Concrete sleeper retaining wall design in Mount Gravatt East

The brief

The client needed structural drawings for new concrete sleeper retaining walls in Mount Gravatt East, Brisbane. The layout has two lengths of wall: one up to 1500 mm high and one up to 800 mm high.

The drawings had to show how each wall is built and supported: the footings, the steel posts, the sleepers and the drainage behind the wall, in a form the builder could follow and that could go forward for approval.

Why it needed a structural engineer

In Queensland, a retaining wall is accepted development, with no building approval needed, only if it meets every condition in Schedule 1, section 3 of the Building Regulation 2021 (the FAQs below list them all). One is that the total height of the wall and the fill or cut it retains is no more than 1 m. Brisbane City Council's retaining wall guide says to contact a licensed building certifier to organise building approval when a wall doesn't meet those conditions.

At up to 1500 mm, the higher wall on this job is over the 1 m limit. The structural drawings size each part of the wall for its height: how deep the piers go, which steel section each post uses, how the sleepers are reinforced and how water is drained from behind the wall.

What we designed

We prepared a six-sheet drawing set: a cover sheet, general notes, a layout plan, the wall detail and schedule, an elevation of the wall, and a typical section for piers near the existing sewer line.

• 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 post, so the sleeper end fits firmly.
• Sleepers. Each sleeper is 200 mm deep and 75 mm wide, typically 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, with 30 mm cover.
• Posts. Intermediate posts are universal column (UC) sections and end-of-wall posts are parallel flange channel (PFC) sections, both sized by wall height in the schedule below. The corner detail also uses a PFC section. The elevation and layout plan show bays of 2000 mm maximum between posts, and the schedule notes cap spacing at 2030 mm post to post.
• Footings. The piers are 450 mm in diameter, in 25 MPa concrete, and go down to hard strata. Footings must be founded at least 200 mm into the natural silty clay, with a verified allowable bearing capacity of at least 150 kPa, and the founding depth is adjusted on site so every pier is embedded in competent natural material.
• Where heights are measured from. Wall height and pier depth are both measured from the top of the pier footing, which sits 100 mm below the proposed ground level.
• Drainage. Behind the sleepers is crushed rock drainage fill (nominally 20 mm to 150 mm maximum size), at least 300 mm wide, with a geotextile layer for its full height. A nominal 100 mm ag drain in a geotextile sock is laid at a grade of at least 1 in 100 and connects to an approved drainage system at its low end, and its upper end is brought up to the surface and capped. An impermeable spoon drain, nominally 300 mm wide and 100 mm thick, runs for the full perimeter of the wall.
• Near the sewer. For piers near the existing sewer line, the drawings set out the sewer's zone of influence. It starts 300 mm below the pipe and at least 600 mm either side of the pipe's centreline, then rises at 1 vertical to 2 horizontal to the surface. Piers there must be founded at least 300 mm below that zone.

The wall schedule, for stiff natural soils and a wall with no fence:

Wall height	Pier depth to hard strata	Intermediate post	End 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
1400 mm	1700 mm	100 UC 14.8	150 PFC
1600 mm	1950 mm	150 UC 23.4	150 PFC

What has to be confirmed on site

The design depends on the ground, so the drawings list what has to be checked:

• Soil class. The foundation design is based on 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 can vary significantly over a short distance. Whatever depth is specified, footings are to be extended as needed into stiff natural soil or rock with at least 150 kPa allowable bearing capacity, which the builder confirms on site.
• Geotechnical check. The general notes require the client or their representative to engage a geotechnical consultant to evaluate and verify the foundation soil conditions before construction starts.
• Soil only. These are earth retaining walls only. They are not designed to carry any structural loads beyond soil retention, and the schedule is for a wall with no fence on it.

The result

The client received the drawing set, issued for approval. It gives the builder the pier depth, post sections, sleeper details and drainage for every wall height, and the soil checks to make before and during excavation. Under the general notes, the drawings are subject to approval by the relevant authority before construction, and no work is to start until written authorisation to proceed has been issued.

Planning a retaining wall in Brisbane?

Our structural design service covers retaining walls in timber, masonry and concrete, and we work across Brisbane suburbs and nearby regions. Send us photos of the area, the heights you need to retain, any survey or site plan you have, and a soil or geotechnical report if there is one. If your certifier asks for engineering certification, our structural reports and certifications page explains when a Form 15 is needed. You can request a quote online or call 07 3155 5013.