Currently the design of tension-type anchor cable based on the assumption that anchoring force uniformly distributed in the anchorage segment, but field test show it unevenly in fact. This study established an elastic mechanical model for the tension-type anchor in rock mass and gained anchoring force distribution expression. The exponential expression shows that anchoring force concentrates in part length of the anchorage segment, and the maximum of it is proportional to the freedom segment length and pre-stressing force, inversely proportional to surrounding rock's stiffness and anchor hole's radius. Then, according to the anchoring force distribution law, the anchorage segment was divided into effective anchorage segment and synergistic anchorage segment two parts. A calculation formula for the effective anchorage length was proposed correspondingly. It was found that this length mainly determined by the rigidity of surrounding rock and buried depth of anchorage segment. Designed grout length must be longer than it. Finally, by referenced to a number of engineering examples, a rational range of grout length for tension-type anchor cable was suggested in soft rock and hard rock respectively.

1 Introduction

Recently, tension-type pre-stressed anchor has been widely used in the reinforcement engineering such as slope, excavation and caverns, for it has many advantages compared to the original one, e.g. large anchoring tonnage, deep anchoring region and simple construction.

In existing national design codes [1], the anchoring force that a certain length anchor segment provides is calculated based on the assumption that bond strength uniformly distributes along the anchoring segment between the grout and rock formations or steel strand. However, many trials [2–3] and theoretical [4–5] studies have shown that the side resistance on anchoring segment is not evenly distributed but concentrated within a small region. So, there is no adequate theoretical basis for the design of anchorage segment in accordance with the assumption, and it is necessary to do in-depth study of the distribution law of pre-stressed anchorage force which can be used as theoretical reference for pre-stressed anchor design.

I.W. Farmer [6], J. Zhongxin [7], H. Siming [8], Y. Chunan [9], etc. put forward the anchor shear stress distribution expression through theoretical analysis. All those theoretical study is based on stress analysis of the whole grouted anchor. Under pre-stressing tension load, the anchorage segment of anchor is embedded deep in rock mass to form a tensile stress concentration area, and the free segment end is locked on the rock surface to forma compressive stress concentrated area. Obviously, for bearing load actively, the tension-type pre-stressed anchor has different mechanical behavior from common grouted anchor.

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