was threatened by the fire but not lost to it. It had three or four cords of firewood stacked in the back yard against a side retaining wall. The uphill neighbor had a driveway that ran along the edge of the retaining wall. The wildfire destroyed the backyard woodpile, the neighbor’s home, and over a thousand other structures in Ventura and Santa Barbara counties.

GEI was asked to examine the retaining wall between the properties and determine if all, some, or none of the wall needed to be replaced due to heat damage from the fire.

Our expert, Dr. Lamb, visited the home and examined the damage to the side retaining wall, the screen block wall between the front and back yards, and the concrete patio deck.

The fire burned away the strongbacks used in conjunction with wall anchors on the retaining wall.

Damaged retaining wall at right, screen wall in left background, and concrete patio to the foreground.

The heat of the fire also caused the wall anchors to expand and, in some places, deform.


Deformed wall anchors protruding from retaining wall.

All of the wall anchors were loose, and none were reinforcing the wall. The damage to the retaining wall extended 24 feet from the screen block wall.


Extent of the damage, as well as a view
of the concrete patio and screen block wall at left.

The retaining wall consisted of standard 16-inch x 8-inch x 8-inch Concrete Masonry Unit’s (CMU) that were solid-grouted. A wall this size likely had a foundation that was 16-inch x 12-inch x 8-inch CMU’s. Because the wall was solid-grouted, it appeared that repairs would require replacing 24 feet of the wall, including the foundation.

The failure of the retaining wall was observed in how it was overturning or leaning. It deflected from vertical by as much as three inches as measured from the top of the wall. This movement appeared to have also caused the failure of the screen block wall that divided the front driveway from the backyard.


Leaning retaining wall.


This CMU screen block wall developed significant cracking to the point that it could be pushed over by hand.


CMU screen wall showing various cracks.

About 13 feet of the screen block wall would also need to be replaced.

Finally, the heat from the fire caused spalling of the concrete patio deck and burned the material used for expansion joints in the slab. This slab covered an area of about 144 sq. ft. and appeared to be two inches thick in most places.


Concrete patio showing spalling and damage to  expansion joints.

In conclusion, our expert determined that 24 feet of the leaning retaining wall, 13 feet of the cracked screen block wall, and the spalling concrete patio would all require replacement due to heat damage from the Thomas Fire.


Expert of the Month:

Kenneth Lamb, P.E., PhD

Dr. Lamb is an Assistant Professor in Engineering at Cal Poly Pomona. He is a Registered Civil Engineer in the State of Nevada with 17 years of experience. His expertise in engineering covers hydrology issues, planned water/waste water facilities and storage facilities ranging from 700,000 to 20 Mgal, design of sanitary sewer pipelines, drainage studies for residential and commercial developments, and flood control issues.


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.

A leaning wall

The 40 year old retaining wall that was on the property line between the two hillside homes was leaning downhill.   Both homes had backyard in-ground swimming pools.   There were claims that the uphill home’s trees were causing the damage to the wall.   Other claims were of soil creep.   Our client wanted to know if the wall was the problem rather than tree roots or soil creep.

GEI’s assignment was two fold: conduct a survey of the common property line to locate the wall in relation to that property line (whose wall was it?), and to inspect the site to identify and determine the predominant cause and approximate age of the damage to the retaining wall.

The properties were visually inspected and photographed while a survey along the mutual property line was in progress.

The uphill home was constructed circa 1962 with later alterations and additions. County Assessor records indicated it was built in 1970, which indicated a major addition to the multi-level home down the descending slope.   The rear yard swimming pool was constructed circa 1977.

The house faced north at the end of a short cul-de-sac.   In the south end of the lot, in the lower rear yard, a pool was constructed.   Property drainage was down slope to the rear yard and pool deck.   There were no water collection devices on the pool deck and drainage was to the surrounding landscaping on the east and south in a down slope direction.   There were eight trees along the rear wood fence line, approximately 2 to 5 feet from the fence.   The trees varied in diameter and distance from the retaining wall.  The wood fence was approximately 12 to 18 inches from the back of the rear south property line retaining wall.   Soil probing with a 0.25-inch diameter metal rod indicated that the landscape soil was moderately loose to a depth of 12 to 24 inches between the trees and wood fence.   No tree roots were encountered during the soil probing.

The property line and wall conditions were surveyed.   The survey map indicated that the south property line was 61.37 feet long and the retaining wall was on the downhill property except for 20 to 30 feet where the base of the wall crossed onto the property line and ran coincident with it to the east corner of the lot.

The down slope home was constructed circa 1939, with an addition circa 1987.  The rear yard swimming pool was constructed circa 1986.   No permit record was found for the retaining wall along the northerly common property line.

The downhill property was developed by leveling a pad into a cross sloping lot, such that a west to east running retaining wall was necessary along the north property line with the rear of the northerly two neighbors’ south property lines to cut into the toe of the ascending slope.   The northerly property line retaining wall dated to circa 1939 to the 1940s and was, therefore, 60 to 70 years old.

The retaining wall was approximately 5 feet in height along the mutual property line.  The wall height was constructed approximately 1 foot above the rear grade on their property.  The wall was constructed of 4-inch thick concrete block with periodic vertical pilasters made with 6-inch concrete block units.  The wall had a decorative 8-inch high lattice block atop the solid faced block.

The wall had a limited number of visible weep holes to remove seepage water and reduce hydrostatic pressure behind the wall and had no surface back drain to remove uphill surface runoff.   The size of the retaining wall footings, steel reinforcement, and number of grouted cells in the retaining wall were unknown.   The wall was leaning over at the top into the downhill property approximately 5 inches with an approximate lean of 8 percent.  The amount, or degree, of wall lean was more or less uniform along the wall.   A depressed planter ran nearly the entire length of the wall between the wall and the swimming pool, patio, and wood steps up to an elevated wood deck.   The planter had been stripped of previous vegetation, as evidenced by remaining roots.

A retaining wall is a structure that holds back soil.  It provides support for vertical changes in surface grade.  The retained material is trying to move forward due to gravity creating lateral earth pressure behind the wall.  The pressure is usually smallest at the top of the retained material and increases as the square of the height toward the bottom of the wall.  The earth pressure will push the wall forward, or overturn it, if the wall is not designed and constructed for the imposed loading.  In addition, any water behind the wall, that is not dissipated by an adequate subsurface drainage system, imposes an additional horizontal hydrostatic pressure on the wall.   Walls designed per Code include adequate surface and subsurface drainage control to mitigate hydrostatic pressure and adequate concrete footings, concrete block thickness, and horizontal and vertical steel reinforcement to withstand the soil and water loadings imposed over the life of the structure.  The usual design of a concrete block retaining wall is 8-inch block, filled with concrete, with both vertical and horizontal rebar.

Tree roots develop and survive where there is adequate oxygen and moisture.  Most active tree roots are in the top 3 feet of soil; the majority is in the top 12 inches.  The more compacted or poorly drained the soil, the closer the roots are to the soil surface.  When roots encounter concrete foundations, or retaining walls adequately designed for soil pressures, the roots will travel parallel with the concrete structure in the softer soil.

The conclusion? The wall was on the downhill property and the predominant cause of the outward deflection of the wall was an original inadequate design and/or construction of the wall.   In addition, the inadequate relief of the hydrostatic pressure behind the retaining wall contributed to the failure.   This had been going on for a number of years and it was unlikely that the tree roots were a significant factor.

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.