Cliff top challenge

The Ohope Escarpment Drainage project was a small but technically challenging project that required an innovative and collaborative approach between the local authority, consultant, contractors and community to solve a long-standing problem of slope stability.

Following heavy rain in July 2004, major landslips on the Ohope escarpment caused major damage to properties and infrastructure. Tonkin & Taylor was engaged by Whakatane District Council to investigate solution to protect properties in the area of greatest risk, at the top of the escarpment. Following geological assessment, Tonkin & Taylor recommended the replacement of on-site soakage with a network of reticulated stormwater and wastewater.

HEB Construction was engaged to undertake the $2.76 million project, which began in September 2007 and was completed in May last year. Project manager David Burns says the project posed a number of challenges.

The project serviced 47 residential properties in the affluent cliff top suburb of Otarawairere – and this in itself presented the first challenge. A significant number of the residents were opposed to the project which meant good relations and a high standard of work was essential. Add to this the simple fact that working in a residential area and having to get equipment into properties and around houses is always difficult.

Wastewater from the properties at the top of the escarpment (which were previously using septic tanks) was routed through the drilled pipelines to an existing wastewater sewer serving properties at the foot of the cliff.

In total, the project involved 9.4 kilometres of pipelines, of which about half were public lines and the rest private connection pipework.

Over two kilometres of the pipelines was installed by directional drilling, a job subcontracted to specialists DDS 2006. Five major inclined drills through the escarpment represent some of the longest inclined pipe installations in the country. In places, there was up to 50 metres cover of rock and soil.

The longest directional drill was 244 metres long through rock with a strength of up to 100MPa, using a 250mm drill to then install a 180mm diameter pipe with a fall of over 90 metres.

And this is where the next big challenge presented itself to the contractors.

“With the directional drilling in excess of 50 metres, the sensor couldn’t be picked up – so we were drilling blind for about 100 metres,” says David Burns.

Drilling from the bottom up, they calculated where the drill should come out, the drill was set in position and got to work. Once they were able to locate the drill again on the sensor they found they were within two metres horizontally of the anticipated position. With 100 metres left to go, the drill was adjusted and arrived at the manhole out by only 50mm.

“This was a major achievement considering we couldn’t see the drill for nearly half of it,” says Burns.

Along the roadside, the trenches were open cut. To allow traffic to continue using the road the spoil was trucked away from the trench.

Another challenge, a health and safety one, arose because a lot of work was on the top of the escarpment – the contractors were working on or adjacent to a cliff face that was a minimum of 45 metres high and comprised unstable ground which was susceptible to further movement.

Health and safety was a very high priority on the project, given the site location and the nature of the work – a high percentage of the drainage trenches were over 4.5 metres deep and a minimum of 2.5 metres wide – and staff were constantly reminded of the dangers of working in and around trenches.

Another major aspect of this project was the very high level of stakeholder involvement and community support that was generated under tight time constraints. Through good early consultation, stakeholders recognised the benefits of a joint approach to the problem and were quickly committed to the projects.

By eliminating on-site stormwater and wastewater soakage, and reducing stormwater runoff, the stability of the escarpment has been improved significantly. This has provided greater security for Ohope, improving safety for the local community as well as enhancing the environment.

The project has resolved the long-standing landslip problem along the Ohope escarpment and is an excellent example of how consultants, contractors and local authorities can work together effectively with the local community and wider stakeholders to achieve an innovative and sustainable outcome.

The project has been strongly praised by all involved and recently netted Tonkin & Taylor a merit award at the Association of Consulting Engineers (ACENZ) Innovate NZ awards.