AASHTO LRFDTUN

Description / Abstract: PURPOSE AND SCOPE

The provisions of these Specifications are intended for the design, evaluation, and rehabilitation of highway tunnels. These Specifications are intended for the design of tunnels constructed using cut-andcover, bored, mined, and immersed tunnel construction methodologies.

Provisions are not included in these Specifications for water conveyance, utility, transit, or rail tunnels or for shafts. For tunnel elements not explicitly covered herein, the provisions of these Specifications may be applied, as augmented by the Engineer with additional design criteria where required.

Construction specifications consistent with these design Specifications are not included. There is a listing of suggested construction specification sections included in Appendix B.

Structures internal to tunnels that support roadways over ventilation plenums, roadways, or other openings in the tunnel shall be designed in accordance with the AASHTO LRFD Bridge Design Specifications (hereafter referred to as the LRFD Specifications) including all applicable interim changes and as modified or supplemented herein. The load effects of these internal structures shall be applied to the tunnel lining, walls, or other supporting members in accordance with these Specifications.

Retaining walls for retained cut approaches to tunnels shall be designed in accordance with the LRFD Specifications.

Support and ancillary structures such as ventilation, control, and administrative buildings are not covered by these Specifications. These structures shall be designed in accordance with local building codes.

These Specifications are not intended to supplant proper training and experience or the exercise of judgment by the Engineer, and provide only the minimum requirements necessary for public safety. The Owner or the Engineer may require the sophistication of design or the quality of materials and construction to be higher than the minimum requirements. The design of tunnels is strongly dependent upon the geologic setting, site conditions, and construction methodology, and this fact is considered in the Specifications. The concept of ground/structure interaction is emphasized for mined and bored tunnels; however, it is also applicable to cut-and-cover and immersed tunnels.

The concept of safety through redundancy and ductility is emphasized for tunnel elements subject to repeated loads and load reversals.

The design provisions of these Specifications employ the Load and Resistance Factor Design (LRFD) methodology. The load factors have been calibrated using structural analysis modeling for a limited number of loading conditions that take into account ground/structure interaction.

These Specifications are modeled after the LRFD Specifications and the AASHTO Guide Specifications for LRFD Seismic Bridge Design. The philosophy and guidance provided in those documents are carried forward and implemented in this document.

Whenever the LRFD Specifications are referenced in this document, the reference is to the latest edition including all applicable interim changes.

The term “notional” is often used in these Specifications to indicate an idealization of a physical phenomenon, as in “notional load” or “notional resistance.” Use of this term strengthens the separation of an engineer’s “notion” or perception of the physical world in the context of design from physical reality itself.

The term “shall” denotes a requirement for compliance with these Specifications.

The term “should” indicates a strong preference for a given criterion.

The term “may” indicates a criterion that is usable, but other local and suitably documented, verified, and approved criteria may also be used in a manner consistent with the LRFD approach to tunnel design.

The load factors specified in Section 3 have been calibrated to provide designs with member proportions consistent with the current practice in tunnel design.

The calibration is based upon analyses performed for a circular bored tunnel. Additional calibration for different tunnel cross-sectional geometry and loadings from a variety of ground conditions would be useful in further validating and refining the load factors.

The primary loads on structural components of tunnels are groundwater and earth loads. For immersed tunnels, loads imposed by transporting immersed elements from the fabrication site to the tunnel location can also govern the design of these tunnels. For pre-fabricated linings used in bored tunnels, construction-imposed loading can govern the design. The determination of groundwater, earth, transportation, and other construction loads varies based on the in-situ conditions, level of testing during subsurface investigations, and ground conditions may have great variation. There are little data available to establish a statistically significant sampling in order to calibrate these Specifications based upon structural reliability theory. As such, judgment and past experience were also used to select the load factors.

These Specifications are an initial attempt to codify and standardize highway tunnel design. As such, as future data that are produced in a systematic fashion in accordance with these Specifications become available, recalibration may be implemented based on statistical evaluation of these data.

The commentary is not intended to provide a complete historical background concerning the development of these Specifications, nor is it intended to provide a detailed summary of the studies and research data reviewed in formulating the provisions of the Specifications. However, references to some of the research data are provided for those who wish to study the background material in more depth.

The commentary directs attention to other documents that provide suggestions for carrying out the requirements and intent of these Specifications. However, those documents and this commentary are not intended to be a part of these Specifications.

The Specifications direct the Engineer to utilize other documents in the development of designs. When this occurs, the most current edition of those documents should be utilized. Those documents referenced in the Specifications are intended to be part of these Specifications by reference.

Unless otherwise specified, the Materials Specifications referenced herein are the AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing.