Luther Forest Technology Campus – Main Access Road Structures
Platinum Award for Engineering Excellence from the American Council of Engineering Companies (ACEC) of New York
The Luther Forest Technology Campus (LFTC) located in Malta, New York is a 1,414-acre site designed specifically for semiconductor and nanotechnology manufacturing and their support industries. A network of roads servicing the campus has been constructed, several of which cross deep ravines which required the construction of stream crossing structures. Support of these structures proved to be challenging, as the structure locations were underlain by deep soft and compressible soils. C.T. Male was retained as the Geotechnical Subconsultant by Creighton Manning Engineers, LLP to analyze these conditions and recommend the most cost effective foundation for the structures. Creighton Manning Engineers, LLP was the prime consultant responsible for the design and construction oversight of the site roadways.
Protection of wetlands and other environmental permitting issues required the stream crossing structures to be wide relative to the size of the streams they would convey. Precast concrete arch culverts were selected over conventional bridges as being the most cost effective type of structure. The culverts were precast in 5-foot long sections and, upon being set in place on their foundation, the joints between sections were filled and a waterproofing membrane placed over the entire length of the culverts prior to their backfilling. Limitations on the differential settlement of these structures were imposed to maintain the integrity of the joints between segments; i.e. prevent “pinching” or separation either of which could induce cracking of the segments at their joints, failure of their waterproofing membranes and potentially progressive loss of ground (backfill material) into the culverts.
A maximum differential settlement one-half (1/2) of an inch from the middle of a structure to its end was established for the two 240-foot long by 36-foot wide open bottom culverts. Complicating the design of stream crossing structures over soft and compressible ground, is the need to construct approach embankments. At the Luther Forest Technology Campus site, the approach embankments were 25 to 28 feet high and were estimated to cause the ground to settle 6 to 9 inches. Such settlement would induce downdrag loads on pilefoundation first considered for support of the structures. The downdrag loads over the 75-foot long piles were estimated to be greater than the pile’s geotechnical capacity, leaving no capacity for the piles to support the structures themselves. In addition, as the embankments were high, their weight would induce lateral squeezing and “pushing” of the soils against the pile foundations. The lateral loads induced by lateral squeeze and the stresses these loads would impose on the structures were difficult to predict and design for. Accordingly, attention was given in design to precompressing the ground under the weight of the approach embankments to eliminate downdrag and lateral squeeze from further consideration. If this could be accomplished, conventional pile foundations could be used to support the structures and the weight of backfill placed over them. Further analysis, however, revealed that spread foundation support of the structures would be feasible if the structures were backfilled with geofoam (expanded polystyrene) with a unit weight of approximately one (1) pound per cubic foot.
To allow for support of the structures by spread foundations, the approach embankments needed to be constructed very close to where the foundations would be located and with side slopes that were very steep. High strength, geotextile reinforced earth embankments were designed and constructed to accomplish these goals. The ground settlement induced by their weight was accelerated to meet the construction schedule through the installation of vertical drains (“wick drains”) spaced at intervals of 5 feet near the toe of the embankments and at 10 feet further behind the embankment toes.
Four (4) such embankments were constructed, with two approach embankments for each structure. Construction was progressed 2 to 3 reinforced lifts at a time, moving from one embankment to the other. This staged construction was followed to allow the pore water pressures induced by each stage to fully dissipate before the next stage of embankment construction. By allowing for this dissipation, the embankments would not overstress the soft ground to the point of inducing global failure of the embankments and would allow the ground to fully compress under each load increment. Sophisticated laboratory tests were conducted on samples of the soft soils to determine their time dependent shear strength parameters. Vibrating wire piezometers were installed to monitor the pore pressure dissipation within these soils. Settlement platforms were used to measure the progression of ground settlements induced by the incremental construction of the embankments. Construction of the embankments proceeded without any delays between stages and the measured ground settlements were within the limits predicted.
Once the approach embankments were fully constructed, the spread foundations for the structures were cast and the structures installed on them. Geofoam, with a unit weight of about one (1) pound per cubic foot, was used as much of the structure backfill. The balance of the structure backfill consisted of lightweight expanded aggregate and sand removed from cut areas of the site. Utility lines crossing the structures, water and stormwater, were supported directly on the geofoam. Settlement of the structures was monitored as the backfill was placed. Upon completing the backfill and paving the road across these structures, a maximum differential settlement of 0.60 inches was measured. This value essentially equaled the maximum considered permissible.
The use of spread foundations over pile foundations to support the structures resulted in a cost savings to the Owner of 2.17 million dollars on a project that had a total cost of 33 million dollars. Settlement of the structure compared favorably with estimated amounts. The maximum differential settlement of one of the structures slightly exceeded the limit recommended by the supplier but with no ill effects apparent.
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