Retaining wall design · North Rothbury NSW
In short: The client needed structural drawings for a proposed concrete sleeper retaining wall in North Rothbury, up to 800 mm high and 30 m long. We designed precast sleepers between steel posts in 450 mm concrete piers, with crushed rock drainage and an ag drain behind the wall, and issued the four-sheet set for approval.
- Service
- Retaining wall design
- Location
- North Rothbury NSW
- Completed
- October 2026
- Wall type
- Precast concrete sleepers stacked between steel posts, each post set in its own concrete pier
- Wall size
- Up to 800 mm high and up to 30 m long, as per the site conditions
- Scheduled wall heights
- 600 mm and 800 mm, for stiff natural soils
- Soil class designed for
- Class M (presumed, to be confirmed by the contractor before excavation)
- Bearing capacity
- At least 150 kPa allowable, in stiff natural soil or rock
- Piers
- 450 mm diameter, 25 MPa concrete, 1000 mm scheduled pier depth for both wall heights
- Posts
- 100 UC 14.8 intermediate posts and 100 PFC end-of-wall posts, no more than 2030 mm apart
- Sleepers
- 200 x 75 mm precast concrete, 40 MPa, 2000 mm typical length, two N12 bars
- Drainage
- Crushed rock fill at least 300 mm wide with a geotextile layer, a 100 mm ag drain and an impermeable spoon drain
- Loads
- Earth retaining only, not designed for structural loads beyond soil retention
- Main standards in the general notes
- AS 3600-2018 (concrete), AS 4100-2020 (steel), NCC 2022 Volume Two
- Drawing set
- Four sheets, issued for approval
The brief
The client needed structural drawings for a proposed concrete sleeper retaining wall at a house in North Rothbury, in the Cessnock local government area of NSW. The layout plan shows one straight run of wall, up to 800 mm high and up to 30 m long, as per the site conditions.
The drawings had to show the builder how the wall is founded, built and drained, and go forward for approval before construction.
Why it needed a structural engineer
In NSW, a retaining wall can go ahead without planning approval only as exempt development, which means meeting every exempt development standard in the State's Codes SEPP. The NSW Department of Planning's page on earthworks, retaining walls and structural supports lists them, such as being at least 2 m horizontally from any other retaining wall on the site, and says all works must be structurally adequate and comply with the Building Code of Australia. The NSW Planning Portal says earthworks and structural supports more than 600 mm above or below ground level may be done as complying development, under the development standards in Schedule 9 of the Codes SEPP.
At up to 800 mm high, this wall goes past that 600 mm figure. The structural drawings set out the piers, posts, sleepers and drainage for each scheduled wall height, for the approving authority to check and the builder to follow.
What we designed
The drawing set has four sheets: a cover sheet, general notes, a retaining wall layout plan and the retaining wall structural detail.
- Wall system. Precast concrete sleepers stack between steel posts, and each post is set in its own concrete pier. Timber packers sit centrally on each sleeper where it meets a steel post, so the end of the sleeper fits firmly.
- Sleepers. Each sleeper is typically 200 mm deep, 75 mm wide and 2000 mm long, made from 40 MPa concrete vibrated in steel forms. It has two N12 bars, one at the top and one at the bottom, located centrally, with 30 mm cover to the bars and 30 mm at the ends.
- Posts. Intermediate posts are 100 UC 14.8 universal column sections and end-of-wall posts are 100 PFC parallel flange channels, for both scheduled wall heights. Internal posts are no more than 2030 mm apart, and an end post is no more than 2030 mm from the internal post next to it.
- End details. UC posts take the sleeper ends at intermediate supports, and PFC posts at wall ends and corners.
- Piers. The piers are 450 mm in diameter, in 25 MPa concrete, with a scheduled pier depth of 1000 mm for both wall heights. The post stops a typical 200 mm above the base of the pier, and the top of the pier sits 100 mm below the proposed ground level.
- Founding. The footings are to be founded at least 200 mm into natural orange, grey or brown silty clay with a verified allowable bearing capacity of at least 150 kPa. The founding depth is adjusted on site so every pier is consistently embedded in competent natural material.
