Precast Retaining Wall vs Cast-in-Place Concrete: Which to Choose

Short answer: Precast retaining walls are made in the factory and simply set in place on site, so they need less time on site: no formwork and no waiting for concrete to cure, and every unit has consistent quality. Cast-in-place concrete suits walls whose height or shape is beyond precast sizes, or walls that must be cast together with another structure. Whichever you choose, the height, size and foundation should be designed by an engineer from the soil conditions and the loads above the wall.

The two methods compared

A study published in PCI Journal (2017) comparing a precast retaining wall with a cast-in-place wall found that precast cuts on-site construction time, reduces risk to workers and gives better-controlled concrete quality. Its limits are that unit size is restricted by transport and lifting, and the joints must be well designed.

Types of precast retaining wall

1. L-shaped retaining wall (Retaining Wall L Series)

• Built-in base slab: the weight of backfill on the base holds the wall steady
• Suits property edges filled higher than neighbours, canal banks and pond edges, and can double as a fence
• Certified Made in Thailand (MiT), with a version to the Department of Highways design
• Dowels can be ordered to extend the wall upward (10 cm dowels for 50–60 cm sizes, 15 cm for 145–280 cm sizes)
• Sizes from 145 cm have tongue-and-groove joints and weep holes

Installation in 4 steps: excavate along the wall line → pour lean concrete → set the units end to end → backfill and level

2. Large retaining wall block

• 100×75×50 cm, 450 kg, placed by machine for fast installation
• Works with geogrid to save on foundations, and with piles for building on newly filled ground
• Also used as walls for storing agricultural produce, and can be moved
• Example installation models for heights of 2–8 m

3. Small retaining wall block

• 50×48.5×25 cm, 70 kg
• Can be built vertically at 90 degrees to make full use of the land, and planted in the openings
• Can be used for water channels or ponds

4. Geo-block

• Lightweight; can be laid by hand without machinery
• Suits narrow sites machines cannot reach, gardens and landscaping
• Square-face and curved-face versions

When to choose cast-in-place

• Walls taller or longer than can be made or transported
• Curved or highly irregular wall lines
• Walls that must be cast as one with another structure, such as basement walls

Four questions to help you choose

1. How high is the soil to be retained, and what loads sit above it? For example trucks, buildings or stockpiles. This sets the model and foundation.
2. Can a crane or excavator reach the site? If not, look at small blocks or Geo-block.
3. Should the wall also serve as a fence? L-shaped walls can be used as precast fences.
4. Is there standing water or groundwater behind the wall? Plan drainage behind the wall.

FAQ

Do precast retaining walls need a foundation?
L-shaped walls sit on a poured lean concrete bed. The foundation for each job depends on height and soil, and should be set by an engineer.

Can I install them myself?
Geo-block and small blocks can be laid by hand; large L units and large blocks need lifting equipment.

How tall are L-shaped walls?
Standard sizes start at 50 cm, and units can be made to the site height. See the size table above.

Enquiries and quotations: CPS, tel. +66 61 403 4448 · LINE @ccp-pavingstone

Products: Retaining Wall L Series · L retaining wall, Department of Highways design · Large retaining wall block · Small retaining wall block · Geo-block

Reference: Design principles of totally prefabricated counterfort retaining wall system compared with existing cast-in-place concrete structures, PCI Journal, Sept–Oct 2017