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Analysis of High Strength Rock Bolt Application in Mining Fields

High Strength Rock Bolt Stock

High strength rock bolts are a kind of material used in mine support engineering. Its scope of use is very wide, and its advantages and disadvantages are different depending on its arrangement. In this article, Longteng will show you about its characteristics, advantages and disadvantages. Longteng Special Steel is specializes in manufacturing mining anchor-rock bolts, such as left-handed anchors , right-handed anchors, etc. If you are in need of mining rock bolt, contact us for quotation now!

Concentrated Point Arrangement - Rock Bolt Arranged Under Column

  • Advantages: It can make full use of vertical force transmitted from the upper structure to balance part of water buoyancy; because the mining rock bolts are concentrated, it is more convenient for the external waterproofing under basement floor; for individual mining rock bolts, the bearing capacity is insufficient. In the case, because there are more mining bolts, there is a strong resistance.
  •  Disadvantages: High strength rock bolts are required to be anchored in hard rock mass, so it is not suitable for soft rock and soil, and damage is often caused by the break of anchored rock mass; due to tightness of local mining rock bolts , construction of mine rock bolts is inconvenient; basement floor beam The board has a large reinforcement.

Evenly Arranged in a Plane, Rock Bolt Under the Basement

  • Advantages: Suitable for all soils and rock masses; basement beam and slab reinforcement is small.
  • Disadvantages: The vertical force transmitted from the superstructure cannot be fully utilized to balance part of water buoyancy; for insufficient bearing capacity of individual mine bolts , due to lesser number of mine rock bolts that can be shared , the resistance is poor in this circumstances; The mining bolts are relatively scattered, which is troublesome for the external waterproofing under basement floor.

Concentrated Line Layout, Rock Bolt Placed Under Basement Floor Beam

  • Advantages: Due to relatively concentrated arrangement of mining bolts , it is more convenient for external waterproofing under basement; for the case of insufficient bearing capacity of individual mining bolts , there are more mining bolts sharing, which has stronger resistance.
  •  Disadvantages: The vertical force transmitted from the superstructure cannot be fully utilized to balance a part of water buoyancy. The rock anchor is required to be anchored in the hard rock mass, not suitable for soft rock and soil; the basement plate has a large reinforcement.

Main Failure of High-Strength Anchor Cage Bolts

Wind Turbine Foundation Cage Completed

Fatigue is a form of failure in which high strength anchor cage bolts are cracked or broken under repeated action of cyclic stress and strain. Fatigue is a process of cumulative mechanical damage and delayed failure. Including crack initiation and crack propagation, occurs at a yield strength below the material and abruptly fails without significant deformation. Therefore, fatigue is one of the most dangerous forms of failure compared to other forms of failure for high strength anchor bolt.

Usually,  connecting bolts on the vehicle are pulled-pull fatigue failure. For example, wind turbines have complex and harsh service environments. They are subjected to dynamic load shocks and corrosive environments. The fatigue fracture of wind turbine foundation bolts is as high as 85%. It is concluded that main failure mode of high-strength anchor cage bolts is fatigue.

Challenges for High-Strength Anchor Bolt Manufacturing

High-end mechanical equipment brings unprecedented opportunities for high-strength bolt manufacturing. It will be difficult to achieve the expected goals without solving the “three major problems”, and it is even harder to say that it has reached the international advanced level. Solving the “3 major problems” needs to take care of problems: first, independent intellectual property rights; second, economic affordability; third, long-life, high-reliability, and structural weight-reduction technologies.

Wind Turbine Anchor Cage Detection

First of all, to change the concept, we must recognize the time and space, China must develop high-end machinery manufacturing, master key technologies and enhance core competitiveness, and abandon those flickering words that change rhetoric , and become second-hander of technology in developed countries, market-changing technology and other errors. The concept from the past “introduction, imitation – backward, re-introduction, imitation…”, liberation from paste, copy, and cottage products, speeding up development of fastener heat treatment is an urgent task.

Secondly, it is a revolution to fully understand “3 major problems” and realize the long life, high reliability and structural weight reduction of high-strength bolts:

  1.  From the “formed” manufacturing of high-strength bolts to the technological revolution of anti-fatigue manufacturing; Solve the “with or without” transformation into a key high-strength bolt to support the sustainable development of high-end machinery manufacturing;
  2.  From the final inspection of indicators to the management revolution of process control; 
  3. From low-tech, passive to high-tech, active practitioners Quality revolution; 
  4. From cheap labor, abuse of the environment and resources to high value-added, humanized, low-emission, environmentally friendly environmental revolution.

