Anchorage, Tieback and Underpinning Systems

Ground Anchors and Anchored Systems

P.J. Sabatini, D.G. Pass, and R.C. Bachus
FHWA-IF-99-015
June 1999

This document presents state-of-the-practice information on the design and installation of cement grouted ground anchors and anchored systems for highway applications. The anchored systems discussed include flexible anchored walls, slopes supported using ground anchors, landslide stabilization systems, and structures that incorporate tiedown anchors.

This document draws extensively from the FHWA-DP-68-IR (1988) design manual in describing issues such as subsurface investigation and laboratory testing, basic anchoring principles, ground anchor load testing, and inspection of construction materials and methods used for anchored systems. This document provides detailed information on design analyses for ground anchored systems. Topics discussed include selection of design earth pressures, ground anchor design, design of corrosion protection system for ground anchors, design of wall components to resist lateral and vertical loads, evaluation of overall anchored system stability, and seismic design of anchored systems. Also included in the document are two detailed design examples and technical specifications for ground anchors and for anchored walls.

Lateral Support Systems and Underpinning

D.T. Goldberg, W.E. Jaworski and M.D. Gordon
FHWA RD-75-128, FHWA RD-75-129, and FHWA RD-75-130

One of the most extensive reports ever written on the subject of lateral support systems and underpinning. The work is divided into three volumes (all of which are included in the download document:)

• Volume 1: This volume is a convenient reference on the design and construction of lateral support systems and underpinning which are often required in conjunction with cut-and-cover or soft ground tunnelling. The design recommendations and construction methods described herein are a summary of the more detailed information presented in the companion volumes of this study. Included in this volume are discussions of displacements, lateral earth pressure, ground water, passive resistance, stability analysis, bearing capacity, soldier piles, steel sheeting, diaphragm walls, bracing, tiebacks, underpinning, grouting and freezing. An overview compares the relative costs of the construction methods used in later support systems and underpinning.
• Volume 2: This report provided current information and design guidelines on cut-and-cover tunnelling for practicing engineers. Included in this volume is a state-of-the-art summary of displacements and lateral pressure. Other topices are basic concepts of soil mechanics, ground water in open cut, passive resistance, design aspects of lateral earth pressure, stability analysis of sheeted excavations, bearing capacity of deep foundations and construction monitoring. Detailed explanations of design methods and literature citations are included.
• Volume 3: This provides specific design recommendations, design considerations, and construction techniques for the construction of lateral support systems and underpinning. The design considerations are presented for each technique or method (soldier piles, steel sheeting, diaphragm walls, internal bracing, tiebacks, underpinning, grouting and freezing.) The factors affecting the design or implementation of these schemes are discussed. Construction techniques are presented, and literature references are provided for those seeking even greater detail. An overview of the construction methods compares the applicability of the techniques and the construction costs of each.

Methods Used in Tieback Wall Design and Construction to Prevent Local Anchor Failure, Progressive Anchorage Failure, and Ground Mass Stability Failure

Ralph W. Strom and Robert M. Ebeling

U.S. Army Corps of Engineers Information Technology Laboratory
ERDC/ITL TR-02-11
December 2002

A local failure that spreads throughout a tieback wall system can result in progressive collapse. The risk of progressive collapse of tieback wall systems is inherently low because of the capacity of the soil to arch and redistribute loads to adjacent ground anchors. The current practice of the U.S. Army Corps of Engineers is to design tieback walls and ground anchorage systems with sufficient strength to prevent failure due to the loss of a single ground anchor.

Results of this investigation indicate that the risk of progressive collapse can be reduced by using performance tests, proof tests, extended creep tests, and lift-off tests to ensure that local anchor failures will not occur and to ensure the tieback wall system will meet all performance objectives; by using yield line (i.e., limit state) analysis to ensure that failure of a single anchor will not lead to progressive failure of the tieback wall system; by verifying (by limiting equilibrium analysis) that the restraint force provided by the tieback anchors provides an adequate margin of safety against an internal stability failure; and by verifying (by limiting equilibrium analysis) that the anchors are located a sufficient distance behind the wall face to provide an adequate margin of safety against external stability (ground mass) failure. Design measures that can be used to protect against local anchor failure are described, along with testing methods that can be used to ensure that anchor performance meets project performance objectives.

Examples are given to demonstrate the yield line analysis techniques that are used to verify that the wall system under the “failed anchor” condition can safely deliver loads to adjacent anchors and to ensure that the failure of a single anchor will not lead to progressive wall failure are. Limiting equilibrium analysis procedures used for the internal and external stability of tieback wall systems are also described. Simple procedures applicable to “dry” homogeneous sites and general-purpose slope stability programs applicable to layered sites (with and without a water table) are also illustrated by example.

