Besides the application of grouting technology in dam bedrock treatment, how to analyze the grouting technology of rock foundation in water conservancy projects?

Paper Keywords: water conservancy project rock foundation grouting technology

Abstract: Foundation treatment engineering plays an important role in the construction of water conservancy and hydropower projects, and is an important link in the construction. This paper introduces the grouting technology of rock foundation and its classification, and analyzes the materials needed for grouting.

There are two kinds of foundations of hydraulic structures: rock foundation and soft foundation, among which soft foundation includes soil foundation and gravel foundation. Due to the change of geological structure and the influence of hydrogeology, the natural foundation has defects in different degrees, and it needs manual treatment to be a reliable foundation for hydraulic structures.

The foundation treatment of hydraulic structures is to reduce or eliminate some natural defects of the foundation, improve and improve the physical and mechanical properties of the foundation, make the foundation have sufficient strength, integrity, impermeability and stability, and ensure the safety, reliability and normal operation of the project. With the development of water conservancy and hydropower construction, higher and higher requirements are put forward for the methods and technologies of foundation treatment. At the same time, foundation treatment is a hidden project. According to the requirements of hydraulic structures for foundation, the hydrogeological conditions are analyzed, the technical and economic comparison is made, and the construction scheme with feasible technology, reliable effect, short construction period and reasonable economy is selected.

1 rock foundation grouting

Rock foundation grouting (referred to as bedrock grouting for short) is to press a certain slurry with fluidity and gelation into the cracks of rock stratum through drilling grouting equipment according to certain matching requirements, and form stones after hardening and cementation, so as to improve the strength and integrity of bedrock and improve its impermeability. The grouting treatment of bedrock should be selected after analyzing and studying the geological conditions of bedrock, building types and grades, confined water head, foundation stress and displacement.

Grouting technology is not only used in dam bedrock treatment, but also widely used in underwater tunnel surrounding rock consolidation, lining backfilling, advanced support, concrete dam joints, building (structure) reinforcement, plugging and so on.

2 Classification of bedrock grouting

The rock foundation of hydraulic structures generally needs curtain grouting, consolidation grouting and contact grouting respectively.

2. 1 curtain grouting

It is arranged in the dam foundation near the upstream water surface to form a continuous impervious curtain wall. Its purpose is to reduce the seepage flow of dam foundation, reduce the seepage pressure at the bottom of dam and ensure the seepage stability of foundation. The depth of curtain grouting is mainly determined by the working head and geological conditions, which is much deeper than consolidation grouting, and some projects have curtain depths exceeding 100 meters. Single hole grouting is usually used in construction, and the grouting pressure used is relatively high.

Curtain grouting is generally arranged before reservoir impoundment, which is beneficial to ensure grouting quality. Curtain grouting is usually arranged in the grouting gallery of dam foundation because of its large amount of work and contradiction with the dam construction period. In this way, the dam body can rise synchronously with bedrock grouting, and provide a certain thickness of concrete weight for grouting construction, which is beneficial to improve grouting pressure and ensure grouting quality. For the grouting curtain of high dam, it is often necessary to penetrate into a large range of rock mass on both sides of the dam, and generally it is necessary to excavate grouting flat holes in layers on both sides. The underground grouting curtain formed by many projects in the dam foundation and the mountains on both sides of the river is much larger than the visible dam water retaining surface.

2.2 Consolidation grouting

Its purpose is to improve the integrity and strength of bedrock and reduce the water permeability of foundation. When the geological conditions of bedrock are good. Generally, consolidation grouting holes can be arranged at the upper and lower stress parts of dam foundation; In the case of poor geological conditions and high dam body, comprehensive consolidation grouting is needed for the dam foundation, even in a certain range of upstream and downstream outside the dam foundation. The depth of grouting holes is generally 5 ~ 8m, and some are as deep as 15 ~ 40m. These holes are arranged in a staggered grid on the plane. Group hole flushing and group hole grouting are usually used.

Consolidation grouting should be carried out on dam foundation concrete with a certain thickness, which can not only prevent the surface of bedrock from bleeding, but also adopt a large grouting pressure to improve the grouting effect, and also take into account the contact grouting between dam and bedrock. If the bedrock is hard and complete, in order to speed up the construction, consolidation grouting without concrete weight can also be directly carried out on the bedrock surface. Only when the concrete strength of the corresponding part reaches 50% of the design strength can drilling and grouting be carried out on the foundation concrete. Or drill a hole in the rock foundation first, embed the grouting pipe, and then grout after the concrete is poured to a certain thickness.

In the consolidation grouting of some dam foundations of the Three Gorges Project of the Yangtze River, leveling concrete is poured, and the strength grade of leveling concrete is the same as that of dam foundation concrete. The pouring thickness is generally 30 ~ 40 cm, which is mainly used to fill low-lying pits and grooves. Consolidation grouting operation will be started after the concrete strength reaches 70% of the design strength.

