An Improved Approach for the Design of Thrust Blocks in Buried Pipelines

In 2010, the ASCE Task Committee presented a draft white paper that included preliminary details of a proposed preliminary framework for the design of thrust blocks and thrust restraint systems to improve current practice. The concepts included in the proposed preliminary framework were formulated by reviewing the historical work and current practice for the design of thrust blocks and thrust restraint systems. Since the publication of the draft white paper, the committee expanded the concepts outlined in the white paper to further explore the development of a unified design practice applicable to all pipe materials, using simplified yet rational assumptions, to solve the thrust restraint problems in buried pipelines. Justifying the need for a new framework for the design of thrust blocks, the white paper noted the shortcomings in current practice: improper handling of (a) the frictional resistance between the thrust block and the soil, and (b) the lateral (passive) soil pressure against the thrust block face. While the primary resistance against the unbalanced thrust force is provided by the lateral (passive) soil pressure against the thrust block face, the frictional resistance between the thrust block and the surrounding soil could be significant in some cases. Accounting for the frictional resistance would help optimize the design, resulting in smaller thrust blocks. All current AWWA design manuals estimate lateral (passive) resistance based on "allowable lateral soil bearing pressures" published for different types of soils; however, the values of these allowable pressures vary widely among the design manuals for different pipe materials. The use of inconsistent terminology and inconsistent recommendations for soil parameters among design guidelines for different pipe materials, without adequate explanation or justification, can cause significant misunderstanding and confusion. The lateral resistance should be calculated accounting for the tolerable movement of a thrust block, which could indeed depend to some extent on the pipe material characteristics, joint designs, and beam stiffness. This paper presents an expanded framework for the design of a thrust block for a horizontal bend in a buried pipeline, documenting the progress made to advance the development of a potentially unified approach. The proposed approach considers the tolerable movement of the pipe, the structural integrity concerns of the adjoining pipe segments and joints, the characteristics of the soil, and an appropriate factor of safety. This improved approach provides a common, yet rational, tool for the design of thrust blocks for all pipe materials.