"3 Major Problems" of High-Strength Anchor Bolts

Wind Turbine Foundation Tower

High-strength bolts are generally made of high-strength low-alloy steel, ultra-high-strength steel and high-temperature alloy. With the development of high-end mechanical equipment, the strength of alloys for high-strength bolts will continue to increase. Material for high strength anchor bolt is featured with high fatigue strength, and light in weight, small in size, long in life, and highly reliable. However, outstanding weakness of high-strength materials is that fatigue strength is sensitive to stress concentration.

The fatigue failure analysis results of a large number of high-strength bolts show that more than 80% of fatigue failures are caused by surface damage, decarburization at head bar intersection, obvious fine cracks in thread processing, or discontinuities in the machining of the tool marks and surface corrosion, quenching. The tissue is uneven for high concentration of stress.

High-strength bolts are generally made of high-strength low-alloy steel, ultra-high-strength steel and high-temperature alloy. With the development of high-end mechanical equipment, the strength of alloys for high-strength bolts will continue to increase. Material for high strength anchor bolt is featured with high fatigue strength, and light in weight, small in size, long in life, and highly reliable. However, outstanding weakness of high-strength materials is that fatigue strength is sensitive to stress concentration.

The fatigue failure analysis results of a large number of high-strength bolts show that more than 80% of fatigue failures are caused by surface damage, decarburization at head bar intersection, obvious fine cracks in thread processing, or discontinuities in the machining of the tool marks and surface corrosion, quenching. The tissue is uneven for high concentration of stress.

Right-handed Rock Bolts Introduction

Right Handed Rock Bolt For Sale

Rock Bolts are generally suitable for reinforcement of mine roadways, bridge tunnels, track slopes, etc. The general rock bolt holes are drilled with a bolt drill, then the appropriate anchoring agent (resin roll) is placed, and then anchor drill is used. The tool is used to drill anchor rod into anchor hole to anchor, and then use tool such as the bolt rig to install nut.

Right-handed anchor, full name of strong-threaded steel resin rock bolt is right (or left) Manufactured by refining and rolling rebar, which is featured with continuous thread and nut can be used for the whole length. Used in conjunction with anchor plate nut and roadway support. The rock bolt is a replacement product of anti-twist anchor and has superior performance.

Right-hand Rock Bolt Technology Details

Rock bolts are classified according its diameter. Generally, most commonly used ones are: 16mm, 18mm, 20mm, 22mm. Achor rod body can achieve best matching with the anchor hole diameter and the diameter of the resin medicine, and has strong anchoring force, high elongation and safety. Reliable, high quality, low consumption, easy to use and so on.

Slope Anchor - Right Handed Rock Bolt

Bolt material description: high-strength rock bolt material adopts elongation of not less than 15%, yield strength of not less than 335MPa, breaking strength of not less than 490MPa, and material of 20MnSi is heat-sealed, and compressive pressure is 20 tons. To 50 tons, straightness tolerance of anchor rock bolt is Φ18 to Φ22 is ≤2mm/m, and linear tolerance of anchor rod of Φ16 is less than 4mm/m. The load capacity of rod tail should be not less than 75% of base metal breaking force. 

Use Rock Bolts to Increase Dam Stability

Grouted rock bolts are installed in the interface between rock mass and concrete in concrete dam structures as an additional safety measure. They can theoretically contribute to increase both the stability against sliding as well as overturning. One can however realize that they cannot increase the bearing capacity of the dam foundation since the rock bolts are inactivate as long as they are not exposed to tensile stresses.

Correct Understanding of High Strength Anchor Bolt

High-strength anchor bolts are the most common construction component in steel construction, and all steel structure engineers will find it familiar and think they know it well. However, this may not be the case. Today we start with the most basic concepts and bring you a new understanding of high-strength steel bolts.

What is a High-strength Anchor Bolt

High-Strength Friction Grip Bolt, literally translated as: high-strength friction pre-tightening bolt, English abbreviation: HSFG. It can be seen that the high -strength bolts mentioned in Chinese construction are short for high-strength friction pre-tightening bolts . In daily communication, the words “Friction” and “Grip” are simply abbreviated, but it has caused many engineers and technicians to understand the basic definition of high-strength bolts, which has caused misunderstanding.

Bolts with material grades exceeding 8.8 are “high-strength anchor bolts”?

The core difference between a high-strength anchor bolt and a normal bolt is not the strength of the material used, but form of force. The essence is whether to apply pre-tightening force and use static friction to shear. 
In fact, in British standard, high-strength anchor bolts (HSFG BOLT) mentioned in American standard are only 8.8 and 10.9 (BS EN 14399 / ASTM-A325 & ASTM-490), while ordinary bolts contain 4.6. 5.6, 8.8, 10.9, 12.9, etc. (BS 3692 11 Table 2); thus , material strength is not key to distinguish between high-strength bolts and ordinary bolts.

Correct Understanding of "High Strength"

According to GB50017, calculate the tensile strength and shear strength of a single ordinary bolt (Class B) 8.8 and high strength anchor bolt 8.8.

Strength Grade Designation4.64.85.65.86.66.88.810.912.914.9
Tensile Strength ( R m), Min.40405050606080100120140
Yield Stress (R e ), Min.243230403648
Stress at Permanent Set Limit (R 0.2 ), Min.6490108126
All stress and strength values are in kgf/mm 2 units.

Through calculations, we can see that the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts under same level.

So how are high-strength bolts called "high tensile "?

In order to answer this question, we must start from the design working state of 2 bolts, study the law of elastoplastic deformation, and understand limit state of design failure.

Deformation Curve of High Tensile Bolt

Stress-strain curve of ordinary bolt and high-strength bolt under working condition

Common Bolt: screw itself plastically deformed beyond a design allows the screw to be cut bad. 
Ordinary bolt connection, relative slippage occurs between connecting plates before shearing force is started, and then bolt rod and connecting plate are in contact, and elastic plastic deformation occurs, and shearing force is received. 
High-strength Bolt: The static friction between effective friction surfaces is overcome, 2 steel plates are relatively displaced. The design consideration is damage. 
High-strength bolt connection, friction is firstly subjected to shearing force. When load is increased until friction is insufficient to resist shearing force, static friction is overcome and connecting plate is relatively slipped (limit state). However, although it is broken at this time, bolt rod and connecting plate are in contact with each other, and elastoplastic deformation of bolt can be utilized to withstand shearing force. 

High-strength Anchor Bolts Have Higher Bearing Capacity

From calculation of a single bolt, design strength of high-strength bolts is lower than that of ordinary bolts. The essence of its high strength is that node does not allow any relative slip during normal operation, which is, elastoplastic deformation is small and joint stiffness is large.

Compare of High Strength Bolt and Ordinary Bolt

It can be seen that the node designed with high-strength bolts does not necessarily save number of bolts used in the case of a given design node load, but its deformation is small, rigidity is large, and safety reserve is high. It is suitable for main beam, and position where the node stiffness is required to be large, which is in line with basic seismic design principle of “strong node and weak member”. 

High-strength Bolt Strength is not the design value of its own bearing capacity, but the rigidity of design node, high safety performance and strong resistance to damage.

Anchor bolts shall be commercially smooth and free of burrs, laps, seams, cracks, and other injurious manufacturing defects that would make them unsuitable for the intended application.

Washers used on anchor bolts, installed in holes with dimensions greater than oversize or short slot as defined by the Research Council on Structural Connections, require design consideration. (For guidance refer to Specification for Structural Joints Using ASTM A 325 or A 490Bolts.)

Differences Between Soil Nails And Rock Bolt

Rock Bolting in Mining
Roof Bolting - Rock Bolt
Roof Bolting Design
Soil Nail Install
Soil Nail Install

Stress Mechanism

  • The soil nail is passively stressed, that is, after a certain deformation of the soil body, the soil nail is stressed, thereby preventing the soil from continuing to deform;
  • The rock bolt is actively stressed, which means, by pre-stressing the anchor rod, the excessive deformation of soil structure is restricted before foundation pit is not excavated;

Field of Load

  • The soil nail is full-length force, but direction of load force is divided into two parts. Potential slip surface divides soil nail into two parts. The direction of first half is directed to the direction of potential slip surface, and load direction of back part is directed back of potential slip surface.
  • The rock bolt is free end of front half and forced section of  rear half, so sometimes mortar is not filled in front half of rock bolt.
Rock Bolting in Mining

Working Mechanism Difference Between Soil Nail and Rock Bolt

The soil nail is a soil reinforcement technology, which uses the densely arranged reinforced body as the soil reinforcement means to improve the strength and self-stability of the soil to be reinforced;

The rock bolt is an anchoring technology, and the load of the unstable rock mass in the surface layer is transmitted to the deep stable position of the rock and soil body through the tension rod, thereby realizing the stability of the rock mass to be reinforced.

Rock Bolting in Mining

When soil structure is deformed, soil nail provides resistance with this deformation, and the force characteristics are the same as those of rock bolt. It’s just that it’s full length. The direction of the two broken forces divided by sliding surface is the same, both pointing into  slope. Under action of prestressing, rock bolt is actively stressed, and always provides the resistance in the slope. With the loss of prestress and the stop of deformation of slope, it degenerates into a soil nail.

High Strength Anchor Bolt and Other Bolt used in Wind Turbine Project

Wind Blade Stud Bolt
Wind Turbine Anchor Bolt Types

Wind turbines use high-strength bolts and nuts with many varieties and special materials, and the special working conditions of fasteners must have good mechanical properties. At present, there are roughly three types of fasteners used in domestic wind turbine projects:

High Strength Anchor Bolt is applied to wind turbine foundation system and wind tower structure. Characteristics: excellent anti-delay fracture performance and long fatigue life.(Fatigue Test: not damaged after 12 million test). The first step of wind power project is the foundation work, anchor bolt provide support and resistance to the tower and also forms a solid foundation to the whole structure, high strength anchor bolt is the fastener between wind turbine and concrete anchor cage structure.

Wind Turbine Foundation Bolt

Quenching plus tempering or other surface treatments at different temperatures will impart different performance properties to the fastener. Under certain conditions, the quality of raw materials will affect the selection of heat treatment process parameters, and directly affect the comprehensive mechanical properties and safety of high-strength fasteners for wind power. If your project need high quality anchor bolt, you can contact us now for price and details.

Wind Blade Stud Bolt
Wind Blade Stud Bolt

It is the bolt used on the tower of wind turbine. It mainly uses GB/T 1228~1231 DIN6914~6916 and large hexagonal steel structure connecting bolts such as DAST.

Bolts used on wind turbines, mainly includes: hexagonal head bolts, hexagon socket bolts, hexagons nuts and washers such as GB/T 5782, GB/T 5783, GB/T 70.1, GB/T 6170, GB/T 97, etc.

Bolts that connect wind turbine blades, mainly non-standard stud bolts and T-shaped round nuts for some products.

Bolts and screws are 10.9, a few are 8.8; nuts are 10, 8; the hardness of the gasket is 35-45HRC, these varieties must be quenched and tempered to achieve the specified mechanical properties.

Overview of Wind Turbine Anchor Cage Install Process

Wind Turbine Foundation Anchor Cage Install

Preparation Work of Anchor Cage Install

  1. According to the list of prestressed anchor bolt assemblies in the anchor bolt foundation drawing, count the number of parts and perform visual inspection. Check whether the upper and lower anchor plates are deformed; whether the anchor bolts are damaged or the anchor bolts are bent, and the unqualified products are removed. It is strictly forbidden to use.
  2. After transporting the required parts to the site, they should be placed in a flat place with cork support to prevent deformation of the upper and lower anchor plates and damage to the anchor threads.

Installation of Lower Anchor Plate

Lower Anchor Plate Install Step
  1. According to the list of prestressed anchor bolt assemblies in the anchor bolt foundation drawing, count the number of parts and perform visual inspection. Check whether the upper and lower anchor plates are deformed; whether the anchor bolts are damaged or the anchor bolts are bent, and the unqualified products are removed. It is strictly forbidden to use.
  2. After transporting the required parts to the site, they should be placed in a flat place with cork support to prevent deformation of the upper and lower anchor plates and damage to the anchor threads.

Installation of Upper Anchor Plate and Anchor Bolt

Upper Anchor Plate and Bolt Install

The installation of the anchor bolt is divided into a positioning anchor bolt and a common anchor bolt.

  1. Use the crane to hang the upper anchor plate to a certain height, and then put the positioning anchor bolt symmetrically on the inner and outer bolt holes of the upper anchor plate. The anchor bolt penetrates the upper anchor plate and is provided with a temporary steel nut. Screw the blackened half nut on the lower end of the anchor bolt (flat end), then insert the PVC sleeve (length according to the drawings) into the anchor bolt, and then put the heat shrinkable tube on the PVC sleeve (position anchor bolt sleeve two Segment heat shrinkable tube). Screw in the nylon adjusting nut (steel nut is not allowed) at the upper end of the positioning anchor (cone end).
  2. After the anchor bolt is worn, the crane slowly hoists the upper anchor plate and the positioning anchor bolt, and moves to the upper side of the lower anchor plate, and the positioning anchor bolt and the lower anchor plate bolt hole are correspondingly inserted, and the gasket is placed under the lower anchor plate. Tighten the black nut. The nut requires a tightening torque of 300 N ̇m.

Ordinary Anchor Bolt Installation

Ordinary Anchor Bolt Install Step

After the positioning anchor is installed and leveled and aligned, the ordinary anchor bolts are installed in a diagonal order. The upper anchor plate of the upper end of the anchor bolt (the end of the cone) is inserted first, and the end of the half nut is inserted into the lower anchor. Plate, the same method tighten the nut (black nut) and add the gasket. The nut requires a tightening torque of 300 N.m.

 A single heavier anchor is passed over the upper anchor plate, penetrates the lower anchor plate, and fits into the PVC casing. It is important to note that after the anchor bolt penetrates the lower anchor plate, all the gaskets under the lower anchor plate should be padded, and the lower nut should be tightened to 300N.m. The concealed project acceptance shall be carried out before the local cushion under the lower anchor plate is poured, and the inspection may be carried out after the inspection and acceptance of the visa and confirmation (no omission and tightening).

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