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How to choose the right precast block and why it matters (a lot).

When it comes to precast concrete retaining wall blocks, your choices are virtually limitless, from 20 lb minnows to 5,000 lb juggernauts. With so many options, it begs the question – how do you know which precast block to pick for your retaining wall project?

Precast concrete block wall systems provide excellent value for money. They are durable, require less technical knowledge than other options, and their modular design facilitates quick construction. In addition, the flexibility provided by a modular design enables precast block retaining walls to absorb vertical frost heaves effectively without requiring a deep foundation that penetrates below the frost line. However, they have their limitations – if you’d like to learn more about precast concrete block wall benefits and restrictions, please read our article about gravity retaining wall systems.

You can build two types of retaining walls with precast concrete blocks: gravity retaining walls and reinforced retaining walls. The first type is gravity retaining walls, which rely solely on sheer mass to hold back the soil. The second type, reinforced retaining walls, rely on their mass plus additional reinforcement (typically geogrid) to hold back soil pressures. For the sake of simplicity, in this article, we will discuss the suitability of different precast concrete blocks for gravity retaining wall systems.

The general principle here is that the heavier the mass, the sturdier the retaining wall and the taller you can build. So, for example, a retaining wall built with a 6″ tall, 8″ wide, and 12″ deep precast block is sturdier than a retaining wall built with a 6″ tall, 8″ wide, and 9″ deep precast block.

To keep things simple, let’s assume that your retaining wall is on flat ground, is situated on good soils, and has no load surcharges on top of the retaining wall. Under these conditions, you can use practically any precast retaining wall block to build your retaining wall with, as long as the wall is less than 1 meter (3′ 3″) in height.

Suppose access to your site is restricted. Consider using a smaller retaining wall block (90 lbs or less) that you can place without heavy equipment. As mass is vital for stability, you must reinforce a small block retaining wall with geogrid if you plan on building a wall taller than 1 meter (3′ 3″). However, suppose your site conditions offer lots of space for heavy equipment to maneuver. Choose the largest precast block you can find, as this will make the construction of the retaining wall progress much faster and leave you with a more robust wall.

A key point to remember here is that for a retaining wall block to function as intended by its manufacturer, you must follow the recommended construction methods to the letter, regardless of how short the retaining wall might be. If you forgo any necessary steps during the construction of the retaining wall, then it will most likely fail before the end of its intended lifespan.

For retaining walls ranging in height from 1-1.2 meters (3′ 3″ – 4′), a precast concrete block heavier than 210kg (463 lbs) is your best bet. However, if your wall is taller than 1.2 meters, you must increase the mass of the precast block exponentially. Therefore, you will have to use precast blocks weighing 800 kg (1,760 lbs) or more for gravity retaining walls ranging from 1.2 (4′) to 1.7 meters (5′ 6″) tall.

For retaining walls taller than 1.2 meters (4′), even marginal increases in the height will affect its stability. Therefore, you should consider your precast concrete block choice carefully. Even under optimal site conditions and using the largest precast blocks available, you can only achieve a maximum height of about 3.5 meters (12′) for a gravity retaining wall system. Therefore, having a qualified structural engineer design the retaining wall is a good idea when it is over 6′ (1.8m) in height. Also, a geotechnical engineer should ensure that the soil conditions can support the weight of a gravity retaining wall system.

Generally speaking, building a gravity retaining wall without additional reinforcement will cost less than building one with reinforcement, even when you have to use massive precast concrete blocks and large equipment to move them. Another benefit of using a large precast block is the speed of construction. The larger the block’s dimensions, the fewer “lifts” it will take to complete the retaining wall.

A word of caution here – not all large precast concrete blocks are created equal. Some precast concrete blocks are manufactured with leftover concrete from construction projects. Although cheaper to purchase, they will most likely disintegrate before the intended lifespan of the retaining wall as they have been made from concrete churning in a cement truck for hours. Therefore, only use these blocks if your load-bearing structure is temporary.

We provide in-house design services and have extensive experience working with every city in the Greater Hamilton and Greater Toronto areas. We also offer stamped design drawings, slope stability assessments, arborist reports, and letters of completion.

Contact us today for a free consultation. We would happily provide an innovative solution for your retaining wall project.

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