Application of Long Span Reinforced Concrete Beam and Slab Structure in the Bottom Structure of Stope

1 Project Overview

Suichang gold in mineral Predevonian metamorphic NEE configuration alteration zone, a build gold silver deposit account vein quartz rock type is shallow mesothermal sulfide depleted. The ore type is rich in gold-silver-depleted sulfide quartz veins, the average grade: Au: 11.5g / t; Ag: 190.27g / t, the ore body inclination is generally 45 ° ~ 60 °, the thickness is 0.33 ~ 18m Etc., is a thin ~ medium thick inclined ore body. The ore body and surrounding rock are affected by faults and veins after mineralization, and local joint fissures are developed. The surrounding rock of some ore bodies is parallel to the size of the ore body and the smooth surface is poor. The ore is vein quartzite , f=16～18, and the surrounding rock is dominated by biotite oblique gneiss , f=16～18.
The mine is underground mining, and the Pingshuo-blind shaft development system is adopted. The stage height is 40m. The mining method is a comprehensive method of retaining ore. The ore blocks are arranged along the strike. The length is generally 25~50m, and the height of the bottom column is 5~6m. The height is 3~4m, and the width of the column on both sides of the mine is 3~4m. The bottom structure is mainly in the form of two common funnels and electric gutters. For the higher grade nuggets, the top and bottom columns and the inter-column are harvested before the mine, and then artificial columns are constructed. Up to now, it has accumulated nearly 30,000 m3 of concrete and more than 70,000 tons of ore. The artificial pillars constructed in the past have a span of less than 8m. The average thickness of the artificial pillars in the stope is about 10m, which is the first large-span artificial pillar of the mine.
The 26512 ore block is the top nugget of the west end of the V-1 ore body. It is located on the east side of the L3 line, bounded by fine rock in the east and bounded by a group of faults in the west.
In the middle section of +260m, the strike length is about 50m, the horizontal thickness of the ore body is 2~14m, there are 1~2 stones in the mine, and the ore body extends to +280m. The length of the nugget in the middle section of +280m is about 45m, and the thickness of the ore body is 1.4~4.5m. The thickness of the ore body is thin at both ends. The overall shape of the ore body is 200°∠50°. More stable. The average grade of the nuggets is: Au: 14.69 g/t; Ag: 171.13 g/t.
2 artificial false bottom design
According to the geological shape of the ore block and the location relationship of the surrounding mines, combined with the comprehensive recovery of the pillar resources, the artificial false bottom is constructed at the bottom of the middle section of +260m. Because the ore body is thick in the middle section, and the surrounding rock joints of the ore body are relatively developed, if the reinforced concrete slabs used in the mine are designed, the bearing load of the ore and the stability of the components themselves cannot be met. Therefore, the inverted "T" beam and plate combination structure is used for design.
The length of the mine is 50m, the section height is 40m, the loose rock accumulation height is H=35, the ore body inclination angle is α=50°, the ore rock bulk density γ is 26.75kN/m3, the internal friction angle is 38=38°, and the loose ore is The friction coefficient of the stope wall is tgθ=0.78, the true thickness of the stope is calculated as H0=7.5m, the average thickness of the unsteady rock stratum on the stope is B=4m, and 200# concrete and I and II grade steel bars are selected, Ra= 107.8MPa, Rw=137.2MPa, safety factor n is 2, concrete weight q=23.52kN/m3.
2.1 Beam calculation
The beam is calculated according to the structure of the point column. The point column is generally located in the middle of the stope to reduce the span of the stope and support the ground pressure. The external force is mainly from the loose ground pressure and deformation ground pressure of the surrounding rock. This is used as a basis for structural strength calculation at design time.
2.1.1 Load calculation
The vertical column bottom plate is designed to be placed, and the pressure is the component of the gravity of the unstable rock mass of the upper plate in the axial direction of the vertical point column. According to the theory of area bearing hypothesis, the ground pressure value can be calculated by equation (1):

Where: Q———ground pressure value, N;
S———the roof area occupied by the point column, m2
γ————The weight of the surrounding rock of the plate, N/m3
B———the average thickness of the unstable rock mass, m;
α———The inclination of the ore body.
That is: Q = SγBcosα = 687.78MN.
2.1.2 Strength calculation
The strength of the beam is also calculated according to the point column. The beam is designed as a reinforced concrete structure. To ensure its stability, according to the empirical formula Sp/Sc=(D/H0) 1/2, the beam design strength calculation formula can be derived (see Formula (2)):

Where: Sp———beam design strength, N/m2;
Sc———Strength of concrete cube block, N/m2;
D———the shortest side length of the rectangular section point column (or the circular section point column diameter), m;
H0———point height, m;

D/H0———The aspect ratio of the point column is determined empirically.
Namely: Sp=(D/H0)1/2·Sc=48.21MPa
2.1.3 Determine the sectional area of ​​the beam
According to the average stress of the point column is less than or equal to its allowable stress condition, the following formula (3) can be obtained:

