Case of the Month: Driveway Cracks

May 1, 2015 by

A main water line belonging to the City of Thousand Oaks burst in front of the insured’s residence. The homeowner then claimed that the water flow cracked his concrete driveway and rear patio slabs, did damage to the exterior stucco, and also resulted in cracking of the concrete floors of his residence. GEI was engaged to investigate the predominant cause and age of the observed damage.

Our expert visited the site twice in the presence of the homeowner. The property was a single-family, timber-framed building, constructed circa 1987, and was aligned principally facing the east-west street to the south.

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The street was undergoing repairs from the burst water service at the time of first inspection, so our expert was able to view the main water line.

Ventura County Fire Department (VCFD) reports showed that the call was received at 5:41 p.m. The incident duration was 1 hour 47 minutes.

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California Water Company, responding to the VCFD call for aid, notified the VCFD that they would be on-site within 30 minutes. Therefore, the total probable time of water flow is estimated at 1.0 to 1.5 hours. The line size that broke (asbestos-cement pipe prior to hydrant and after street valve) was 6 inches in diameter. The line pressure is required by Code to retain 20 psi at 1,500 gallons per minute flow from the hydrant for backflow prevention.

However, the hydrant outlet size was smaller than the broken line size. For that reason, our expert considered the total flow to have been greater than 1,500 gpm. The expert estimated that a flow of approximately 3,000 gpm was more appropriate, based on a 6-inch pipe flowing at half the maximum capacity and at a pressure of 120 psi (half capacity based on review of broken pipe in the trench at scene). Such a flow would release approximately 270,000 gallons. This value appeared reasonable and within a half-order of magnitude, because the start of flow was not known, and pressure conditions may not have achieved the maximum for the system.

Damages noted by the expert on his first visit included:

· Cracked and shifted stamped concrete driveway

· Cracked and shifted front door concrete steps

· Stucco damages to the front and west sides of the residence and the garage

· Cracked and shifted concrete to the rear patio and pool surrounds

Our expert was asked to return to the site because the homeowner noted further damages of cracking and shifting concrete. The additional damages on the second visit 20 days later included:

· More cracking and shifting of the driveway concrete

· Further cracking and shifting of the front door concrete steps

· Cracking of the concrete in the secondary entryway

· Cracking of the concrete in the kitchen floor, hallway floor, and stairs to the living room

· Cracking of the stucco to the driveway surround wall

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The damage to the kitchen and hallway floors indicated loss of support for the slabs from the sub-soils.

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The property site was generally sloping, and it was likely that the residence was built, at least partly, on consolidated fill. While the driveway and residence presented a relatively impervious surface to water intrusion down into the sub-soils, it is also recognized that the driveway and front door concrete step, together with the patio and pool surround slabs, had shifted and cracked following the excess flows. That shifting and cracking were recent, based on the expert’s observations at the site (no old grass or weeds in the cracks, freshly cracked concrete having sharp and undisturbed edges, and no wear to the edges of the shifted panels of concrete). Because the damage was present to the interior of the residence, it appeared likely that some further water-based consolidation of sub-soils beneath the kitchen/hallway grade slab had occurred.

Based on the foregoing, it was the expert’s opinion that the predominant cause of the damage seen was the water intrusion resulting from the burst fire hydrant service outside the residence. The approximate age of the damage cause was consistent with the date-of-loss.

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