Case Studies In Earth Retaining Structures

The Experimental Media and Performing Arts Centre (RPI EMPAC) is currently being constructed on the Rensselear Polytechnic Institute campus in Troy, New York State. It is being built on a series of terraces cut into the sloping ground that stands above the Hudson River. The slopes are formed of lacustrine clays, and there is evidence of landslipping of these deposits in the area. During the ground investigation particular attention was given to identifying possible pre-existing shear surfaces. The foundation system evolved into a series of retaining walls, stepping down the slope, restrained by permanent anchors and propping slabs. The terracing employed over steepened the slope and increased the effects of the groundwater. Consequently at all stages of the project overall stability had to be carefully controlled. Early analyses, solely relying upon limit equilibrium methods, indicated that the benefit of the permanent ground anchors was grossly over-estimated. A methodology was subsequently developed, incorporating discrete embedded piles, to resist down slope forces and prevent global deep seated movement. For the temporary work the contractor adopted the use of king post walls, utilising the permanent anchors to assist in the resistance of the temporary loads. Problems occurred with these walls in freezing conditions. The anchors had to be partially destressed to prevent overloading.

The design of anchored earth retention systems for providing lateral support of adjacent ground is a highly empirical approach involving geometry and stiffness characteristics of the wall support system and retained earth. Typically, when designing an earth retention system, simplifying assumptions are required to reduce an indeterminate soil-structure interaction problem into a simple structural system for analysis, whereby movements of the wall and surrounding ground are limited within acceptable levels. This paper presents a case history of an anchored earth retention system for stabilizing a bridge abutment during construction. Factors considered in the overall design and construction of the system and the impact of these on ground movement are presented.

Geogrid-reinforced mechanically stabilized earth (MSE) walls are used for bridge approach/abutment applications. The bridge structure is typically supported on pile foundations constructed through the reinforced mass. The common construction technique is to build the MSE wall after the piles or the pile sleeves are installed. An alternative construction technique is to drive the piles through the constructed reinforced mass. This alternative offers significant construction advantages over the traditional approach. Full-scale demonstration/testing was conducted to investigate the feasibility of this alternative construction technique. The constructability of driving steel H piles through an HDPE geogrid reinforced MSE wall was demonstrated. This paper documents the wall construction and materials, instrumentation, and monitoring results of the full-scale demonstration/testing. The design implications and the advantages of this proposed construction technique are summarized.

Tieback anchors for a sheet pile retaining wall failed due to corrosion resulting in the near collapse of the wall. The sheet pile retaining wall, with a sloping backfill, was designed with drainage backfill and grouted anchors angled down at 20 degrees from horizontal. Corrosion protection was specified for the sheeting and the anchors. Part of the wall was parallel to and within about 10 feet of the owner's building. Finite element analysis was used to evaluate staged construction and to estimate the lateral forces imposed on the adjacent building foundations. The contractor deviated from the specifications in several respects and the owner accepted a credit in return for eliminating corrosion protection from the sheeting and anchors. Two years after completion of the wall, the owner notified the engineer that several anchors had failed. Investigation revealed that up to 25% of the anchor's cross section was lost in just two years due to corrosion. Installation damage was also identified as a possible contributing factor in the failure. The problem was corrected by excavating to expose the anchors, replacing the corroded length of the anchor rods, providing corrosion protection, replacing a portion of the backfill with approved material, installing isolation pads under the anchor bearings, and implementing drainage improvements.