Where: A
p———the sectional area of ​​the point column, m2;
n———Safety factor.
That is: Ap≥n·Q/Sp=28.53m2 According to this, it is determined that the beam is a rectangular beam with a sectional area of ​​2×3m2, and the total sectional area is 5×6=30m2, and the aspect ratio is 0.2. The Sp is 48.21 MPa and the safety factor is 2.1.
Referring to the requirements of the elastic beam reinforcement of the axial compression members, the longitudinally stressed steel bars are arranged as 20×Φ28 for each beam, and are evenly arranged along the section, and the reinforcement ratio is 0.21%.
2.2 board calculation
The main function of the artificial false bottom is to isolate the loose ore in the upper and middle sections. Therefore, the calculation of the load of the false bottom should be based on the static pressure of the loose ore deposited on the upper part.
Starting from the concept of stress transfer, according to the theory of ground pressure of Taishaji, when the dip angle of the ore body is not large, the loose ore is not in close contact with the surrounding rock, and the friction is negligible. It is assumed that the full-height gravity of the loose ore is all loaded on the false bottom except for the friction against the contact surface of the bottom plate. At this time, the average pressure of the false bottom can be calculated by the formula (4).

It can be seen from equation (4) that the size of P is only related to H under the condition that α, γ, and θ are constant, and is independent of thickness, but the smaller the ore body thickness is, the smaller the P value is; and when H exceeds a certain value After the impact on the P worth size is minimal. It can be seen that the formula (4) is suitable for the case where the thickness of the ore body is large and the pile height of the loose ore is not too large.
Take 1m wide plate as the calculation unit and calculate the plate thickness as 1.5m. According to formula (4), the average pressure P=26375×103×35×(sin50°-cos50°×0.78)=2.48MPa; Component self-propelled uniform load q=23.52×103×1.5×sin50°=0.27MPa; maximum bending moment Mmax=1/8(P+q)·H02=1/8(2.48+0 .27) × 7.52 × 105 = 19.34 MN / m; according to this calculation, the cross section of the reinforcement is 60.29 cm 2 , taking 10 × Φ 28, and the upper and lower layers are equally arranged.
Based on the above calculation results, the artificial false bottom design uses five 2m × 3m specifications (width × height) beams combined with 1.5m thick plates (see Figure 1).

3 construction methods

(1) Site excavation and leveling. Firstly, according to the quality data of the mining site, the ore body of the artificial false bottom elevation is cut and harvested in advance, and the upper mining height is controlled according to the beam plate design elevation and the construction work space, and some loose ore is released after the upper mining reaches the predetermined elevation. Most of the loose ore is used as a false-bottom casting cushion, and the site is leveled in combination with the actual inclination of the upper and lower plates of the stope (in principle, the beam-slab section should be perpendicular to the surrounding rock of the upper and lower plates. If the false-bottom span is large, the slope can be appropriately reduced).
(2) Eye hole construction and surrounding rock reinforcement. After the site is leveled, the eye hole calibration is carried out according to the orientation of the steel structure along the direction of the stope. The position of the eye hole and the depth of the eye must be designed and constructed to prevent the structural changes of the components from affecting the mechanical properties. If the surrounding rock cracks during the construction of the eye hole, the resin anchor is used to strengthen the surrounding rock to ensure that the rib is in the stable rock layer.
(3) Component erection. Before the components are erected, a layer of tarpaulin should be laid on the loose cushion to prevent the slurry from being poured during pouring. According to the component layout diagram, the component is erected and supported, and the unloading port is reserved according to the specified position, and the self-flowing pipe and the pedestrian passage of the working face to the position of the upper mixing concrete are laid.
(4) Concrete pouring. After all the above work is prepared, the concrete construction will be carried out according to the corresponding “Construction Quality Acceptance Specification for Concrete Structure Engineering”. Due to the limited construction conditions of the stope and the long construction period, the concrete construction joints need to be treated before each shifting operation. .
4 effects analysis
Since the completion of the construction of the false bottom in 2008, the upper ore body of the stope has been continuously recovered, and a total of 17,450 tons of ore is mined. Due to the enterprise's ore production plan, the upper part of the loose bottom is stored for about 6 years. During the period, there was no change in the structure of the artificial false bottom, and the top and bottom plates of the goaf above the false bottom did not appear to fall, which played a positive role in the maintenance and management of the top and bottom of the goaf. From the analysis of the recovery effect of resources, if the ore pillar is reserved, nearly 4,500 tons of ore resources will be lost. After the artificial false bottom replacement, the amount of mineral metal Au: 51.7kg and Ag: 856.2kg will be recovered. Bottom construction cost, increase the economic efficiency of the enterprise by about 12.93 million yuan.
5 Conclusion
With the increase of mining years, the company's resources are increasingly exhausted, and the resource reserves are quite severe compared with the existing production capacity of the mine. In 2005, it was listed as a resource crisis mine by the Ministry of Land and Resources. Over the years, the company has attached great importance to the conservation and comprehensive utilization of mineral resources, and has tried to improve the resource recovery rate. It has adopted a combination of low-grade corner ore and high-grade ore bodies to mine and apply (reinforced) concrete to replace various ore pillars. During 2009-2011 alone, the recovery rate of mining reached more than 99%. The total investment for raising the “three rate” project reached 34.654 million yuan, increasing the product gold by 218.106kg and silver 2927.15kg; increasing the enterprise income of 6476 .98 million yuan; economic benefits of 30.115 million yuan, achieved a good economic and social benefits.
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