Intake investigation work ends

Monday 6 November 2017 | Published in Local


Intake investigation work ends
The results from the land investigations conducted by Coff ey principal geotechnical engineer Peter Marchant (far right) and Coff ey engineering geologist Matt Illingworth for McConnell Dowell will provide a picture of the sub-soil conditions and nature of the ground where structures will be constructed at each water intake site as part of the Te Mato Vai Stage 2 project. 17110312

Land investigation work at the 10 Rarotonga water intake sites marked for upgrades under the Te Mato Vai Stage 2 project has been completed.

The results from the investigations, conducted by Coffey geotechnical engineers and geologists for McConnell Dowell, will provide a picture of sub-soil conditions and nature of the ground where structures will be constructed at each water intake site.

One of the key elements engineers are looking for is the “bearing capacity” of the ground.

Bearing capacity is the ability of the soil to support the loads applied to the ground.

This can be factored into the foundation design of structures such as filters and tanks.

The bearing capacity of the soil can be improved by a variety of methods including adding gravel to the soil or compacting the soil, to ensure the ground can support the structures for their full design life.

The design for all structures will allow for conditions encountered at each of the test pits. Forty of them were excavated for testing before being reinstated and ground conditions where test pits were excavated indicated that the sites were suitable for the planned structures. Results from the recent land investigations are consistent with findings from previous geotechnical investigations.

The geology of Rarotonga is that of an eroded volcano where soils can be complex and variable, with extensive boulder deposits.

When the location of proposed intakes, tanks, filters and auxiliary structures is confirmed, the design will be completed for all of the intake sites. Site layout plans will then be presented to landowners for consent to proceed with construction.

In addition to the meetings, site visits can be provided for landowners where they can be shown where structures will be located, and what trees and vegetation will be removed to make construction easier.

Full justification for the location of structures and proposed works will be provided and discussed.

At the meetings there will be opportunities for landowners to discuss the designs with the Cook Islands Government, the local Te Mato Vai project management unit, contractor McConnell Dowell and the contract supervision engineers, GHD.

There will be subtle design differences for each site, but generally, the concept will be the same for all 10 intake sites.

Intakes in Rarotonga capture water from the streams and take it to settlement tanks – some of which are bigger than others.

Settlement tanks allow solids and sediment to drop out of the water before it is taken to a sand filter for purifying, and then to storage tanks. Trunk mains take the water from the storage tanks to the ring mains constructed by CCECC as part of Te Mato Vai Stage 1.

Some intake sites will not include storage tanks due to space constraints on site, otherwise the process will be the same for all 10 intake sites, making operation and maintenance of the system consistent and simple.

The design phase for structures at each intake site is expected to be completed by March, 2018. However the design phase is staggered and the design for some sites will be complete before the end of this year.

Construction at the water intakes is scheduled to begin in March 2018 subject to obtaining landowner consent for the works. It will continue for around 18 months.

- MW