How to Deal with Easy-to-collapse Formations? Self Drilling Anchor Bolt Construction Guide

Time:2023-04-28From:sinorock View:

With the development of economy and society, the degree of development and utilization of underground space is increasing, and the scale and depth of various deep foundation projects are increasing. At present, new technologies and techniques of foundation support emerge in an endless stream, but the pile-anchor support form is still one of the widely used support forms. After long-term engineering practice, ordinary prestressed anchor bolts have mature technology, reliable technology, convenient construction, and low cost. They are currently widely used. However, under some complex geological conditions, such as filling and soft soil, there may be holes and necking. Insufficient anchor rod length, poor grouting effect and other problems lead to increased construction difficulty and even affect the safety of the support structure and the stability of the slope. So how to deal with these complex and easy-to-collapse formations, the following will answer your questions.

Types and Characteristics of Common Collapse Strata

1. Soft soil and other easy-to-collapse formations

In soft soil, newly accumulated backfill soil and newly deposited loose fine sand soil layer, if the groundwater level is high, even if the borehole is protected by mud, it is difficult to maintain the stability of the hole wall, and it is easy to cause collapse. Because this type of formation is mostly distributed in the shallow layer of the ground surface, with the gradual increase of the amount of landslides, the ground around the borehole will collapse, often causing the casing to sink. In severe cases, it will also pose a threat to drilling equipment and operators.

2. Sand and silt strata

When drilling holes in fine sand or silt layers, if the groundwater level is high, it is necessary to use mud to form holes in the retaining wall. Due to the lack of cohesive force between particles, this type of formation is easily scoured under the action of water flow. The sand particles scoured by the hole wall further reduce the wall protection effect of the wall protection mud. It is more likely to cause problems such as neck guards and even collapsed holes.

3. Gravel layer, pebble layer

It is very difficult to drill holes in sand, gravel, pebble and broken zone formations. Such formations are called mechanical dispersion formations. Due to the lack of cementation between the particles, the hole walls are prone to collapse during drilling. For such formations with mud or high-viscosity slurry wall protection, the key to solving the problem is to increase the cementation force between the well wall particles. Appropriate infiltration of dry slurry into the double-wall formation can significantly enhance the cementation force between sand and gravel, thereby enhancing the stability of the hole wall.

4. Dissolution formation

Dissolution formations are most typical of sodium chloride salt formations, and others include potassium salts, gypsum, Glauber's salt, trona, etc. This kind of stratum is also called water-soluble stratum. They will dissolve when encountering the water in the pore-forming fluid, and the pore wall will be dissolved away. As a result, the pore diameter is often oversized and collapsed.

gravel

Introduction of Self-drilling Anchor Bolt

The self-drilling anchor combines drilling, grouting, and anchoring into one process. It can be cut or lengthened arbitrarily according to the needs of the project, and the construction is very convenient. The use of hollow thick-walled steel pipes of equal cross-section instead of steel bars can ensure the tensile strength of the bolt. Grouting through the anchor bar body can improve the grouting density of the bolt, thereby improving the grip force between the rock bolt and the rock and the durability of the rock bolt. The self drilling anchor bolt system has the characteristics of fast construction speed, large anchoring force, good anti-corrosion performance, light weight, simple operation, and can provide anchoring force in time. It is especially suitable for weathered rock, crushed stone layer, backfill layer, gravel layer and other stratum working faces that are difficult to make holes but need to be supported in time.

self drilling anchor system

Self-drilling Anchor Bolt Construction Guide

1. Preparation before construction: Check whether the functions of the drilling rig are normal, whether the drill bit and the rod body are damaged, and whether the anchor bar is unblocked, etc. After ensuring that there is no problem, thread the hollow anchor body to the drill bit;

2. Drilling and grouting: Connect the hollow anchor bar and the grouting pipe to the drilling rig through the rotary grouting adapter, and perform grouting during the drilling process to achieve simultaneous completion of drilling and grouting;

3. Extension anchor bolt: If the length of a single self-drilling anchor bolt does not reach the design drilling depth, the length of the extension coupler can be used to meet the design requirements;

4. Install plates and nuts: After the grouting is completed, install plates and nuts in sequence on the part of the self-drilling anchor bolt exposed outside the rock close to the construction surface to ensure the anchoring effect.

Summary

The characteristics of loose soil and insufficient bearing capacity of the mud-and-rock stratum have caused a series of difficulties for pile foundation construction. When encountering weak strata such as mud and stone, according to the actual situation, comprehensively consider various factors, choose safe, economical and effective treatment methods, achieve early prevention, take measures in place, solve in time, and have obvious effects, so as to ensure pile foundation construction quality. If you have any problems during construction, please don’t hesitate to contact sinorock for bolting solutions at sinorock@sinorockco.com.

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