- Drainage. Behind the sleepers is crushed rock drainage fill, nominally 20 mm to 150 mm maximum size and at least 300 mm wide at any point, with a geotextile layer for its full height at the back. A nominal 100 mm ag drain in a geotextile sock is laid to a minimum uniform grade of 1 in 100, with nominally 300 x 300 mm of coarse aggregate for the full length of the pipe. It connects to an approved drainage system at its low end, and its upper end is brought up to the surface and capped.
- Spoon drain. An impermeable spoon drain, nominally 300 mm wide and 100 mm thick, runs along the full perimeter of the wall.
- Soil only. The structural detail notes that this is an earth retaining wall only, and it is not designed to carry any structural loads beyond soil retention. The fence bracket drawn at the top of the post is marked "if required", to be used as per site and manufacturer's specifications.
The wall schedule, for stiff natural soils:
| Wall height (W.H) | Pier depth (P.D) | Intermediate post | End-of-wall post |
|---|---|---|---|
| 600 mm | 1000 mm | 100 UC 14.8 | 100 PFC |
| 800 mm | 1000 mm | 100 UC 14.8 | 100 PFC |
What has to be confirmed on site
The design depends on the ground, so the drawings list what the builder and the client must check:
- Soil class. The foundation is designed for a presumed Class M soil classification. The contractor should confirm it before excavation, and the design will be revised if actual site conditions differ.
- Founding depth. Ground conditions may vary significantly over short distances, so the founding depths of the piers should be adjusted to suit them. Whatever depth is designed or specified, all footings must extend into stiff natural soil or rock with a minimum allowable bearing capacity of 150 kPa.
- Geotechnical check. The general notes ask the client or their representative to engage a geotechnical consultant to evaluate and verify the foundation soil conditions before construction starts.
- Groundwater. If groundwater is found at the design depth of the piers, the contractor must promptly contact the engineer for advice.
- Dimensions. The builder confirms all dimensions, existing conditions, obstructions and limitations on site before ordering or delivering materials, and reports any discrepancies to the engineer straight away.
The result
The client received the four-sheet drawing set, issued for approval. It gives the builder the pier size and depth, the post sections and spacing, the sleeper details and the drainage for each scheduled wall height, along with the soil checks to make before and during excavation.
Under the general notes, the drawings are subject to approval by the relevant local authority or council before construction, and no work is to start until written authorisation to proceed has been issued. On completion, if required, the contractor certifies that the as-built works match the approved issued-for-construction drawings and specifications.
Planning a similar retaining wall in the Hunter?
Our structural design service includes retaining wall drawings sized for your block's soil and slope. Send us photos of the area, the height you need to retain, any survey or site plan you have, and a soil or geotechnical report if there is one. Our service areas page lists the Newcastle suburbs we cover; if yours isn't listed, call us, as we may still be able to help. For an existing wall showing movement, cracking or drainage problems, our structural inspections include retaining wall inspections. You can request a quote online or call 02 4091 1900.
Photos and drawings
From the report
Sample pages from what the client received (personal details removed).
Frequently asked questions
Do I need approval to build a retaining wall in NSW?
Not if the wall is exempt development, which means it meets every exempt development standard in the State's Codes SEPP. The NSW Department of Planning lists standards such as not being built on a heritage item, being at least 40 m from a body of natural water, not redirecting surface or ground water or sending sediment onto an adjoining property, and being at least 2 m horizontally from any other retaining wall on the site. All works must also be structurally adequate, installed to the manufacturer's specifications and comply with the Building Code of Australia.
Can a retaining wall over 600 mm be complying development in NSW?
It can be. The NSW Planning Portal says earthworks and structural supports more than 600 mm above or below ground level may be done as complying development, and that Schedule 9 of the Codes SEPP sets the development standards for excavation, fill, retaining walls, structural support and drainage. The NSW Department of Planning describes complying development as a combined planning and construction approval, determined by a council or an accredited certifier, that can be issued in as little as 20 days.
What does a Class M site mean?
Under Part 4.2 of the NCC 2022 Housing Provisions, foundations are classified in accordance with AS 2870. The explanatory Table 4.2.2 in that Part describes Class M as moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes, and notes that Class M is divided further based on the depth of expected movement.
