The earthquake, which occurred near Darfield, located 40 km west of Christchurch City, was caused by a fault that has been named the Greendale Fault – a previously unknown fault under the Canterbury Plains.
The rupture on the fault broke through the ground surface and created a 29 km long east-west running scarp in the land between the Hororata River and Railway Road near Rolleston. There has been up to 4.6 m of horizontal and 1.5 m of vertical permanent offset recorded across the fault at the ground surface.
Liquefaction occurred in some specific areas where there were saturated, unconsolidated soils. This caused water and silt or sand to be ejected to the ground surface, resulting in subsidence lateral spreading of the ground. This has led to damage to houses and underground services.
There are a number of different methods being considered for reducing the likelihood of liquefaction and its effects on buildings in the future – Trenchless Technology is one essential method of repair.
Trenchless assists recovery
The earthquake wreaked havoc on the underground wastewater reticulation throughout the region. Large sections of sewer network were blocked with sand as a result of ground liquefaction and substantial pipe and joint damage was caused by the ground movement.
ProjectMax, New Zealand’s leaders in trenchless knowledge, worked closely with Waimakariri District Council in the badly affected town of Kaiapoi for six weeks following the earthquake. The company helped to co-ordinate the unblocking, pipeline CCTV inspections, and emergency repair to over 20 km of affected gravity wastewater network in the town
(30 per cent of the Kaiapoi’s reticulation) leading to the return to operation of the gravity sewer network.
The initial challenge was to clean the pipes of silty sand that had “÷blocked’ the pipes. The ground surrounding the pipes had liquefied during the earthquake and had “÷flowed’ into the pipes through the damaged pipe joints and housing
service lines.
The earthquake had also artificially raised the groundwater table, and this was also contributing to the ongoing inflow of sand being washed into the pipes through the damaged sections. The groundwater that was gushing into the pipes was eroding the surrounding soil forming sinkholes, damaging the roads and putting nearby structures at risk.
To enable the network to become operational again and provide sanitary services to Kaiapoi’s residents as quickly and cost-effectively as possible, (temporary port-a-loos were supplied on the roadside as a short-term measure), it was important to undertake repairs to remove and prevent collapses in the pipes, and prevent the inflow of groundwater into the network.
Due to the depth of the pipes, the difficult ground conditions and the high groundwater table, undertaking repairs by traditional open excavation would have been both time consuming and very expensive.
Most of the damage to the public reticulation was limited to the pipe joints caused by longitudinal forces pulling and pushing the pipe sections together.
Director of ProjectMax Steve Apeldoorn said “Because of the nature of the pipe failures, ProjectMax were able to recommend and implement CIPP patch repairs as a Trenchless Technology solution that enabled fast, and by comparison more cost-effective, repairs limiting the need for excavation.”
In total, 113 patches were installed.
Waimakariri District Council are now moving from the emergency response phase to the recovery phase and are now undertaking planning for the longer term asset renewals or replacements to return the wastewater network to its pre-earthquake condition.
ProjectMax continues to assist the council by providing pipeline condition assessment analysis and renewal options using Trenchless Technology methods where appropriate and applicable.