What Is a Slope Failure?

Posted by on Aug 16, 2016

A slope failure typically occurs after heavy rains when the water adds weight to the soil and acts as a lubricant making it weaker and more likely to slide.  Sain Associates recently completed a slope repair construction project on Golson Road in Autauga County, just outside of Prattville.

This 300’ section of county road has historically had slope problems over the last 25 years.  The county maintenance crews have done numerous repairs to the road, but never had the resources necessary to permanently  fix the slope problem.  Eventually the county was successful in getting federal funding that allowed for a large scale repair to be designed and constructed.

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Here is an example of the site back in 2014 prior to our project starting. The left side of the road had been repaired many times.  The slope just below the roadway had water seeping out which caused the existing material to become very weak.   A geotechnical engineer did the soil analysis and made the slope repair recommendations which Sain drafted up and put into a construction plan set.

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This picture shows the site as we started construction in December of 2015.  It was the rainy, cold season and that’s just not a good time for construction, especially doing work like this.  You can see the water collecting at the bottom of the slope.  The further we excavated down, the more water came in and the slope became even more unstable.  After struggling to make progress in the wet weather, in March things began to dry up a bit and the contractor modified his system to increase production.

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This image shows all the materials we were working with.  On the far right is the roadway.  The middle orange colored dirt is new material we are placing in 8” lifts and compacting to specifications.  Next is the black geofabric liner which encases the large riprap stone.  These large stones are 24” – 36” each.  We placed about 11,000 tons of this in a layer about 15’ wide by 25’ high.  This stone buttress serves as an anchor to hold up the slope and also allow water to drain out of the slope to the drains that were installed.  The left side of the photo shows the existing gray material that was on the site when we began.  This material was very weak and spongy when it got wet; it  was excavated within 80’ of the roadway and removed off site, replacing it with the orange material brought from a nearby pit that has stronger engineering properties.

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Productivity went well in April and May.  This photo shows the orange fill material nearly back up to the roadway elevation.  This fill was approximately 40’ deep and every lift that was placed was compacted and tested using a nuclear gauge to check that the density met specifications.

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These last 2 photos show the finishing touches being done on the project in June.  After backfill over the buttress was done, the contractor graded the site smooth, placed topsoil and spread grass seed.  The pavement structure included 6” of crushed aggregate base with 3” of hot mix asphalt.

Large slope failures are not too common of a problem in our area.  The geotechnical engineer we worked with knew of about five other examples across the state over the last 10 years.  For this type of project, the design stage lasted about 18 months, and then the construction took another 9 months.

Sain’s role during the construction process was to do Construction Engineering and Inspections.   Sain had an inspector on the project every time the contractor was working, and as project manager I would typically make weekly visits.  Along with inspections, another important function of CE & I is to keep track of the quantities of bid items for making monthly payments to the contractor.  On the management side, Sain worked with the contractor and the geotechnical engineer to develop several changes in the plans during the wet weather to try to progress the work.

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