Retaining Wall with Box Culvert

VIDEOS, Monster Walls
By Doug Blackburn
Sep 18, 2008 - 4:52:12 PM

Today we’re in Colerain, Ohio, at the Stone Creek Shopping Center and we’re taking a look at one of the walls that we designed.  There are a number of retaining walls here comprising about 32,000 sq.ft. of retaining wall.  What we’re going to talk about specifically is the large drainage pipe that comes through the center and the bottom of this retaining wall.  We’re going to talk about a number of things that need to be taken into account when a large drainage pipe comes through a retaining wall.


We’re at the bottom of the retaining wall now and you will notice that this is a really large box culvert structure.  The first thing to consider is that this box culvert needs to be designed to handle the weight of the retaining wall above it.  Typically, this is not the responsibility of the retaining wall designer, but the retaining wall designer may be asked to give calculations or estimations of the weight of the wall bearing on the structure.  The person who designed the box culvert will ensure that this box culvert could handle the weight above.  The wall designer needs to concern himself with two important things with respect to this box culvert and with this creek.  The first is called rapid drawdown and the second is loss of soil material adjacent to the box culvert.  Rapid drawdown is a condition where this box culvert is filled with water from the creek.  So there is water 10 to 12 feet deep in here as this creek is running nearly full.  That water will permeate back into the retaining wall structure.  When the creek water goes down, there is still water pressure inside the wall.  That water needs to be collected and removed as rapidly as possible.  The best way to do that is to use an all gravel or other free draining granular material in the lower region of the wall below some certain elevation.  Typically, that’s the 100-year flood.  Additionally, there needs to be a significant number of 6- to 8-inch drainage pipe outlets from behind the wall.  Remember, those drainage pipe outlets need to be protected with a sock or other filtering device so you don’t lose the retaining wall backfill out of those pipes.  The second thing we’re going to take a look at is ways to prevent loss of soil where the retaining wall meets the edge of the box culvert.  What we’re gonna do is take a walk up here and I’m want to show you something in this area.


Because we’re in an environment where there will be a lot of water and because the material making up the reinforced zone of the retaining wall typically consists of finer particles such as silt and sand, we need to protect against loss of those particles along this interface where these blocks meet this headwall structure.  The best way to do that is by putting a relatively thick non-woven geotextile (it’s a filter fabric) along the backside of the block and running a couple of feet along the inside of this structure.  So, if you can imagine behind this block, there is a piece of fabric running down the structure and it turns and goes behind the block.   Behind that filter fabric then, you have drainage gravel, then you have your sand for the reinforced zone.  That filter fabric prevents loss of soil through these very small, tiny gaps and cracks.

Another thing that has to be performed when constructing retaining walls around headwalls is to make sure that all gaps are filled in with non-shrink grout, concrete or a good strong mortar.  This small area was filled in with some concrete because there was no way to shape and cut the block exactly to fit in there.

Today, we looked at a retaining wall that had a relatively large box culvert coming through the retaining wall.  Some important things we talked about are making sure that the box culvert is designed to handle the load of the retaining wall and making sure that, as the designer, you consider rapid drawdown and that you take preventive measures so that you don’t lose soil material where the retaining wall meets the box culvert.