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RE: Segmented Retaining Wall Performance Specifications

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> From: Rich Lewis [mailto:sea(--nospam--at)] 
> Sent: Monday, January 03, 2005 11:15 AM
> To: seaint(--nospam--at)
> Subject: Segmented Retaining Wall Performance Specifications

First, as a disclosure, I do not specify MSE walls.  I am, however, the
"design" part of design-build MSE walls.

> I am putting together a bid package for a segmented retaining
> wall.  The package is a performance specification bid and
> construction.  I am not personally designing the wall.  I want
> to get enough information on the documents so that the
> proprietary vendors will be bidding on an “apples to apples”
> basis.

> I would like to get some feedback from some of you who have put
> these performance specification type packages together.  I would
> like to hear about what kind of information should be included
> and what shouldn’t.  If you have done this type of package and
> realized you should have included “such and such” information
> and didn’t, I would appreciate knowing it.  If you have been
> burned by a contractor because you didn’t have the right
> information on your documents, I would like to hear it.

> My documents include a generic specification and the following
> drawings:
> • Site plan showing grading contours and wall location
> • Typical wall section showing battered wall and fabric
>  embedments.
>  They are minimum embedments with the design engineer increasing
>  them as required for wall design.

If you put dimensions on this "typical" detail, make sure you CLEARLY
INDICATE that the reinforcement embedments are minimum only, and that the
final lengths are to be determined by the specialty EOR.

> • Typical section also shows surcharge loading and drainage
>  aggregate and filter fabric.  Also, footing is the compacted
>  crush stone and a note requiring stabilizing existing soils
>  below wall footing.

You should allow an un-reinforced concrete footing, which is preferred by
most contractors.  Specify the allowable bearing pressure at the toe of the
wall if it's critical.

> • An additional section at a lake shoreline showing riprap at
>  base of wall, filter fabric below riprap and all the items on
>  the first section.

Is the toe of the wall going to be submerged?  If so this should be clearly
indicated as it has rather large design implications, such as using the
buoyant unit weight of the all soils.  Also, the issue of drainage during
rapid draw down should be addressed. 

> • Wall sections and elevations where box culverts and large
>  concrete pipe penetrate wall.

It's amazing how many engineers forget about things like this.

> • Typical block unit detail showing nominal dimensions.

If you (or the architect or owner) want to use a specific type of block,
then clearly indicate it.  Otherwise, you don't want to limit the contractor
to specific block dimensions if you don't have to, because the "hungriest"
local contractor might have an agreement to use a specific model that
doesn't match your dimensions.  You do, however, want to include something
showing either minimum sizes or a list of acceptable blocks.  Otherwise, a
contractor might show up and try to build a 30' tall wall out of 4" units
from Home Depot.

> • Typical details of convex and concave radius corners
> • Typical details for 90 degree corners, interior and exterior.
> • Geofabric lap details for the 2 corner conditions noted above
> • Typical step footing detail
> • Typical cap details.

These are all typical details, and any competent contractor doesn't need
them.  However, including them on the drawings to help weed out the
incompetent contractors, and as something to point to if things go wrong,
isn't such a bad idea.

> • General notes giving information about responsibility for
>  design, generic notes and geotechnical design information.

Geotechnical information is essential if you want an accurate bid.
Otherwise, the bidder will assume "typical" values and hit you with a change
order if the actual site has worse soil.

Also, a huge sticking point is the responsibility of global stability
design.  Some Civil EOR's try to force the wall designer to take full
responsibility, but this is not reasonable.  As a wall design engineer, I
can only control the materials and construction quality inside the
reinforced zone of the wall.  The Civil EOR has control over the toe and
backfill slope, toe and backfill material, foundation material, and presence
of external loads (traffic or building).  If global stability is an issue,
the Civil EOR (or hired geotechnical engineer) should design the entire
slope to prevent failure, then give the wall designer minimum values for the
wall design (such as toe embedment or reinforcement length).

That being said, I do perform rough global stability checks and alert the
CEOR when it looks to be an issue.  The "lake" wall section you mentioned
implies that it might be.  I do not, however, take design responsibility.

> There is also a written specification section for the project
> manual.

> I got a lot of this information from NCMA design manual. I think
> the one I had was the 1st edition and I understand there may be
> a 2nd edition out now.

The current edition is the 2nd (1997).  Also the other books mentioned by
Harold Sprague.

> Please feel free to email me privately if you don’t want to post
> on the list server.

Other suggestions:
 - Require the design to be sealed by a PE registered in the project state.
 - Require a competent contractor.  This sounds like a pretty large project,
so require references from other similarly sized projects.
 - Require a plan and profile view of each wall, showing face area, top and
bottom of wall elevations, and reinforcing grid type and length.
 - If you plan on actually reviewing and checking the design calculation,
require a sealed calculation package.
 - Require periodic testing (material and compaction) of the foundation,
reinforced, and retained backfill material.  The foundation testing is to
discover errors in the original design assumptions, and the other is to make
sure the contractor is providing what you specified. 

Hope this helps.  Reply if you have any more questions or comments.

Jason Kilgore, PE, SE
Retaining Wall Design, Inc.
Kansas City, Missouri

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