Lake Cargelligo Water Supply Reservoir & Backwash Tank

Project Description

Lake Cargelligo is a small town located approximately 200 km to the west of Parkes.

The Water Treatment Plant is located on the Lake Cargelligo – Euabalong Road (Lachlan Valley Way). The existing 4 ML capacity drinking water storage reservoir at the Lake Cargelligo Water Treatment Plant was in poor condition and therefore Lachlan Shire Council (LSC) sub-contracted PCI to construct a new 2 ML capacity reservoir near the existing reservoir. The existing reservoir was also repaired upon the completion of the new reservoir.

In addition to this new 2 ML reservoir PCI constructed a 50 kL capacity tank of the same height to provide backwash water for the filters.

Project Details

Lake Cargelligo Water Scheme
Lachlan Shire Council
Construct 2ML RC reservoir and 50kL steel backwash tank.
28 weeks
2015
$2.2m

Case Study

1.1 SCOPE OF WORKS

The work under this contract consisted of supply of all materials, plant, equipment and labour for the construction of a 2 ML Reservoir and a 50 kL Backwash Tank and associated works at the Lake Cargelligo Water Treatment Plant (WTP). Main components of the works are given below:

Construction of a reinforced concrete reservoir of 2 ML capacity (13m internal diameter and 15.8m high) including internal pipeworks, roof and access
Construction of a welded steel tank of 50 kL capacity (2.14m external diameter and 16.2m high) including internal pipework, and roof1
Construction of a connecting overhead platform between the reservoir and the tank
Construction of roof access to reservoir
Supply and installation of external pipework
Provision of electrical conduits and cable pits (civil works only)
Construction of gravel access around reservoir/tank
Construction of fence and gate
All other associated minor civil works such as access hatches, water level indicators, and inspection hatches etc., site restoration etc. not included above but for the proper completion of the works; and
All temporary works such as environmental management etc., as necessary.

1.2 CO-ORDINATION & WORKMANSHIP

To achieve a successful project, the Contractor co-operated and maintained a close coordination of the construction activities with Lachlan Shire Council as the Principal, Superintendent’s representative, operational personnel of the Lake Cargelligo WTP, other contractors and any other personnel who were directly involved or interfaced with contract works.

The Contractor was responsible for ensuring that all operations and methods of construction, supply materials and standards of workmanship, design and work methods were in conformity with the Contract, the Building Code of Australia, relevant Australian Standards and all lawful requirements of Commonwealth, State, Territory and local legislation and subordinate regulations, ordinances, by-laws, orders and proclamations.

1.3 WORKS BY OTHERS

All electrical and instrumentation works including:

Supply and installation of level transmitters (2-off), float type level switches (4-off) and associated cabling, and
Supply and installation of electro-magnetic flow meter and associated cabling.

All necessary civil works for the above works (e.g. cable pits, brackets, conduits etc.) were done under this contract.

2. SCHEME OPERATION

2.1 2 ML RESERVOIR

The Lake Cargelligo Water Treatment Plant (WTP) would start/stop when the existing reservoir calls for water. The existing reservoir operated to 98% of the total storage capacity, however considerable leakage was observed through the construction joints. Upon the completion of the new reservoir and the tank, the existing reservoir was taken out of operation for repair works. The existing reservoir was put back in operation after the repairs. Therefore, operating water levels at both reservoirs (existing and new) was set out as follows:

WTP Start – RL 195 m
WTP Stop – RL 198.8 m

2.2 50 KL BACKWASH TANK

The purpose of the 50 kL Backwash Tank was to provide water for backwashing of filters as and when required. The backwash water fed into the filters needed a certain amount of head in order to clean the filters effectively.

Water level in the 2 ML reservoir (and the existing reservoir) was expected to fall significantly during high demand days in the town and therefore could not be used for backwashing. As such, the water level in the backwash tank needed to be maintained at a higher level all the time. In order to achieve this, the inlet to the Backwash Tank was set 1 m below the inlet in the 2 ML reservoir together with a one-way inter-connection between the tank and the reservoir. The top of the inlets was set as follows.