Exposed and re-consolidated grouting of intake tower foundation rock of Xiaolangdi Water Control Project on the Yellow River: 30 ~ 40 cm cushion concrete is poured on the bedrock surface; Under the protection of cushion concrete, the surface layer of grouting shall be consolidated for 3m first, and then the lower rock mass shall be grouted after the cover plate is formed on the bedrock surface.

In principle, consolidation grouting for dam foundation of Ertan Hydropower Station starts from grouting without cover weight and ends with cover weight. Grouting without cover is carried out under the condition of bare rock, mainly for rock mass with a hole depth of less than 3m, and for rock mass with a hole depth of more than 3m, it is led to the concentration place behind the dam through a connecting pipe, and grouting is not carried out until the dam foundation concrete is poured. The bedrock grouting of the same section must be carried out in the order of consolidation grouting first and then curtain grouting.

2.3 contact grouting

Its purpose is to strengthen the bonding ability between dam concrete and dam foundation or bank shoulder and improve the anti-sliding stability of dam body. Generally, grouting is carried out by pre-drilling concrete holes or pre-embedding grouting boxes and corresponding piping systems on the contact surface, or by combining the same grouting. Contact grouting should be carried out after the dam concrete reaches a stable temperature to prevent concrete shrinkage and cracking.

3 Grouting materials

Grout for rock foundation grouting shall generally meet the following requirements:

3. 1 Slurry should have good groutability in the grouted rock stratum, that is, it can be poured into cracks, crevices or holes under a certain pressure and filled tightly.

3.2 After the cement slurry hardens into stones, it should have good impermeability, necessary strength and cohesive force.

3.3 In order to facilitate the construction and increase the diffusion range of the slurry, the slurry should have good fluidity.

3.4 The slurry should have good stability and low water separation rate.

Cement grouting is the most common one in salt-based grouting. The cement slurry injected into bedrock is made of cement and water in a certain proportion, and the cement slurry is in suspension. Cement grouting has the advantages of reliable grouting effect, simple grouting equipment and technology and low material cost.

The variety of cement used in cement slurry should be determined according to the grouting purpose and the erosion of environmental water. In general, ordinary portland cement or portland dam cement with a grade of not less than c45 can be used, and sulfate-resistant cement can be used if acid resistance is needed. Slag cement and pozzolanic silicate cement are generally not suitable for use because they have the disadvantages of fast water separation, poor stability and low early strength.

The fineness of cement particles has a great influence on grouting effect. The finer the cement particles are, the more tiny cracks can be poured into them, and the more complete the hydration of cement is. The requirement of curtain grouting for cement fineness is that the screen allowance passing through 80m square hole screen is not more than 5%. Cement for grouting shall meet the quality standards, and expired, caked or unqualified cement shall not be used.

For the stratum with fracture width less than 200 meters, it is generally difficult to pour cement slurry made of ordinary cement. In order to improve the groutability of cement slurry, since 1980s, many countries have developed various kinds of fertilizer superfine cement, which have been widely used in engineering. The average particle size of superfine cement particles is about 4m, and the specific surface area is 8000 cm2/g ... It not only has good groutability, but also has advantages in stone strength, environmental protection and price, and is especially suitable for grouting tiny fractured bedrock.

Add some additives (such as accelerator, water reducer, early strength agent and stabilizer, etc. Adding cement slurry can adjust or improve some properties of cement slurry, meet the specific requirements of engineering for slurry, and improve grouting effect. The type and dosage of additives should be determined by experiments.

Cement clay slurry, cement mortar and cement fly ash are made of cement slurry mixed with human clay, sand and fly ash, which can be used for bedrock grouting with large grouting amount and low requirements on stone strength. This is mainly to save cement and reduce the cost of materials. This slurry is mainly used for grouting gravel foundation.

When encountering some special geological conditions, such as faults, fracture zones, micro cracks, etc. And it is difficult to meet the engineering requirements by using ordinary cement slurry, chemical grouting can also be used, that is, grouting slurry made of polymer materials such as epoxy resin, polyurethane and polycondensate. The material cost is relatively high and the grouting process is complicated. Chemical grouting instrument plays an auxiliary role in bedrock treatment. Generally, cement grouting is carried out first, and then chemical grouting is carried out on this basis, which can not only improve the grouting quality, but also be economical.

References:

Zhou Yihong. Introduction to water conservancy and hydropower engineering construction [m]. Wuhan: Wuhan University Press, 2004.

[2] Cao Feng. Building construction technology [m]. Zhengzhou: Zhengzhou University Press, 2007.

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