Concrete sleeper retaining wall replacement in Kariong

The brief

The client was replacing a retaining wall at their property in Kariong, on the NSW Central Coast, and needed structural drawings for the new wall. The new wall is a concrete sleeper retaining wall: precast concrete sleepers stacked between steel posts, with each post set in its own concrete pier.

The wall is up to 1200 mm high, and its length is set to suit site conditions. The drawings had to show the builder how to found, build and drain the wall, and they were issued for approval.

Why it needed a structural engineer

At up to 1200 mm high, this wall is above the 600 mm figure in the NSW complying development rules, and above both heights at which Central Coast Council's planning controls call for a professional engineer's design.

• NSW planning rules. In NSW, a retaining wall needs no planning approval only if it meets 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 the main standards 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.
• Central Coast Council. Kariong is in the Central Coast Council area. Chapter 2.1 of Council's Development Control Plan 2022 says retaining walls more than 600 mm above or below existing ground level within 1 m of any boundary, or more than 1 m above or below it anywhere else, must be designed by a professional engineer. This wall is above both heights.
• The building code. The Building Code of Australia is Volumes One and Two of the National Construction Code (NCC). NCC 2022 Volume Two, clause A6G11 classifies a retaining wall as a Class 10b structure, a non-habitable building or structure.

Requirements differ by state and council. Our council approval and compliance page sets out what is generally required.

What we designed

The drawing set has five sheets: a cover sheet, general notes, a retaining wall layout plan, the concrete sleeper wall details and a typical elevation.

• Wall build-up. Precast concrete sleepers stack between steel posts. Timber packers sit centrally on each sleeper where it meets a post, so every sleeper end fits firmly.
• Sleepers. The sleepers are 200 mm deep, 75 mm wide and typically 2000 mm long, in 40 MPa concrete vibrated in steel forms. Each has two N12 bars, one top and one bottom, located centrally, with 30 mm cover to the bars and 30 mm at the ends.
• Posts. Intermediate posts are 100 UC 14.8 sections and end-of-wall posts are 100 PFC sections at every scheduled height. The schedule allows up to 2030 mm between posts, and the layout plan and elevation show the posts at 2000 mm maximum.
• Piers. Each post is set in a 450 mm diameter pier of 25 MPa concrete. The top of the pier sits 100 mm below the proposed ground level, and the wall height and pier depth are both measured from it. The post stops 200 mm short of the base of the pier.
• Sleeper ends. The sleeper ends slot into the UC section at intermediate posts and into a PFC section at wall ends and corners, with blocking pieces on the retained face. The end details set a maximum gap for each section size.
• Soil only. The wall retains earth only. It is not designed to carry any structural loads beyond soil retention.

The schedule, for stiff natural soils and a wall constructed with no fence, sets the pier depth and post sizes for each wall height, and lists no double-sleeper height for any of them:

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
1000 mm	1200 mm	100 UC 14.8	100 PFC
1200 mm	1350 mm	100 UC 14.8	100 PFC

Drainage behind the wall

• Drainage fill. Crushed rock, nominally 20 to 150 mm in size, fills the space behind the sleepers, at least 300 mm wide at any point.
• Geotextile. A geotextile layer runs the full height of the wall at the back of the crushed rock.
• Ag drain. A nominal 100 mm diameter ag drain, wrapped in a geotextile sock and bedded in a nominal 300 x 300 mm band of coarse aggregate along its full length, is laid to a uniform fall of at least 1 in 100.
• Outlet. The low end of the ag drain connects to an approved drainage system. Its upper end is brought up to the surface and capped.
• Spoon drain. A nominal 300 mm wide, 100 mm thick impermeable spoon drain runs the full perimeter of the wall at the top of the retained ground.
• Stormwater. Stormwater is to be properly drained and connected to the house drain.

Our guide to retaining wall drainage design explains what the ag drain, the free-draining gravel and the geotextile filter each do.

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. The footings go at least 200 mm into natural orange, grey or brown silty clay with a verified allowable bearing capacity of at least 100 kPa, as set by the note on the detail sheet. The founding depth is adjusted on site so the piers sit consistently in competent natural material.
• Ground variation. Ground conditions can vary significantly over a short distance, so pier depths vary with them. Whatever depth is specified, the footings are extended as needed into stiff natural soil or rock with an allowable bearing capacity of at least 150 kPa, confirmed by the builder on site.
• 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. Our soil testing and geotechnical coordination service can arrange this.
• Dimensions. The builder confirms all dimensions, existing conditions, obstructions and limitations before ordering or delivering any material, and reports any discrepancies to the engineer.

The result

The client received a five-sheet drawing set for the replacement wall, issued for approval. It gives the pier depth and post sizes for each wall height from 600 to 1200 mm, the sleeper specification, the sleeper end details at posts and corners, and the drainage behind the wall. Under the general notes, the plans stamped and approved by the building surveyor or relevant authority are the ones used for construction.

Replacing a retaining wall on the Central Coast?

Our retaining wall repair and replacement engineering includes full replacement designs, and our retaining wall design and engineering certification service provides engineering drawings for approval. Concrete sleeper walls are one of the retaining wall types we engineer, and we design walls for sites across the Central Coast. Send us photos of the existing wall, the heights you need to retain, any survey or site plan you have, and a soil or geotechnical report if there is one. You can request a quote online or call 07 2146 4066.