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Burns Cooley Dennis, Inc. (BCD) provided geotechnical engineering services related to the investigation, design, and construction of the slide stabilization system at the Signal Hill landslide on U.S. Highway 61 in Vicksburg, Mississippi.

Evidence of landslide activity at the site was initially observed in 1977 shortly after construction of the roadway.  Repair of the landslide was classified as an emergency due to the threat of road closure and the probability of accelerating movement if left unattended.  An earthen berm was recommended for stabilization of the slide due to the simplicity of construction and ease of developing plans.  The berm required approximately 175,000 cubic yards of fill and the acquisition of approximately 10 acres of additional right-of-way.

Construction of the berm in 1978 reduced the rate of movement dramatically.  However, small intermittent movements continued requiring occasional resurfacing of the southbound lanes.  The rate of movement increased significantly in 2004 and an in-depth investigation of the landslide was initiated by the Mississippi Department of Transportation (MDOT).  BCD was retained to assist in the field and laboratory investigations, with the evaluation of alternative remediation techniques, and to prepare the detailed design of the recommended stabilization system.

BCD recommended a permanent ground anchor/buried anchor block stabilization system.  This system was constructed without acquiring additional right-of-way, without impacting existing high pressure gas lines, and was contracted and constructed in a relatively short time frame.

Role of Other Consultants

Other important members of the design/engineer team included the MDOT geotechnical branch, ABMB Engineers, Inc., Advanced Engineering Resources and Hayward Baker Inc.

MDOT completed soil borings, installed geotechnical instrumentation, and made geologic interpretations of the subsurface.

ABMB Engineers, Inc. performed the land survey.

Advanced Engineering Resources served as structural engineer for the project.

Hayward Baker, Inc. (HBI) was the specialty geotechnical contractor.  HBI provided the
design of the anchors and anchor blocks and installed the anchors.

Original and Innovative Applications of New or Existing Techniques
The Signal Hill landslide is approximately 1,500 feet long, extends approximately 800 feet west of Highway 61, and covers an area of about 20 acres.  The escarpment of the slide crosses the southbound lanes at two locations, placing about 1,000 feet of roadway within the active slide area.  The escarpment extends to and follows the shoulder of the northbound lane for a distance of about 200 feet.  The maximum depth of sliding is about 70 feet below the present ground surface.

Geologic interpretations of soil boring and downhole geophysical data revealed an extremely complex system of progressive landslides.  It is postulated that landslide activity began in the late Pleistocene epoch (10,000 – 20,000 years B.P.) and failure progressed from west to east.  Sliding was most likely triggered by degrading of the valley west of the landslide by Hennesseys Bayou. The majority of the slide plane was found to be in high plasticity clays of the Catahoula and Bucatunna formations.  Instrumentation installed and monitored during this investigation revealed that the velocity of the slide is proportional to groundwater levels.

Several constraints had to be considered in the selection of a stabilization system:  1) It was important that the system be designed and constructed in a short time frame.  Large movements of the landslide could threaten the northbound lanes and would result in increased costs for reconstruction of the southbound lanes; 2) The stabilization system needed to be confined to the existing right-of-way.  The acquisition of additional right-of-way would result in significant time delays and expense; 3) High pressure gas lines located just west of the toe of the slide could not be impacted by construction; 4) A residence is located within the limits of the landslide south of the existing right-of-way; and 5) Topography in the project area is relatively severe and limits alternatives for new alignment of the roadway.

A number of possible methods of repair were considered.  These included realignment of the roadway, lowering the grade of the southbound lanes, subsurface drainage, a drilled shaft retaining wall, an earthen berm, and permanent ground anchors.  Permanent ground anchors were selected based on cost and conflicts of the other options with the constraints described in the preceding paragraph.

The design resulted in five rows of anchors evenly spaced from the crest of the slope to the toe of the slide with an anchor capacity along each row of 25 kips per foot.  The anchors were bonded in limestone layers within the Glendon formation which underlies the high plasticity clays.  At the surface, each anchor was attached to a reinforced concrete anchor block dimensioned to limit the contact stress between the anchor block and soil to 2,500 lbs per sq ft.  The anchors were inclined at 45 degrees from the horizontal.  Anchor lengths ranged from about 250 ft to 115 ft.  The design capacity of each anchor was 465 kips.  A total of 250 anchors were installed with a design capacity of 465 kips per anchor.

The anchors basically consist of high strength cables grouted into a borehole.  Each anchor includes a bonded zone and an unbonded zone.  Within the unbonded zone, the cables are greased and encapsulated in plastic sheaths to prevent bonding to the grout.  The cables are in direct contact with the grout in the bonded zone.  Grouting is accomplished by pumping through a tremie tube that is bundled with the cables and extends to the bottom of the borehole.  Two levels of corrosion protection are provided – first by the grout and second by a corrugated plastic casing extending the full length of the anchor.

Every anchor was load tested to verify capacity.  After testing, the tension in the anchor was “locked off” by means of wedges at the head of the anchor.  The load in each anchor was also measured 28 days after “lock off” to determine whether any de-tensioning had resulted due to settlement of the anchor block.

Groundwater levels and ground movements were closely monitored during construction.  Construction sequences were adjusted to minimize ground movements.

Future Value to Engineering Profession
The ground anchor/buried anchor block system is a unique and cost-effective method for slide stabilization.  It can be used in areas where limited right-of-way and/or topography preclude the use of conventional stabilization methods and in cases where stabilization forces need to be evenly distributed over large areas.  The use of buried anchor blocks allows the stabilized slope to be restored to its original configuration with no visible evidence of the stabilization system.

Social, Economic and Sustainable Design Contributions
The U.S. Highway 61 by-pass at Signal Hill in Vicksburg, Mississippi is heavily traveled with cargo/freight and passenger vehicles.  The highway is crucial to Mississippi’s tourism and economic development being a major thoroughfare within the state and an access to surrounding states and the Southeast.

Complexity
The Signal Hill landslide is actually a complex composition of numerous landslides that most likely initiated 10,000-20,000 years B.P. in the late Pleistocene epoch.  Sliding has occurred primarily in high plasticity clays of the Catahoula and Bucatunna formations.  These clays exhibit strain softening behavior when sheared, and displacement can occur at different velocities over a wide range of stress.  Analysis of landslides involving high plasticity clays is complicated by these time dependant and stress dependent phenomena.  The interaction of multiple slides also requires special consideration in the design of the stabilization system.  Stabilization of an active landslide is further complicated by the potential for significant displacements during construction.

Exceeding the Owner/Client Expectations
BCD recommended the ground anchor/buried anchor block stabilization system to accommodate a number of constraints at the project site.  More common methods of slide stabilization were rejected because of cost and conflicts with site constraints.  Use of the ground anchor/buried anchor block stabilization system allowed the design and construction to be completed in a relatively short time frame, did not require the acquisition of additional right-of-way, avoided impact of nearby high pressure gas lines, and was the most economical method to achieve the desired increase in factor of safety.