Bedding and Fill Heights for Concrete Roadway Pipe and Box Culverts

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  • Bedding and Fill Heights for Concrete

    Roadway Pipe and Box Culverts

    Chai H. Yoo

    Frazier Parker

    Junsuk Kang

    Highway Research Center

    Auburn University

    Auburn University, Alabama

    June 2005

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    The investigation which led to the results presented herein was supported by Alabama Department of

    Transportation Project Number 930-592. The authors gratefully acknowledge the financial support and

    the guidance provided by the advising council.


    The opinions and conclusions expressed or implied in the report are those of the authors. They are not

    necessarily those of the funding agencies.

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    The overall objective of the proposed research is to investigate the fundamental interactions that

    take place during the process of excavating a trench, preparing the bedding, installing the conduit, and

    then placing and compacting the backfill. The materials and procedures used will significantly affect the

    conduit performance. The imposed loading will be greatly affected by the relative settlement of the soil

    prism directly above the conduit. An improved understanding of these fundamentals will be essential in

    order to develop technically better and more economical specifications for both designers and contractors.

    The specific objectives of this research were :1) to increase our understanding of the fundamental

    soil-structure interactions; 2) to compare the ALDOT Specifications, 2002 with current Standard

    Specifications of AASHTO and ASCE in three areas (material, design, and construction) and to update

    the ALDOT Specifications, 2002 where necessary; 3) to develop improved design guides which include

    design guides and examples, drawings, tables for Maximum Fill Heights, proposed equations and graphs

    for vertical earth load reduction rates, and instructions for recommended Finite Element Analysis

    programs (SPIDA, CANDE-89); and 4) to develop detailed construction guides, including imperfect trench


    This research focused on obtaining a comprehensive understanding of fundamental soil-structure

    interactions, especially the influence of bedding and fill heights for the behavior of rigid conduits (concrete

    roadway pipe and box culverts). To meet the objectives of this research project, the following main tasks

    were carried out: 1) Review ALDOT Specifications, 2002 and neighboring states procedures for installing

    roadway conduits; 2) Review the literature, including technical papers, manufacturer and trade

    association recommendations, and FHWA(1999)/AASHTO(2002) Specifications/standard practices; 3)

    Compare existing ALDOT design procedures with AASHTO and ASCE design procedures; 4) Develop

    Standard Installations for concrete pipes and box culverts that take into account backfill material

    properties; 5) Develop improved and expanded design guides for imperfect trench installations for rigid

    conduits; 6) Conduct finite element analyses to identify and verify the proposed backfill procedures and

    backfill material requirements. One of the most commonly used programs for the analysis of roadway

    conduits, CANDE-89 and the comprehensive finite element computer program for the analysis and design

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    of buried concrete pipe, SPIDA were used. ABAQUS version 6.3.1 and MSC/NASTRAN (2004), general

    finite element programs were also used to simulate and analyze a finite element model of the soil-

    structure system; and 7) Develop a recommended set of rigid conduits installation procedures that include

    Special Provisions, Special Highway Drawings, Maximum Fill Height tables without imperfect trench

    installation, and Maximum Fill Height tables with imperfect trench installation, including computer codes.

    Numerical analyses focused on the interaction of bedding and fill heights for deeply buried rigid

    conduits under embankment installation and imperfect trench installation. The magnitude of the earth load

    transmitted to conduit is largely affected by the installation method and bedding type. Bedding under a

    conduit has generally been compacted in order to control the conduit grade by minimizing settlement after

    construction. The results of computer modeling indicate that uncompacting the middle bedding reduces

    both the load on the conduit and the invert bending moments. However, the sidefill area should be

    compacted in order to provide support to the conduit and to provide an alternate vertical load path around

    the bottom of the conduit. The results from the field tests and the finite element analyses show the

    maximum bending moments are greater in an untreated sidefill than in a treated sidefill, which means that

    the sidefill area of the concrete pipe contributes significantly to supporting the earth load.

    The analyses of the concrete pipe were based on four AASHTO Standard Installations developed

    by ACPA and adopted by AASHTO. A Type 4 Standard Installation has no construction or quality

    requirements. A Type 1 Standard Installation requires the highest construction quality. Required

    construction quality is reduced for a Type 2 Standard Installation, and reduced further for a Type 3

    Standard Installation. The results from finite element analyses and ACPA show that the difference in the

    earth loads between Type 1, Type 2, and Type 3 is less than 5%. Proposed Installations used in this

    study consist of a combination of TREATED and UNTREATED installations, which conform to Type 3 and

    Type 4 AASHTO Standard Installations, respectively. Therefore, Proposed Installations are an effective

    installation method for maximizing bedding effect with the least effort for construction quality.

    AASHTO specifications include no provisions for the requirements and geometry of sidefill for box

    culverts. However, this study showed that the characteristics of foundation and compaction of side fill

    have a significant influence on the behavior of box culverts. In this study, the effects of different

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    foundations and compaction levels on the side fill of box culverts were evaluated and quantified using

    finite element analysis.

    The primary objective of the numerical analysis was to evaluate the effect of the main variables

    and the effectiveness of imperfect trench installation. The width, height, location, and properties of soft

    materials were used as the parameters for imperfect trench installation in this study. The results of the

    analyses showed that reduction rates are highly affected by Modulus of Elasticity (Es) of soft materials,

    which means the reduction rate is a function of Es. Based on the results of numerous parameter studies

    with Finite Element programs, proposed equations for the reduction rates were derived for both concrete

    roadway pipe and box culverts by means of a linear regression method.

    The primary results of this study are as follows: 1) The behavior of concrete roadway pipe and

    box culverts is more significantly affected by the installation practices for the bedding, haunch, and lower

    side than by foundation characteristics such as yielding or unyielding; 2) Optimum geometries for the soft

    zones of an imperfect trench installation were developed by numerous parameter studies for the height,

    the distance from the top of the conduit, and the width of the soft zone. Optimum geometries of the soft

    zone were proposed as Geometry I and Geometry II for concrete pipe and box culverts. The upper half of

    the pipe is surrounded by soft material in Geometry II for pipes and in Geometry II for box culverts, the

    whole sidewall is surrounded by soft material. Geometry II is more effective than Geometry I for earth load

    reduction. Therefore, only Geometry II was used in the Special Provisions and Special Highway Drawings

    of ALDOT Specifications, 2002; 3) Total vertical earth loads or bottom loads on the box culverts are

    composed of the top earth load, dead load, and shear force on the sidewall. Therefore, the design loading

    of box culverts should be based on the bottom pressure. AASHTO provisions for the design loading of

    box culverts are unconservative, as AASHTO provisions do not consider the shear force on the sidewall;

    4) TREATED sidefill for the box culverts is effective in reducing the shear force occurring on the sidewall

    of box culverts. AASHTO has no provisions for the requirements and geometry of sidefill for box culverts.

    For convenience in the installation and design process, installations for box culverts are proposed

    similarly to those of pipes in this study; and 5) Imperfect Trench Installation for box culverts increases the

    shear force on the sidewall due to the reverse arching effect of imperfect trench installation as well as the

    reduction of top earth pressure and total vertical earth load. The preventive method for the increase of

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    shear force, Geometry II for the soft zone, is designed to maximize the reduction effect of the vertical

    earth load due to the imperfect trench installation. Increase in the shear force due to the imperfect trench

    method can be prevented through the installation of soft material between the sidewall and soil. Using

    Geometry II for the soft zone of box culverts is thus highly effective in relieving any increase of