HRP930932A2 - Safeguarding of underground works and inclinations by using propylene fibres microreinforced concrete and solid tressed anchorage - Google Patents

Description

The field of technology

The invention belongs to the scientific field of civil engineering discipline: geotechnics and building materials.

B) The invention made it possible to solve the following technical problems:

B.1. Increasing the general (global) and local stability of rock slopes;

B.1.1. Rigid prestressed anchors enable an increase in global (general) and local stability by ten percent due to the dowel effect compared to prestressed anchors of low stiffness and the same cross-section.

B.1.2. By joining the rock and a layer of micro-reinforced shotcrete (with propylene fibers) with rigid anchors, a more favorable stress-strain condition is achieved in the zone of the reinforced slope, i.e. the tunnel's primary lining.

B.1.3. With rigid prestressed anchors, multiple anti-corrosion protection is relatively easily achieved, which is important from the point of view of the durability of such constructions.

B.1.4. Increasing the shear resistance parameters of the MAMB layer with polypropylene fibers.

B.1.5. Increasing the elasticity modulus of the ideformability of the MAMB layer with polypropylene fibers.

B.1.6. Increasing the resistance to shrinkage cracks of the MAMB layer with polypropylene fibers.

B.1.7. Preventing the appearance of microcracks in the MAMB layer with polypropylene fibers.

B.1.8. Increasing the homogeneity of the MAMB layer with polypropylene fibers.

B.1.9. Increasing the abrasion resistance of the MAMB layer with polypropylene fibers.

B.1.10. Increasing the corrosion resistance of the MAMB layer with polypropylene fibers.

B.1.11. Increasing the seismic load resistance of the MAMB layer with polypropylene fibers.

B.1.12. Increasing the resistance to dynamic load (impact) of the MAMB layer with polypropylene fibers.

B.2. The economic efficiency of the lining made of MAMB with polypropylene fibers is reflected in the following:

B.2.1. The constituents that make up the coating give the following relationship:

C1:C2:C3 = 2.2:7.7:1

where is:

C1 - price of shotcrete with reinforcing mesh 50x100x5 mm

C2 - price of shotcrete with steel fibers,

C3 - price of shotcrete with polypropylene fibers.

B.2.2. The economy of shotcrete coating is not only in the amount of constituents but also in the following:

B.2.2.1 Significantly lower labor consumption (approx. 4 times compared to a layer of the same thickness of classically reinforced shotcrete.

B.2.2.2 There is less material consumption (MAMB) and less rebound when spraying,

B.2.2.3 Considerable shortening of the time necessary for casting a layer of shotcrete compared to classically reinforced shotcrete.

B.2.2.4 Advantage in the organization of work in a confined space compared to classically reinforced shotcrete.

B.2.2.5 The working conditions are safer (more favorable) compared to conventional reinforced shotcrete.

B.2.2.6 Rigid prestressed anchors when connecting the rock and the MAMB layer provide a more rational construction compared to classically reinforced shotcrete and classical (non-prestressed) anchors.

B.2.2.7 Bonding of rock and layer of MAMB (with polypropylene fibers) is used in the following geotechnical works:

- securing steep rock slopes,

- insurance of underground excavations,

- ensuring excavation for the foundation of buildings,

- supporting structures.

State of the art

Micro-reinforced shotcrete (MAMB) has been accepted worldwide for several years as an efficient way of strengthening rock after excavation, either for work on the surface or for underground structures, and even as a definitive lining.

Steel fibers in microreinforced shotcrete have almost completely replaced steel reinforcing mesh.

Several companies have published publications on the state of MAMB modification with steel fibers. The latest is Tunnelling the World published by the company 'IDramixll - Belgium (1991 edition). This publication presents the historical development and provides a reference list of the constructed objects.

The application of MAMB with steel fibers has already been adopted in a large number of technically advanced countries.

However, the use of polypropylene fibers in concrete is known only to prevent the appearance of microcracks in young concrete, and for this purpose fibers with a thickness of approximately 0.020 mm are used.

Description of the solution

GENERAL VIEW

The subject of the invention is a new procedure for securing underground works and slopes using micro-reinforced concrete with polypropylene fibers and rigid prestressed anchors.

It is a construction that uses all the most favorable properties of the materials from which it is composed, namely: steel rods - rigid anchors that absorb tensile forces and MAMB with polypropylene fibers, which absorbs tensile, bending and shear stresses in the elements between the anchors.

According to this, the new structure was created by joining rock that can take on high compressive stresses and a layer of micro-reinforced shotcrete (with polypropylene fibers) with rigid prestressed anchors. This procedure improves the mechanical properties of a wider zone of the rock mass whose thickness roughly corresponds to the length of the anchors, and which achieves a favorable stress-deformation state in the shear zone, i.e. around the underground opening. The technology and stages of execution of the supporting anchor structure are shown in Fig. 2.

The technical presentation of the new technology is given in Fig. 2, where the basic goals of anchoring in the zone of influence of rigid prestressed anchors and the influence of the MAMB layer are highlighted.

The influence of rigid prestressed anchors can be seen from Figures 3 and 4 and equation (2).

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As Tb increases, Fs increases

As Tdow increases, Fs increases

Tdow increases with increasing geot stiffness. anchor.

INFLUENCE OF MICRO-REINFORCED JET CONCRETE (MAMB)

From an extremely fragile material, by adding polypropylene fibers, concrete becomes an elastoplastic, tough material. For this purpose, smooth and profiled (X-section) polypropylene fibers with a thickness of 0.1 to 2.0 mm are used according to the technology described in this patent application. In order to improve the adhesion of such fibers and concrete, a polymer additive is used. The working diagram of the MAMB loaded in bending has a characteristic shape according to Figure 5.

