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.