Pretension Load Research Articles

The effect of surface profile, rock strength and pretension load on bending behaviour of fully grouted bolts

Fully encapsulated rock bolting has, in recent years, become a universally accepted system of ground reinforcement in mining and tunnel construction. The application of bolting systems extends both to rebar as well as cable bolting. The effectiveness of the bolt application has been studied in shear, both by laboratory tests as well as by numerical modeling. A specially constructed double shearing apparatus (DSA) was used to examine the shearing behaviour of a bolt installed perpendicularly across two joints. The experimental study was complemented with three-dimensional numerical analysis. Parameters examined include, the effect of reinforced material on tension/compression zones along the sheared bolt, shear resistance, shear displacement and induced strains and stresses during bolt bending process. The study was undertaken at both free load and pretension conditions. The conclusions drawn from the study were the level of bolt resistance to shear was influenced by bolt profile configuration, the strength of the rock or medium influenced the level of load generated on the bolt and the increased bolt pretension contributed to increased shearing load of the bolted medium. The numerical simulation of the bolt/medium interaction and deformational behaviour were found to be in close agreement with the experimental test results.

A new method for cerclage wire fixation to maximal pre-tension with minimal elongation to failure

Objective. To develop and test a new cerclage wire tightening technique, yielding reproducibly maximal pre-tension, load to failure and minimal elongation to failure of the wire junction. Design. Laboratory bench study. Background. Cerclage wire fixation is difficult to perform by hand with reproducible quality and tightening tension, which are required for optimal performance. Methods. With the new technique, 1, 1.25 and 1.5 mm steel wires were passed through a 9 × 18 mm steel tube, grasped using a modified ASIF wire-tightener and tightened by twisting until spontaneous failure of the wires in the tube. These fixations were compared to wires pre-tightened by hand to controlled high or low pre-tension using the simple twist, the knot twist and twist secured against untwisting, loaded to failure on a testing machine. The tests assessed pre-tension, ultimate failure load and elongation to failure. Results. Wire twists performed with the new technique were always perfectly symmetrical and may be tightened to maximal pre-tension without weakening of the wire. The twist secured against untwisting combined high stiffness with high failure load. The knot twist elongates to an unacceptable degree, unlike the secured twist. The simple twist untwists under little tension. Conclusions. The new technique allows to obtain maximal pre-tension and thus minimal elongation to failure of simple wire twists, without having to worry about breaking the wire at the base of the twist due to over-tightening. Relevance Cerclage wire fixation is an effective and cheap method to perform osteosynthesis. For adequate performance, maximal pre-tension, symmetrical twisting and high load to failure are necessary. The here presented technique combines all of these pre-requisites in a simple fashion.

203 引張荷重下におけるスポット溶接強度の検討(O.S.1-5 強度・磨耗・応用解析)(O.S.1 材料の変形・損傷・破壊の解析と評価)

203 引張荷重下におけるスポット溶接強度の検討(O.S.1-5 強度・磨耗・応用解析)(O.S.1 材料の変形・損傷・破壊の解析と評価)

Ground response curves for rock excavations supported by ungrouted tensioned rockbolts

A numerical analysis has been developed for the design of ungrouted tensioned rockbolts as support of excavations under axisymmetric conditions. The bolts dimensions (length, cross-section, longitudinal and circumferential spacings), their stiffness, their pre-tension load and the delay of installation are taken into account. Moreover, the method effects three main improvements in the usual theory, taking into consideration: 1. the reaction force transferred to the rock mass in the bolts anchoring zone, 2. the elastic recompression of the carrying ring surrounding the excavation due to the bolts preload, and 3. the relative displacement of the bolts ends which has a repercussion on their tension. Since the usual rock-support interaction analysis is only available when the rock mass and the support behave independently, an alternative solution has been explored for the bolting system (since it cannot be considered as an internal support). It consists to include the effect of the rockbolts into the ground reaction curve. In this paper, the principles of the analysis are explained and a numerical application is taken.

Evaluation of adhesion characteristics of nylon‐6 tire cord to natural rubber

AbstractThe adhesion characteristics of Nylon‐6 cords to rubber matrix, using Resorcinol Formaldehyde Latex (RFL) solution, were evaluated at various conditions. The heat‐setting studies of RFL dipped cords in the temperature range of 180–210°C showed a loss in tensile properties. However, a minimum in the loss was observed when a pretension load was given to the cords. Percent shrinkage of RFL‐dipped cords increased with the increase in temperature of heat setting. Adhesion behavior of Nylon‐6 cords to rubber matrix showed some improvement under relaxed conditions in the entire temperature range studied. With the application of suitable tension to cord, adhesion behavior can be made constant. This has been attributed to the higher extent of shrinkage of cords under relaxed conditions providing greater cord surface area. This in turn, leads to increased cord to rubber matrix.

Support Systems For High Wellhead Loads

Abstract This paper outlines the method presently in use by Shell Canada Resources Limited to support high wellhead loads at surface. Wellhead load is the total of all casing and tubing loads hung at surface. This load is normally carried by the surface casing and ultimately transferred to the earth by the surface casing cement. On deep wells the load can exceed the compressive strength of the surface casing. On wells with inadequate surface casing cement, the load can cause surface casing buckling and wellhead subsidence. Either failure can have catastrophic consequences to pressure integrity or production capability. Repair of wellhead support failure can be costly. An example of a subsidence problem is discussed. Guidelines are presented jar the compressive capacity of surface casings. If anticipated wellhead load exceeds the capacity of available surface casing, or if a poor surface casing cement job is anticipated a landing base casing bowl is designed to transfer load to the conductor pipe. The conductor pipe must be adequately sized to support the wellhead and transfer the load to the earth via a cement to formation shear bond. Geotechnical properties of the conductor hole formation must be known. This paper outlines considerations necessary for the design of a wellhead support system. Introduction Adequate support of loads imposed on the wellhead has not been a problem in Canadian onshore drilling in the past. Prior to 1978 Shell Canada had drilled many wells in the 4000 m to 5000 m depth range, and had no apparent problems with wellhead support. With the advent of deeper exploration wells and an increasing tendency to pre-tension casing strings as they are hung off it was recognized that wellhead load could exceed the capacity of standard surface casings. A wellhead subsidence problem on an exploration well highlighted the fact that inadequate surface casing cement can also cause wellhead support problems. Methods outlined in this paper are the result of a study undertaken to provide solutions to the problem of wellhead support. Wellhead Load Wellhead load includes the as-cemented weights of all casing strings (excepting surface casing and liners), tubing string weight, the weight of the wellhead and BOP equipment, thermal or pressure induced loads and any pre-tensioning loads applied to casing strings when they are hung off. Intermediate and production casing strings are often pre-tensioned (hung off with a load higher than their as-cemented weight) in order to prevent buckling at the cement top. Buckling can occur with changes in fluid densities, thermal expansion or changes in surface pressure. Casing pre-tension loads can be a significant portion of the total wellhead load. The following example gives a wellhead load breakdown for a typical foothills well: Calculation (Available In Full Paper) Load Transfer Wellhead load is transferred through the casing bowl to the surface casing, and/or from a landing base to the conductor pipe. It is transferred from the pipe to the cement via the casing - cement bond. The load is transmitted through the cement sheath by virtue of its shear strength.

Precompression Cracking Method for Fracture Toughness Test (2 nd Report)

It has been found that a sharp crack is formed on releasing the compression load applied to in excess of a certain value at a low temperature as liquid nitrogen temperature (-196°C), and it has been verified that this precompression cracking phenomenon can apply as a notch tip preparation method to fracture toughness testing.In this report, the suitable notch tip shape and proper preload magnitude for precompression cracking in specimens for fracture toughness testing are examined. Concerning the notch tip shape, 0.1 mm notch tip radius and 3060° opening angle are suitable for V shaped notch tip, and 0.2 mm width and 2 mm length are suitable for narrow width saw cut notch tip. Concerning the preload magnitude, the simple estimation method of precompression load in order to obtain a needed crack length and the determination method of proper pretension load are proposed.The precompression crack extension characteristics in welded joints are also examined experimentally using electrogas welded joint of SM 41 A, submerged arc welded joint of SM 50 B and shielded metal arc welded joint of HT 80. Cracks with a front sufficiently straight in shape are obtained in all joints by the precompression cracking method, although the crack extension length of HT 80/SMAW joint is changed largely in thickness direction by the fatigue cracking method. The precompression induced crack extension length can be estimated roughly by the proposed simple method.