OA17887A - Undercut excavation method with continuous concrete floors. - Google Patents

Abstract

The present invention provides a technique in undercut excavation that allows a continuous steel reinforced concrete floor to be set up or installed over a large width and length and installing continuous steel reinforced concrete floors in any subsequent lifts. Using the present invention, the continuous concrete floor can be extended at a later date if the stopping area is extended at some future date.

Description

1. Field of the Invention

This invention relates to a method for excavation from the top down, usually known as undercut excavation using concrète floors that become a roof for the next lower level of excavation. More particularly the invention relates to how to develop a continuous concrète floor using only standard size 5 m x 6 m drifts openings in the top lift or with some modification, continuous floors in the second and subséquent lower levels.

2. Discussion of the Prior Art

There are many descriptions of conventional undercut-and-fill mining methods in the mining literature, however, probably one of the best is to be found in the article entitled: Undercutand-Fill Mining at the Frood-Stobie Mine of the International Nickel Company of Canada, Limited by J. A. Pigott and R. J. Hall published in The Canadian Mining and Metallurgical Bulletin for June, 1961, Montreal, pp. 420-424.

It is also already known to mine ore by an undercut-and-fill method while providing concrète floors that serve as a roof for the subséquent eut on a lower level. For example, in an article entitled Kosaka Mine and Smelter published in the Mining Magazine—November 1984, page 404, a method called underhand eut and fill using an artificial roof is disclosed. According to this method, the cross-cuts are back17887 ο n .du filled by first installing a layer of reinforcing steel mesh near the floor, followed by pumpina in a 500-600 mm thickness oncrete mix and, when it is dry, the length of the mining block th

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I rv x- XU.S. Pat. No. 5,522,676

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I17887 the ground, and blasted with explosives to break the ground around the posts without,- however. damaging the posts rhemsfi'ves. This fa~ilitat.es excavation under the concrète floor/roof thereafter

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ed in US Patent No ·· i .i further support to the concrète roof and thi.

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The concrète posts arc f course, ri g overload and fail particularly during seismic events, such as a rock burst or may produce massive energy releases.

U.S. Patent No.

5,944,453 provided

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22,b/6 by providing protection against rapid loadinq rp'H.

(a) drilling holes of ground;

Οι êXCêSSlvæ lOSdo ό.ΠΘ tO the holes, these posts having their bottom ends ~ 'Z>

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(d) pouring of the posts, and •u ion, with the résilient '-f £= 4- H i ri

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Π baC kfilling is a monolithic 5 m n r» mine that

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Cold joints are formed when concrète is backfilled against concrète that has previously hardened or set directed to a further improvement in the undercut excavation xJ f t

522,676 and No. 5,944,453 by providing a method ot pouring in the excavation

U.S

Patent

SUMMARY OF TUF technique in undercut

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Γ7 ontinu.ous concrète floor can be extended at ? fl xJ -, 4- X. U.CL UC _a

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concrète floorS.

floor eteuably is invention to

A continuous

in the rock on the first lift of

A further object of this invention is to create a continuons concrète floor in a simple and efficient manner starting from of 5 ore bodies with a plan area of 10m x

100m Ol laryer to t-s 4- 1— xJ kJLJL· il U.

use the continuous concrète floor in the undercut ex —rr-i’- cemented backfill while allowing the concret rocl the posts below to fail.

In the development nd elastic pads hâve shown r'.c.o-f-o b. o-rm -F.-'.

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A still further object of this floors on subséquent

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invention will be apparent from the following description there·:

DRAWINGS

The invention will now be described. b

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FIG. 1 is a top plan view of a computer model of an excavation having a sériés of parallel drifts to be excavated according to the method of the présent invention.

FIG. 2 is a partial section view of the excavation of Fig. 1.

FIG 3 is a detailed view of a form and sand fill utilized around the base of the walls of a drift in accordance with one embodiment of the invention.

FIG 4 is a detailed view of a concrète floor poured over the sand fill of FIG 3 and with the form removed in accordance o embodiment of the invention.

reinforcing layer before

Γ î VT ! o nu. vmw k? 4form of Fig 3 and sand fill as used around the par-ί ph nroximity to the walls of the drift.

FIG 7 is a detailed view of the periphery of the concrète floor of Fig 6 showing the sand fill and a ramp after the form of Fig 3 is removed. and

FIG 8 is a top plan view showing a part of the periphery of a concrète floor not in proximity to the --7=11 = of - drift with reinforcing steel exposed.

ial section view of an excavation according to the présent invention wherein i->nd-=r performed under continuous concrète floors on the lifts above the lift being excavated.

