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1. What is a cave-in?

A cave-in occurs when groundwater infiltrates into a separated pipe joint or into a crack in a stormwater pipe and the groundwater carries soil with it. Over time, as the soil around the pipe infiltrates into the stormwater pipe, the ground surface or pavement above the pipe sinks. Cave-ins also occur around manholes and catch basins where pipes enter the structure, between the basin and the concrete lid, and where the installers of the basin failed to fill temporary weep holes in the walls of a basin.

2. What is the process?

When we receive a phone call about a cave-in, we ask the caller several questions to determine the location and public safety risk caused by the cave-in. If the reported cave-in is large and within the right-of-way, we will request that the stormwater crews fill the cave-in immediately without doing a field investigation. Otherwise, we conduct a field investigation and dispatch a work request to Stormwater to perform a first-stage repair. A first-stage repair is done by digging down into the hole, placing filter fabric in the hole, and backfilling with stone and/or soil to create a level surface. If the cave-in re-appears, we ask that the citizens call the Call Center (757-382-CITY) to report it. We will generally refill a cave-in at least twice before adding it to the list for cave-in repair. We do this to ensure that further settlement is not the result of the fill material settling.

In general, cave-ins that are less than four feet deep and not in difficult-to-reach locations are repaired by City crews. If the cave-in is large, located in a high-traffic area, or under a building foundation, it is placed on the contractor's list.

Cave-ins are repaired by several methods:

• Exposing and wrapping the failing joint with filter fabric
• Grouting the joints from inside the pipe
• Slip-lining a section of pipe
• Repair or replace the pipe or drainage structure

We often run a pipeline video camera through the sections of the pipe where cave-ins are observed to determine the extent and cause of the pipe failure. In many areas with older stormwater piping, we find that multiple joints within a section of pipe have cave-ins. For this situation, slip lining or pipe replacement is often the most cost-effective repair method. If only a few joints have failed, excavating around the leaking joints and wrapping them with filter fabric, or pressure grouting the joint from within the pipe, are the preferred repair methods. The filter fabric prevents soil from being carried into the pipe by groundwater flowing into the leaking pipe section.

3. How does the City prioritize the list of cave-ins to be repaired?

We rank the priority of the cave-in requests as follows:

• Any cave-in that is close enough to a building or other structure that will either cause structural damage or a significant public safety risk (such as in the travel lanes of a major roadway) is given the highest priority.
• The next criterion we look at is the length of time the cave-in has been in our system. The oldest cave-ins are given priority over newer ones.

4. How much does it cost to repair a cave-in?

Depending on the depth, pipe size, location, etc., the cost of repairing a cave-in varies from a few thousand dollars to $100,000. The average cost is $20,000 to $30,000.

5. What is the City doing to prevent future cave-ins?

In the mid-1990’s Public Works began requiring that all new stormwater piping placed in the City have every pipe joint wrapped with filter fabric. Public Works engineering inspectors monitor building activities to make sure that this is done. The Stormwater operations group provides assistance to the engineering inspectors by running a video camera through newly installed stormwater piping to look for defects before the City accepts the stormwater piping into the City system.

6. What should I do if I see a cave-in?

If you suspect a cave-in, call 757-382-CITY (2489) and report the problem. This will start the cycle as described above.