Simplified Procedures for the Design of Tall, Flexible Anchored Tieback Walls

Robert M. Ebeling, Muluneh Azene, and Ralph W. Strom
ERDC/ITL TR-02-9
November 2002

In practice, the procedures used to design flexible tieback wall systems differ from those used to design stiff tieback wall systems. In the design of flexible tieback wall systems, apparent pressure diagrams are commonly used to represent the maximum loads the tieback wall system might experience during construction. Apparent pressure diagrams used in an equivalent beam on rigid supports analysis are demonstrated in this report. Analyses are performed for flexible wall systems in both cohesionless and clay soil. Flexible wall systems include a soldier beam–wood lagging system and a sheet-pile system. Wall heights of 25, 35, and 50 ft (8, 11, and 15 m) are evaluated.

Apparent pressures are developed on a “total load” approach using limiting equilibrium procedures. Apparent pressure diagrams are nonsymmetrical in shape, as recommended in FHWA-RD-97-130 (“Design Manual for Permanent Ground Anchor Walls,” Federal Highway Administration). Designs are provided for two performance objectives: “safety with economy” and “stringent displacement control.” A factor of safety of 1.3 is used for the safety with economy designs for which displacement control is not a significant concern. A factor of safety of 1.5 is used for the stringent displacement control designs, for which it is assumed that displacements must be minimized to prevent settlement-related damage to nearby structures.

Comparisons are made between the safety with economy and the stringent displacement control designs for the wall heights indicated above.

Simplified Procedures for the Design of Tall, Stiff Tieback Walls

Ralph W. Strom and Robert M. Ebeling
ERDC/ITL TR-02-10
November 2002

Methods used in the design of flexible and stiff tieback walls are described. Methods applicable to the design of stiff tieback wall systems are illustrated by example. Important in the design of stiff tieback wall systems is the consideration of construction sequencing effects. Illustrated by example are the equivalent beam on rigid supports method and the equivalent beam on inelastic supports method.

Both the equivalent beam on rigid supports and the equivalent beam on inelastic supports analysis methods consider construction sequencing effects. The equivalent beam on rigid supports method uses soil pressure distributions based on classical methods. The equivalent beam on inelastic supports method uses soil springs (nonlinear) to determine earth-pressure loadings and preloaded concentrated springs (nonlinear) to determine tieback forces. Soil springs are in accordance with the reference deflection method proposed in the Federal Highway Administration’s “Summary report of research on permanent ground anchor walls; Vol II, Full-scale tests and soil structure interaction model” (FHWA-RD-98-066). Soil springs are shifted after each excavation stage to account for the plastic soil movements that occur during excavation. The software program CMULTIANC, newly developed to facilitate the equivalent beam on inelastic supports construction-sequencing analysis, is illustrated in the report.

The results from the equivalent beam on rigid supports and equivalent beam on inelastic supports analyses are compared with each other and to the results obtained from other tieback wall analyses. The results are also compared with those obtained from apparent pressure diagram analyses. The apparent pressure diagram approach is common to the design of flexible wall systems.

State of the Practice in the Design of Tall, Stiff, and Flexible Tieback Retaining Walls

Ralph W. Strom and Robert M. Ebeling
U.S. Army Corps of Engineers Information Technology Laboratory
ERDC/ITL TR-01-1
December 2001

In tieback wall design, the determination of anchor loads and wall forces requires knowledge about the interaction between the wall and the soil during successive stages of excavation, as well as after completion of the project. Interaction between the wall and soil is difficult to predict. As a result, simple methods of analysis have been developed for use in the design of various tieback wall systems. These methods may, or may not, require a construction sequencing analysis.

This report describes state-of-the-practice analytical methods used to evaluate tieback wall performance and to design the tieback wall and ground anchor system. Analytical methods include equivalent beam on rigid support methods, beam on elastic foundation methods, and finite element methods.

The applicability of the various design methods with respect to various tieback wall systems frequently used on Corps projects is described in the report. Tieback wall systems covered in the report include vertical sheet-pile systems, soldier beam systems with wood or concrete lagging, secant cylinder pile systems, reinforced concrete slurry wall systems, and slurry wall systems constructed using soldier beams and concrete lagging.

Analysis methods depend on whether the tieback wall system is stiff or flexible. The report describes the characteristics of stiff and flexible tieback wall systems and indicates how the analysis method selected can be influenced by wall stiffness.