New Substructures Beneath Existing Bridges

EXECUTIVE SUMMARYTo minimize traffic impact during accelerated bridge construction (ABC) projects, it issometimes desirable to construct the new substructure underneath an existing bridge prior to itsdemolition and road closure. Installing a new substructure under an existing bridge createschallenges during construction, primarily due to the low overhead space and stability concernsfor the existing foundation.A preliminary literature search revealed that documentation of the technical details for methodsthat can be used during the construction of new substructures underneath existing bridges are notreadily available. This limited availability of documentation indicates the methods are not likelyto be used by state departments of transportation (DOTs), local agencies, consultants, orcontractors.The objectives of this project included three main focus areas: Document through a literature search and survey, methods (including but not limited tomulti-splicing and micropiles) for constructing new substructures beneath existing bridgesEvaluate proven methods in terms of design considerations, constructability, and costDocument the project findings and develop method selection recommendations/guidelinesA literature review was first completed to obtain knowledge about different foundation methodsand their application in ABC projects. The findings are presented as brief case studies in thisreport.In the next step of this study, a survey was developed and distributed via email to investigatemethods utilized by other DOTs in the construction of new foundations on ABC projects. Thesurvey included questions about the common methods for bridge foundation construction.Key FindingsFoundations for bridges are divided into two main categories: shallow foundations and deepfoundations. Shallow foundations include spread footings and mats. Driven piles, drilled shafts,continuous flight auger piles, and micropiles are categorized as deep foundations. Each methodhas its own advantages and limitations.Spread footings can be an economical and practical approach for bridge foundations, but thismethod is not applicable for high structural loads or weak soil conditions. Although groundconditions can be improved by different methods to accommodate spread footing foundations,that may not have economic justification.ix

Driven piles with various types of material, cross sections, and driving methods are one of theapproaches in bridge foundation construction. Through-deck pile driving is a method that can beused on ABC projects when there is restricted space under the existing bridge.Drilled shafts can provide proper axial and lateral resistance to loads induced from thesuperstructure. However, they can only be installed outside of the existing bridge footing, whilethere is also some newer equipment for spaces with low headroom.Continuous flight augur piles are another approach to install piles for bridge foundations that canbe utilized under existing bridges.The last method from the deep foundation family is micropiles. Micropiles have a small diameterbut can resist significant axial loads and moderate lateral loads. The installation equipment formicropiles is relatively small and can be mobilized easily. However, the cost of micropilesusually exceeds other piling systems.ConclusionsAccording to the recorded information from the email survey, 47 DOT respondents opened thesurvey link and 19 states answered at least one of the questions. According to the survey results, state DOTs use common methods for bridge foundationconstruction on ABC projects, and there is no new method in useUsing a soil strengthening method is not used in most statesThe continuous flight auger method does not appear to be a common method among stateDOTSThe average height for minimum headroom for micropiles is nearly 13 ftThe average height for minimum headroom for a drilled shaft is nearly 27 ftThe average height for minimum headroom for steel pile driving is nearly 35 ftThe disruption of through-deck piling on traffic flow is evaluated as highImplementation Readiness and BenefitsDocumenting proven techniques for constructing new substructures beneath existing bridges andcomparing their design considerations, constructability, and costs will help engineers determinethe most appropriate application of these techniques. This could help engineers make moreconsistent, efficient, and cost-effective decisions and reduce the risk in using the techniques onABC projects.x

CHAPTER 1: INTRODUCTION1.1BackgroundTo minimize traffic impact during accelerated bridge construction (ABC) projects, it issometimes desirable to construct the new substructure underneath an existing bridge prior to itsdemolition and road closure. Installing a new substructure under an existing bridge createschallenges during construction, primarily due to the low overhead space and stability concernsfor the existing foundation.A variety of approaches have been used to either construct new or rehabilitate existing bridgeswith new foundations. Some examples include use of spread footings, use of micropiles, anddriving piles through holes in the existing deck. Each method has advantages and limitations.A preliminary literature search revealed that documentation of the technical details for methodsthat can be used during the construction of new substructures underneath existing bridges are notreadily available. This limited availability of documentation indicates the methods are not likelyto be used by state departments of transportation (DOTs), local agencies, consultants, orcontractors. Therefore, documenting proven techniques and comparing their designconsiderations, constructability, and costs are necessary to determine the most appropriateapplication of these techniques. This will assist engineers in attaining more consistent, efficient,and cost-effective decisions, and reduce the risk in using the new techniques.1.2Objective and ScopeThe objectives of this project included three main focus areas: Document through a literature search and survey, methods (including but not limited tomulti-splicing and micropiles) for constructing new substructures beneath existing bridgesEvaluate proven methods in terms of design considerations, constructability, and costDocument the project findings and develop method selection recommendations/guidelines1

