While constantly pressed to provide longer bridge spans, to support heavier loads, and to cross wider openings, engineers also strive to minimize the costs of bridge construction. As a result, many advancements have been made in bridge design both in the superstructure and substructure designs.

In the last 10 years, the rate of change with respect to bridge substructure design has accelerated for a number of reasons. One is the renewed focus on bridge design with the deterioration of existing structures, many of which are over 50-years old, and the need for replacement spans.

Some of the most significant changes in bridge design have resulted from the evolution of design codes. AASHTO design codes today specify increased loading requirements compared to the past. AASHTO requirements for seismic design also have advanced along with our understanding of technical issues, such as liquefaction. As a result, the lateral design loads are much higher for comparable structures today, with more emphasis on earthquake and scour conditions. This, in turn, results in significant changes with respect to substructure design.

Coupled with design code changes, there are other practical aspects of design. Many of the new bridges today are replacements of existing structures. In most cases, the replacement bridge has to be built while maintaining traffic on the existing structure. This results in either an alternate alignment or staged construction. With an alternate alignment, the subsurface conditions are usually less favorable, assuming the original designers optimized the bridge alignment, length, and cost. With staged construction, there are typically design issues related to the interaction of existing and new foundations. These issues include the effects of installing new foundations, as well as new loads imposed on existing foundations.

Another very significant change in the recent past is the influence of environmental factors on design and construction. Requirements that impact the excavation, handling, and disposal of river bottom sediments are continually more restrictive. Consequently, substructure design and construction techniques are being developed and modified to lessen the need for excavation. Another aspect of environmental impacts is the tightening of requirements regarding equipment access across wetlands and mud flats. In some cases, these restrictions have a major affect on the substructure design and construction.

Foundation designs meet requirements

In response to increased engineering demands, the designers and constructors of bridge substructures are developing new foundation approaches. In most places, the large cofferdams and massive pile groups, previously used as the staple for bridge foundations, are long past.

Today’s substructures are smaller, less intrusive, and capable of resisting greater loads. These foundations involve minimal footprints to reduce the excavation of possibly contaminated materials. They are less intrusive, because in many cases there is little to no construction below the water line, and, therefore, no need for a cofferdam. They can resist greater loads because higher capacity methods (drilled shafts, load-bearing elements, rock anchors, and so on) are being employed that were not available to our predecessors.

High-capacity substructures

Haley & Aldrich is involved in the design of three major bridges that demonstrate some of these significant changes. In Boston, construction of the Charles River Bridge, which is the signature structure for the Central Artery/Tunnel project, is scheduled to be completed in 2002. The superstructure of this bridge is unique in many ways including the fact that when complete it will be the widest cable-stayed structure built. The substructure is unique also.

Due to a combination of unusual subsurface soil conditions (fill overlying loose sands) and seismic design requirements, the foundations for the bridge have been designed to resist large lateral forces that may result under design earthquake loading. These forces include those associated with lateral spreading of the loose sands.

The foundations (8-ft.-diameter drilled shafts with 7.5-ft. rock sockets drilled approximately 25 to 40 ft. into the underlying bedrock) are high-capacity units capable of supporting vertical loads of 5,000 kips each. The foundations were installed using slurry methods and included the use of cross-hole sonic methods to assess the integrity of the concrete placed using tremie methods in each drilled shaft.

The location of the 10-lane bridge with a span of 1,407 ft. was dictated in part by the presence of the original bridge structure that has to remain in operation, as well as the existing subway tunnel that the tower legs straddle.

In two other Central Artery design areas, Haley & Aldrich designed monoshaft foundations (8- to 10-ft.-diameter drilled shafts) that form single element foundations. The monoshaft foundations transition into the substructure pier without any pile cap. These elements were developed due to inadequate plan space to construct conventional foundation caps for load transfer from the pier to the foundation.

Another example of bridge substructure innovative design is the New Chelsea Street Bridge, also in Boston. This replacement bridge is a vertical lift span of 450 ft. that will replace an existing bascule bridge on the same alignment. The greater length of the new span is driven by the need to maintain the existing span coupled with today’s ability to construct higher-capacity structures and foundations. The foundations for the new bridge will be drilled shafts (3 to 5 ft. in diameter, rock sockets of 5 to 30 ft.). Drilled shafts were chosen in part to address environmental concerns, and to build foundations with minimal excavation without a cofferdam. Drilled shafts were also selected due to the significantly different ground conditions at each of the two tower legs, and tight deflection tolerances.

Future subsurface design