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Sl. 5. The characteristic shape of the MAMB working diagram tested according to ASTM C-1018

Tests of the tunnel lining and the lining of the slopes made of MAMB with polypropylene fibers were carried out. The load-bearing capacity of the coating was tested on the derived surfaces using hydraulic presses.

These tests have shown

- that the load capacity of such cladding is equal to or greater than the load capacity of classic cladding with steel reinforcing mesh or cladding reinforced with steel fibres;

- that the behavior of the lining after the appearance of cracks at the point of maximum force moment is characteristic of elastoplastic materials.

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Sl. 6. Diagrams from the load-bearing test of the MAMB tunnel lining

And lining without reinforcement (d=7.5 cm)

II lining reinforced with steel nets (d=10 cm)

III cladding reinforced with steel fibers (d=14 cm)

IV lining reinforced with polypropylene fibers (d=14 cm)

BRIEF DESCRIPTION OF THE BASIS OF CALCULATING STRESS, DEFORMATION AND DIMENSIONING OF PRIMARY EXCAVATION INSURANCE

CALCULATION OF STRESS 1 DEFORMATION

The results of stress analysis around underground openings according to the theory of elasticity can be limited only to rock of exceptional quality, i.e. that which is marked as exceptionally good according to the "Q" and "RMR" classifications. For all other categories, it is necessary to take into account the fact that during the deformation of the massif, cracks open, crushing and creeping of the crack filling in the contacts of the layers, and especially the appearance of partial or complete degradation of the geotechnical characteristics of the massif, even though the rock within individual blocks retains the mechanical properties of compact rock . The behavior of rock as a massif is characterized by a non-linear relationship between stress and deformation, the occurrence of creep, plastification or viscous flow, i.e. an increase in deformation without a further increase in stress. The measurements carried out near the underground rooms show that the relationship between stress and deformation is very complex, that anisotropy is almost regularly expressed, so that the actual phenomena are far from what the results of the theory of elasticity give.

After blasting, regardless of whether it is an excavation in the entire profile or a partial excavation, the circumferential ring of rock with a different depth from the edge of the profile is disturbed. In this zone, the rock is full of visible and invisible cracks. The material can be scattered to a greater depth, sometimes released from stress and thereby brought to an unstable state, and in any case mechanically degraded compared to the intact rock. Careful contour blasting and "presplitting" will somewhat reduce the harmful effects on the quality of the rock. After the opening of the profile, the primary state of stress changes to a secondary one, and the greatest concentration of stress would occur next to the opening, if the edge ring of the rock is not mechanically weakened. Plasticization and viscous flow occur, as well as slippage. Usually, these processes are limited only to a smaller zone along the profile and are stopped by primary insurance. Deformations should be limited by insurance to the size that the rock can still take without crushing and destruction. The tendency of NATM (New Austrian Tunneling Method) is to avoid "heavy"-"Strong" subgrade and causing high reactive pressure in the subgrade, while allowing rock deformations with the maximum possible use of the bearing capacity of the rock itself.

A subgrade with a large deformation capacity - low stiffness will produce a lower reactive pressure on the rock, which will require greater convergence. In contrast, a rigid underlayment or lining installed immediately after excavation will prevent plasticization of the rock, but with the realization of a large pressure on the rock.

These phenomena are explained by Rabchewich's diagram of the relationship of convergence and possible subgrade pressure given in Fig. 7.

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Instead of stress distribution according to the theory of elasticity, Kastner proposes an analysis of rock stress based on the limit state of equilibrium, according to the Mohr-Coulomb theory of strength, which is also acceptable for all materials that have lower tensile strength than compressive strength (fig. 8).

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The rock and subgrade form a statically indeterminate system in which mutual influences are determined by common deformations. An important factor in this is the deformability of the subgrade, because a compliant subgrade will follow the realization of convergence while realizing a lower reactive pressure. The equilibrium state occurs precisely when the subgrade reaction is equal to the internal pressure required to achieve the same convergence.

The elastic deformations that occur during the transition from the primary state of stress to the secondary (due to the opening of the profile) are practically instantaneous, so these deformations are not prevented by primary securing.

DIMENSIONING OF PRIMARY INSURANCE

The reactive pressure of the primary insurance is:

Pa=Ps+Pb+Pm

where is:

Ps - limiting (reactive) anchor pressure

Pb - limiting (reactive) pressure of the shotcrete shell

Pm - limiting (reactive) pressure of steel meshes in a layer of shotcrete

The limiting (reactive) pressure of the anchors is:

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where is:

As- cross-sectional area of anchors (m2)

bvd - limit of large steel deformations (kN/m2)

e1 and e2 - distance between anchors (m)

The limit (reactive) pressure of the shotcrete shell is:

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where is:

bpr - concrete prism strength (kN/m2)

d - thickness of shotcrete layer (m)

ar- radius of the concrete shell (m)

The limit (reactive) pressure of the steel mesh (in the shotcrete layer) amounts to

:

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Therefore, the total reactive pressure from the shell of reinforced concrete amounts to:

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Claims (1)

1. The procedure for securing underground works and cutting using polypropylene fibers of micro-reinforced shotcrete (MAMB) and rigid prestressed anchors, which is indicated by mixing smooth or profiled polypropylene fibers with a length of 0 0.1 to 2.0 mm into the shotcrete mixture and polymer additive to increase the adhesion of fibers and concrete, and the thus obtained elastoplastic concrete shell is connected to the rock massif using pre-stressed steel anchors, forming a unique load-bearing structure.