CHAPTER 2: LITERATURE REVIEW AND EVALUATION2.1IntroductionABC provides a cost-effective approach to rapidly replace an existing bridge and reduce theimpacts on the mobility and safety of transportation systems. Due to the overall poor condition ofbridges in the nation and ever-increasing demands of the traveling public, ABC has beenattracting more and more interest over the past decade.With ABC projects, it is sometimes necessary to build the new substructure beneath an existingbridge. This requires the contractor to deal with challenges associated with limited overheadspace and in ensuring the stability of the existing foundation system. Several approaches havebeen used with the goal of solving these problems.When soil conditions allow, spread footings are ideal given that excessive headroom is notrequired during construction. Spread footings can be built using traditional constructionapproaches. However, bedrock is seldom near the surface in Iowa and the utilization of spreadfootings are therefore not ideal in Iowa.Piles are economically preferred in most cases in Iowa. However, the vertical clearanceunderneath a bridge is typically not enough to allow driving of traditional piles.While multiple splicing of common-length piles seems a potential solution to reduce the requiredheadroom, the current Iowa Department of Transportation (DOT) Bridge Design Manual doesnot allow pile splicing of piles with a length shorter than 55 ft and does not allow multiplesplicing of piles with a length between 56 ft and 110 ft. A review of technical papers and reportsand other states’ practices will compile current information on approaches, techniques, andperformance data related to multiple-pile splicing.This chapter provides a general definition of accelerated bridge construction. Then, differentsolutions for bridge foundations are reviewed and assessed for ABC projects. To investigateprobable new methods in foundation construction related to ABC projects and also gatherinformation about common methods, a survey was distributed to state DOTs. The last section ofthis chapter is devoted to the survey and its results.2.2Accelerated Bridge ConstructionABC is a bridge construction method that uses innovative planning, design, materials, andconstruction methods in a safe and cost-effective manner to reduce on-site construction time(Culmo 2001). The key components of an ABC project, which make it different fromconventional bridge construction, are reduction of on-site construction time and returning thebridge to service within a short period of time.2

2.3Conventional Construction Foundation Solutions for ABC ProjectsWith conventional construction for ABC projects, the foundation is the first structural elementconstructed and its performance has a significant effect on the overall performance of thestructure. Generally, foundations are classified into two broad groups: shallow foundations anddeep foundations (Coduto et al. 2011). Shallow foundations include spread footings (footer orsimply footing) and mats. Driven piles, drilled shafts, continuous flight auger piles, andmicropiles are categorized as deep foundations. The following sections briefly review differentfoundation types and assess the feasibility of each type for ABC projects.Shallow FoundationsSpread footings are a general type of shallow foundation used in bridge construction. Comparedto deep foundation methods, generally, spread footings are more economical. If the geomaterialand ultimate loading conditions are appropriate, spread footings can be one of the favorablechoices for bridge foundations.The main factors that make a spread footing suitable for ABC projects are that it does not need alarge space or tall equipment to be constructed. However, shallow foundations are restricted instructural loads and ground conditions.This method is not appropriate for situations in which the foundation experiences large uplift orlateral loads; also, it is not suitable for foundations subjected to large settlement, liquefaction, orscour. More detailed information about usage of spread footing foundations is provided inImplementation Guidance for Using Spread Footings on Soils to Support Highway Bridges(Abu-Hejleh et al. 2014).In some situations, the ground condition is improved to supply adequate soil properties needed touse a shallow foundation. Generally, there are nine methods to improve ground condition(Schaefer et al. 2016): Vertical drains and accelerated consolidationLightweight fillsDeep compactionAggregate columnsColumn-supported embankmentsSoil mixingGroutingPavement support stabilizationReinforced soil structures3

Driven PilesWhen one or more upper soil layers are highly compressible and too weak to support loadstransmitted by the superstructure, piles are used to transfer the load to underlying bedrock or adeep strong soil layer. The piles provide efficient resistance to both lateral and vertical loads.Driven piles are the most commonly used deep foundation approach and one of the provenfoundation systems used for transportation projects. Piles can be categorized by different aspectssuch as material, cross-section, and driving method. Primary pile materials include steel,concrete, and timber. However, plastic piles, including various composite materials such aspolymer composites, polyvinyl chloride (PVC), and recycled materials, are used nowadays inspecial cases.Considering their materials, piles can have various cross sections. Steel piles are generally eitherpipe piles or rolled steel H-section piles. Shell-type cross sections, such as Z or U profiles, aretypes of sheet pile. Pipe piles can be driven to the ground with their ends open or closed; openend pipe piles are available in diameters that range from 8 in. to 160 in. and closed end pipe pilestypically range from 8 in. to 30 in. (Hannigan et al 2016). Typically, favored sections for H-pilesare in 12 in. to 14 in.Concrete-driven piles, with rectangular or octagonal cross sections, can be fabricated usingordinary reinforcement or pre-stressed cables. Prestressed piles can either be pre-tensioned orpost-tensioned. Pre-tensioned piles are usually cast to their full length in permanent casting beds.Post-tensioned piles are usually manufactured in sections, most commonly cylindrical, andassembled and stressed to the required pile lengths at the manufacturing plant or on the job site.Reinforced concrete piles are more susceptible to damage during handling and driving becauseof tensile stresses compared to prestressed piles. However, reinforced concrete piles are easier tosplice

efficiency, safety, reliability, and sustainability while improving the learning environment of stu-dents, faculty, and staff in transportation-related fields. Disclaimer Notice The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein.