Christies Beach Wastewater Treatment Plant Refurbishment

Introduction

Incon was engaged to provide structural engineering design, verification, and construction-stage support for refurbishment works at the Christies Beach Wastewater Treatment Plant (CBWWTP). The project involved complex demolition and modification of existing reinforced concrete infrastructure, requiring detailed engineering assessment to ensure structural integrity and safe execution.

Project
description

The refurbishment works focused on the removal and modification of suspended concrete slabs over grit removal units and primary sedimentation tanks (PST), alongside associated structural interventions including lifting design, monorail review, and manway assessments.

The existing structure comprised:

Suspended reinforced concrete slabs (180mm–300mm thick)
Spanning up to ~3.9m (grit tanks) and ~3.1m (PST tanks)
Primary reinforcement spanning in one direction (short span behaviour)

The demolition methodology required controlled slab cutting, lifting, and staged removal, while maintaining stability of the remaining structure throughout construction.

Challenge

Slab propping from below required personnel entry into tanks, which was restricted due to safety risks from residual material, preventing conventional demolition approaches
Cutting sections of slab risked compromising load paths and structural behaviour, particularly due to one-way spanning action.
Existing penetrations were insufficient for effective pump-out, requiring additional slab openings without weakening the structure.
Design of lifting points, block removal strategies, and interaction with monorail systems required precise coordination.

Solution

Incon delivered a comprehensive and practical engineering solution integrating analysis, demolition methodology, and temporary works design:

Verified structural performance using conservative assumptions (20MPa concrete, 230MPa steel), confirming:
Replaced conventional penetrations with 600mm wide strip cuts, enabling:

Effective tank pump-out access
Preservation of slab structural behaviour
Controlled redistribution of loads

Engineered requirement for:

Minimum 1m intact slab between cuts
Temporary support beams with defined bearing lengths
Sequential cutting aligned with structural spanning direction

Developed safe lifting strategies including:

Integrated lifting points for slab sections
Coordination with demolition methodology
Engineering review of monorail and lifting systems

Addressed uncertainties in existing conditions through conservative design and staged methodology.

Key Engineering Value Delivered

Enabled safe demolition within confined and high-risk environments
Eliminated need for unsafe internal propping, improving worker safety compliance
Maintained structural stability during staged slab removal
Delivered efficient demolition methodology, reducing time and operational disruption
Integrated design, verification, and construction support into a cohesive solution
Provided cost-effective and buildable engineering outcomes

Supported efficient staged demolition with minimal disruption

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