Collapsed culvert investigation

Project

Risk address

Detached property built 1900s

Drainage system

The culverted watercourse runs through the boundary of the property and under the garage to the rear of the property. The culvert does not service the building. The customer is responsible for the repair and maintenance of the culvert under riparian ownership.

Reason for investigation

A hole appeared under the garage following the collapse of the existing culvert.

Extent of damage

It is presumed leakage from the culvert washed away the culvert ground support causing the culvert to collapse and fracture.

This caused the culver to drain into the surrounding ground. This washed away a section of the culvert, its supporting material and material from above the culvert, creating the void. The culvert pipe is now visible in the void.

culverted watercourse - excavation back fill

Action taken

The void has created an environment too unstable to undertake cost-effective and safe repairs. It was decided to take down the garage to reduce risk to the civils teams. It was decided that a civil engineer would then visit the site to review next steps.

Civil engineer’s visit

The higher-level timber garage was demolished and all timber removed from site. A lower-level retaining masonry wall and storage area was also demolished.

During this process, the contractor allowed large volume of masonry debris from demolition to enter void which has hampered assessing the scope of works and repairs to the culvert.

On the day of visit, it had been raining and our engineer could hear culvert flows running beneath hole filled with masonry. The estimated depth of pipe from lower level was 2.0-3.0 metres.

The culvert pipe work is no longer visible. The void is also open to the elements now the garage has been demolished. As a result, rainwater is also entering the void.

culverted watercourse - excavating garage

Excavation risks

Gas cannisters

An additional concern brought to our attention is that 2 LPG bottles are missing from lower-level storage area and believed to be within void now buried by masonry.

This adds additional health and safety risks to the works and great care needs to be taken whilst removing demolition debris left by the previous contractor.

Ground conditions

During our inspection, ground beneath the garage and concrete structure was found to consist of granular fill type material (slag/ash type).

Due to the location of the void, on a sloped embankment, with a variable water level caused by the collapsed culvert, great care needs to be taken when excavating any additional material.

Consideration must be given to risks associated with loading material excavation from the void area due to possible slip circle failure of the embankment.

Ground stabilisation

Our engineer has advised that the embankment should be stabilised. This should be done by driving 6.0m sheet piles downhill from position of the garage structure. These sheets are to be driven to a depth in excess of 3.0m. This is to be completed prior to any excavation works.

Excavation method

An excavator with an over-reach arm is needed so the excavator can be set up away from the edge of existing retaining wall above the void in the concrete slab.

Although existing retaining walls of the garage structure are undermined, the engineer has not recommended the removal of any existing concrete/masonry walls until the culvert is repaired and the excavation is backfilled to a depth of 1.0m.

Once the excavation is stabilised, the existing garage/retaining walls are to be underpinned with mass concrete, circa 8.0m in length. Once underpinning is completed, the backfill and compaction of the excavation with type 1 mot to a new formation level can be completed. The existing concrete floor slab can then be reinstated.