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Geogrid connections by R. G. Carroll, Jr & V. Chouery-Curtis and connections for geogrid systems by M. R. Simac
Geotextiles and Geomembranes 9 (1990) 547-551
Discussion Geogrid Connections by R. G. Carroll, Jr & V. Chouery-Curtis and Connections for Geogrid Systems by M. R. Simac The importance of the two papers on geogrid connections by Carroll/ Chouery-Curtis and Simac, respectively, can be dramatically illustrated by the recent failure of a highway bridge abutment in the northeastern part of the USA. The 28 ft (8.5 m) wide deck of the bridge was placed on eight precast concrete box beams. Spanning a distance of approximately 100 ft (30m), these beams are supported by reinforced earth abutments consisting of tied-back precast concrete facing panels. A stream flows between the two abutments. The side of each abutment turns away from the stream in the form of wing walls until the adjacent ground surface is met. The upper photograph of Fig. I shows the north abutment illustrating the octagonal facing panels which form the abutment support system for the concrete beams and one of the wing walls of the same type of construction. While the north abutment appears to be stable, the south abutment suffered a rapid and dramatic failure in the form of a complete blowout of three of the panels. This was accompanied by massive distortion of the eight adjacent panels. Backfill soil was immediately discharged from the open void as is seen in the lower photograph of Fig. 1. The roadway support beams settled, the roadway was closed, and a temporary steel truss was erected to support the box beams as is seen in the lower photograph of Fig. 1. Fortunately, no one was injured in the incident. Clearly earth pressures, and in this case earth pressures superimposed by hydrostatic pressures, are very meaningful and can overstress connections of tie-back reinforcement used to support facing panels of this, and similar types of retaining walls. The cause of this failure was initiated by a very minute detail. The geotextile filter used to span the 1-0 in (25 mm) openings between the edges of the adjacent facing panels was in the form of 12 in (250 mm) wide 547 Geotextiles and Geomembranes (9) (1990)--~ 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
548
Discussion
Fig. 1. Photographs of North Abutment (upper) and South Abutment (lower) showing the stable situation and the failure situation.
Discussion
549
Fig. 2. Photographs of portions of the geotextile filter which were completely glued into a solid mass.
550
Discussion
Fig. 3. Closeup photographs of the failure zone showing the grid reinforcement ripped away from the facing panel connection and hanging freely in the void where soil was released during the failure.
Discussion
551
strips. These strips were glued onto the backs of the facing panels, but during the gluing application the entire width o f the strip was covered with glue, not only the two edges to be attached to the concrete panels. Thus the central part of the geotextile, which spanned the open space between the panels and was to act as a filter, was also glued (see Fig. 2). Laboratory tests of the retrieved geotextile samples showed that the fabric's permeability was essentially zero. Clearly, the intent of the geotextile was not recognized by the installation personnel insofar as its primary function of filtration was concerned. With the geotextile filter being completely clogged by the glue, the hydrostatic pressure behind the facing panels built up to a level which caused the connections between the facing panel and the grid reinforcement to fail in an abrupt and devastating manner. The bridge abutment is a total loss due to a small detail which caused overstressing of the grid connection to the facing panel. Figure 3 shows a close-up of the inside of the failed region showing the grid reinforcement. In this case, the grid reinforcement was a welded steel wire mesh; but the message is equally clear to polymer geogrids. Connections and attention to details, as illustrated in both of the referenced papers, are critically important to the stability of reinforced earth structures and to the safety of persons utilizing these structures. Anon.
Discussion Geogrid Connections by R. G. Carroll, Jr & V. Chouery-Curtis and Connections for Geogrid Systems by M. R. Simac The importance of the two papers on geogrid connections by Carroll/ Chouery-Curtis and Simac, respectively, can be dramatically illustrated by the recent failure of a highway bridge abutment in the northeastern part of the USA. The 28 ft (8.5 m) wide deck of the bridge was placed on eight precast concrete box beams. Spanning a distance of approximately 100 ft (30m), these beams are supported by reinforced earth abutments consisting of tied-back precast concrete facing panels. A stream flows between the two abutments. The side of each abutment turns away from the stream in the form of wing walls until the adjacent ground surface is met. The upper photograph of Fig. I shows the north abutment illustrating the octagonal facing panels which form the abutment support system for the concrete beams and one of the wing walls of the same type of construction. While the north abutment appears to be stable, the south abutment suffered a rapid and dramatic failure in the form of a complete blowout of three of the panels. This was accompanied by massive distortion of the eight adjacent panels. Backfill soil was immediately discharged from the open void as is seen in the lower photograph of Fig. 1. The roadway support beams settled, the roadway was closed, and a temporary steel truss was erected to support the box beams as is seen in the lower photograph of Fig. 1. Fortunately, no one was injured in the incident. Clearly earth pressures, and in this case earth pressures superimposed by hydrostatic pressures, are very meaningful and can overstress connections of tie-back reinforcement used to support facing panels of this, and similar types of retaining walls. The cause of this failure was initiated by a very minute detail. The geotextile filter used to span the 1-0 in (25 mm) openings between the edges of the adjacent facing panels was in the form of 12 in (250 mm) wide 547 Geotextiles and Geomembranes (9) (1990)--~ 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
548
Discussion
Fig. 1. Photographs of North Abutment (upper) and South Abutment (lower) showing the stable situation and the failure situation.
Discussion
549
Fig. 2. Photographs of portions of the geotextile filter which were completely glued into a solid mass.
550
Discussion
Fig. 3. Closeup photographs of the failure zone showing the grid reinforcement ripped away from the facing panel connection and hanging freely in the void where soil was released during the failure.
Discussion
551
strips. These strips were glued onto the backs of the facing panels, but during the gluing application the entire width o f the strip was covered with glue, not only the two edges to be attached to the concrete panels. Thus the central part of the geotextile, which spanned the open space between the panels and was to act as a filter, was also glued (see Fig. 2). Laboratory tests of the retrieved geotextile samples showed that the fabric's permeability was essentially zero. Clearly, the intent of the geotextile was not recognized by the installation personnel insofar as its primary function of filtration was concerned. With the geotextile filter being completely clogged by the glue, the hydrostatic pressure behind the facing panels built up to a level which caused the connections between the facing panel and the grid reinforcement to fail in an abrupt and devastating manner. The bridge abutment is a total loss due to a small detail which caused overstressing of the grid connection to the facing panel. Figure 3 shows a close-up of the inside of the failed region showing the grid reinforcement. In this case, the grid reinforcement was a welded steel wire mesh; but the message is equally clear to polymer geogrids. Connections and attention to details, as illustrated in both of the referenced papers, are critically important to the stability of reinforced earth structures and to the safety of persons utilizing these structures. Anon.