How To Backfill A Retaining Wall

retaining wall with plants in backyard

You have a yard with a slope and you’ve chosen to build a retaining wall. You have picked a wall type and maybe you’ve even poured the footing or prepped for the first course of block.  But now comes the questions. When do you backfill a retaining wall?  How to backfill a retaining wall?  What kind of backfill do you use for a retaining wall? Whether you use a retaining wall contractor or not, it’s a good idea to understand what’s going on in your own backyard.

The question of when to backfill a retaining wall depends on the wall type you are building.  If you are building a poured concrete wall, you want to wait until after the wall is poured to do any of the backfill work.  With a modular block wall, you will want to backfill as you build the wall.

How To Backfill A Retaining Wall Guide With Fresh Green Grass and Ferns Surrounding

How To Backfill A Retaining Wall Step By Step Guide

So, how to backfill a retaining wall? We have designed and backfilled countless retaining walls in the past 20 years. In this guide, we’ll explain the step by step process on how to backfill a retaining wall. We will also call out some important questions to ask yourself before and during the backfill process. Lastly, we’ll identify the common pitfalls people experience when backfilling their retaining wall.

Step 1: Drainage Consideration

First, you need to consider the drainage of the wall.  Does your engineer show a footing drain?  Even if they don’t it’s never a bad idea to install a perforated drain pipe behind the footing which drains to ‘daylight’ lower down the hill.  This helps prevent water from building up behind the wall.  One of the tricks to making sure your drain pipe doesn’t get clogged up quickly is to use filter fabric around the pipe.  Often you can buy rolls of perforated pipe with filter fabric already wrapped around them, these are an easy to use solution.  

Step 2: Backfill Behind The Retaining Wall

Once you have your drainage pipe installed, your concrete wall poured, or the first course of your block wall installed, you can start to backfill behind the wall.  Don’t use the dirt you dug out of the hillside for the backfill.  You want to spend the extra money on this step in the process. That means buying 3/4” or 1 1/2” crushed washed stone for the backfill material.  This will allow the soil behind the wall to drain down to your drain pipe. This is a much better alternative to holding water in the soil immediately behind your retaining wall.  

You can buy crushed stone from sand pits, stone pits, and excavation contractors.  Keep in mind they will be bringing a large dump truck to deliver the stone.  You want to make sure you have a good place to dump it on your site. Also verify there’s a way to move the stone from the dumping location to the wall itself. This is often a great excuse to rent a bobcat or a mini excavator.

Step 3: Backfill And Compact The Wall

Backfill the wall in 6-12” lifts, or as recommended by your engineer.  You can rent a jumping jack or plate compactor from Home Depot to compact each lift.  If you plan on installing a patio, sidewalk or driveway above the retaining wall, you really want to compact the backfill in lifts. 

If you plan on leaving the upper portion of the retaining wall as landscaping, the placement of crushed stone already achieves 90% compaction, which is fine for a landscape.  That said, always refer to your engineering for advice on backfill methods. Don’t be afraid to ask the engineer for clarification.

Step 4: Filter Fabric Layer And Top Layer Of Soil

Now you have the wall nearly backfilled to the top. Next you will want to put down a layer of filter fabric before placing the top layer of soil or the base material for your patio or driveway.  This prevents consolidation of the soil or base material into the air gaps between the crushed stone.  Then you can place your final layer of material and you’ve backfilled your retaining wall.

How To Backfill A Retaining Wall Near Fence And Property Line

One Final Item For Knowing How To Backfill A Retaining Wall

One item to note, you want to double check your engineering to be sure you are grading the soil properly at the top of your retaining wall.  Often engineers will prefer the soil grade away from the top of the retaining wall before sloping up a hill or flattening out.  This prevents surface water from running down to the wall. It also deters water from building up behind the wall itself. In cold climates, this could cause freeze thaw damage. Your risk is heightened especially if you have a stone cap on the wall or the top block of your modular block wall system.