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Rockwork construction

This section addresses the issues associated with rockwork for the protection of waterways and is broken up into two principal parts.

General notes on rockwork

Objectives and principles

The purpose of rockwork is to provide protection and stabilisation of the bed and banks of waterways from erosion and to minimise velocity of stormwater flows. There are a number of types of rockwork; toe and edge rocks, lining rocks, filter/embedment layers and outfall pipe rockwork.

Planning considerations

Rockwork should generally:

  • be made up of angular rock (either quarried or broken up field rock)

  • be made up of well sized, graded, embedded and interlocked rock

  • have all voids sealed to provide stability and resistance to flow velocities and infiltration/undermining

  • form an interlocking mass of rock in which the larger rocks in the mix aren't free to move

The concept drawings aren't a substitute for best practice design. The design engineer should refer to other references for more detailed information on selection of rock sizes and types, rock placement, filter/embedment layer requirements and revegetation.

Rockwork information needs to be provided on the design drawings, such as rock sizes, thickness of rock layers (including filter/embedment layer), extent of rock structures etc.

Toe and edge rocks

Toe and edge rocks are also referred to as leading edge rocks or perimeter rocks. They're situated at the base of a rock structure and are located around the remaining perimeter. The function of toe and edge rocks is to hold the structure together and prevent flows from undermining the structure. 

The following should be considered:

  • the size of toe and edge rocks will depend on site and hydraulic conditions. Edge rocks are generally smaller than toe rocks but are larger than lining rocks

  • toe and edge rocks should be trenched into in-situ ground. Ground should be boxed-out to enable them to be placed before filter layer and lining rocks

  • toe and edge rocks should finish flush with surrounding ground

Lining rocks

Lining rock is the mass rock medium contained by the leading edge rock to armour/protect the earth surface.

The following should be considered:

  • a filter/embedment layer should be used underneath

  • lining rocks should be well graded to form a tightly sealed rock layer which is free of voids and provides resistance to flow velocities and infiltration/undermining

  • in sealing rock voids consideration should be given to using a smaller graded rock mix (also referred to as an infill mix). Depending on availability and construction method adopted, sealing of the rockwork may occur as a pre-mix (i.e. infill mix is combined with larger lining rocks before placement), or by working the infill mix between the larger rocks as they are laid

  • the size and thickness of lining rocks will depend on site and hydraulic conditions. Note, the larger lining rocks in the mix are generally smaller than edge rocks

  • the contractor should use methods for handling and placement of lining rock to avoid segregation of the rock sizes. Rocks shall be carefully placed in position, not dumped directly

Filter/embedment layer

Filter/embedment layers provide protection to the underlying soil base from infiltration of flows and provide a suitable medium for the embedment of the overlaying rock. Granular filter/embedment layers are usually sufficient, however some sandy, dispersive soils may warrant the use of a geotextile filter layer in addition to the granular filter layer.

Where a geotextile filter layer is used, special care needs to be exercised in the laying of the rockwork and consideration given to a thicker granular filter/embedment layer over the geotextile layer to prevent damage to the fabric.

The following should be considered:

  • layer thickness - should generally be 100mm or thicker - depends on type of underlying soil (i.e. more or less dispersive), rock size, and hydraulic conditions

  • use of graded rock - should generally be limited to 0-100mm graded rock. One size ballast rock is considered ineffective as it doesn't resist flow infiltration

Outfall pipe rockwork

Outfall pipe rockwork is used in the transition zone between the end of a stormwater pipe and the receiving waterway or waterbody. It's also often referred to as rock beaching. The rockwork provides a controlled and stable form of discharge to a receiving waterway, minimising disturbance.

Design considerations include:

  • concrete cut off walls should be used to prevent the passage of water under the outfall pipe which can affect the bedding zone

  • need for energy dissipation techniques to reduce velocities and break up flows (e.g. protruding/iceberg rocks in chute leading from outfall pipe to waterway, meandering inverts of chutes, fanning out chutes)

  • orientation of pipe with to receiving waterway

  • amalgamation of outfall pipe and rockwork with surrounding ground

  • transition of rockwork to bed of receiving waterway

  • distance between end of outfall pipe and normal water level of receiving waterway

  • extent of rockwork (length/width of rock chute, need for rockwork on opposite bank of receiving waterway)

  • height of bank rockwork

  • need for bank rockwork to provide erosion protection against overland flow

  • need for safety fence above end of outfall pipe (a safety fence is usually required where there is a vertical drop greater than 0.75 metres)

  • since rockwork for an outfall pipe consists of toe/edge rocks, lining rocks, and a filter/embedment layer, refer to issues considered under those sections

Common mistakes

  • rocks wrapped around end of outfall pipe not set in mortar

  • concrete cut off walls not used resulting in subsidence of outfall pipe

  • rockwork in front of outfall pipe protrudes above invert of pipe, causing a build up of silt/debris in pipe

  • end of outfall pipe not well concealed

  • end of outfall pipe not being epoxy sealed to prevent corrosion of pipe reinforcement

  • plate rocks being set directly on top of outfall pipe, rather than on a mortar pad

  • small, unstable rocks used on top of pipe, instead of large, flat plate rocks

  • bank rockwork doesn't extend high enough

  • bank rockwork is sitting proud i.e. rockworks doesn't finish flush with surrounding ground

  • batters around outfall pipe too steep

  • rockwork doesn't extend to bed of receiving waterway, or to opposite bank of waterway, or to a sufficient depth below water level of waterway, as required

  • since rockwork for outfall pipes consists of toe/edge rocks, lining rocks, filter/embedment layer, refer to issues considered in those sections

Practicalities of rockwork construction

Last updated:
27 September 2017