PREFERRED EMBODIMENTS mined ore and filled stopes with a weak concrète floor on top

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against concrète that has prcrvri des a technique in undercut mining that allows a continuous stpp large width and length. A continuous concrète floor

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in length the floor can be set up

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In accordance concrète floor (for

m) using standard 5 m width y. 6 m height using a mechanical rock cutting machine such as a road header r;

ri η n is used in association with double post mining, support posts or rock below prior to installing are installed into the ore the concrète floor. The

K 4s ? t f or falls of ground.

tall posts, pre-break the of the drift rounds may m lono is directed to how to create a continuous concrète îT-.l x

continuous concrète floor covers a 100 m x 100 m area.

In

ZJ rized by the following advantages:

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When using double post mining, the présent

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a maximum safe mining support width of 5-6 m without falls of cemented rock fill at or near temporary support and the continuous co ·»

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Setting up concrète floors underground requires that the safe

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US Patent No.

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adjoining walls below.

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embodiment the method of excavation of the pre rv top slice 10 at ground 1 for example 5m x 6m x 100 m long

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In the embodiment illustrated the method according to the présent invention of excavating a first lift 16 underneath the comprises the following steps:

(a) A first drift 17 corresponding to the height of the posts

1.3 inserted in the holes 11 in the rock below the top slice 10 and in the embodiment shwn in of said oost.s is excavated.

The width of the drift can vary

SO safely supported bv — j

434 Ί posts 13 or unexcavated

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into adjacent drifts as explained below (b) A second drift 18 the rock below the top slice 10 ^c.

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The width of the drift <

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has

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post holes 21 of x_ 1- - -C post holes résilient éléments 2°·

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i.^- inserted in the post holes 21. The floor 22 of the first drift is backfilled with point below the top of broken rock or ore 25 and graded to a the posts extending above the floor 22 of be (d) rx -τ' the first drift 17.

backfilled to within

A thin plastic a plastic material (e) Then

ΪΛί Ht i ü membrane •f- K o mm of the top of the posts.

mbodiment the thin layer is tl.

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XI the levelled broken rock or ore 25, any other can be used down into the a pattern of that will levelled broken rock reintorcing steel 27

----j_ .t_i d -L provide adéquate strength to rd

17. The reinforcing steel 27 —J 4- u •u c layer per standard then installed around the perimeter

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perimeter walls may vary

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417887

- 14 planking 33 standing on edge the height of the top surface 34 of the concrète floor 35 to be poured above the reinforcing steel 27. In the embodiment illustrated the end 32 is in the shape of an upstanding U-shaped bracket 36. The space 37 5 between the edge of the wall 29 of the drift 17 and the planking 33 is filled with sand 38 so the reinforcing steel 27 is covered. The form 28 when used against the wall of the drift is removed as the concrète floor 35 is poured so the concrète completely covers the sand as described below and shown in FIG 4.

nst th as shown in FIG is used. ±n this embodiment the rorm zü na from planking

33.

Sand 40 fills the space

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Once the concrète floor 35

4- ko ed reinforcing steel from damage a ramp 41 as shown ~ -Ç 4- H embodiment shown so long as they

-1-½ periphery of the concrète floor to be poured to resuit in the

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4- tor shown in FIG 7 with or without the ramp.

(g)

Concrète drift first (h)

AF] ? t c. t or and with a thickness sufficien drift

As noted η ΐπΌΠ r

.......iT JT ” “ the concrète floor 35 when example a thickness of

4- H o-4 4-4the

250 mm.

the before the concrète sets underneath the concrète between the planking 33 and the edge of the wall of the first drift 17.

(i) Steps (c) to (h) above are repeated with the second drift after it is fully excavated along its length.

(j) The first drift 17 and the second drift are tightly filled with cemented rock fill or the équivalent.

(k) Excavate, drill and blast or road header the third drift corresponding to the unexcavated rock or ore 20 between the first and second drifts can be removed up to the edge of the concrète floors 35 in the first drift and the second drift.

(l) When using double post mining, repeat step (c) for the third drift 19, namely once the third drift has been excavated along its length, drilling post holes of predetermined grid, size and length in the floor of the third drift. At the bottom of the holes résilient éléments capable of absorbing shock energy or excessive loads due to ground movement ar

Then concrète posts are inserted into the holes, with the posts having their bottom ends resting on the résilient extending above the floor of the third drift. The floor of the third drift is backfilled.

with broken rock or ore and graded to a point below the top of the posts extending above the floor of the third drift. The broker rock or ore for example may be backfilled to within 50 mm of the top of the posts.

n c Z.

(m) Remove the sand 38 covering the ends of the reinforcing steel 27 from under the concrète floor 35 of the first 1 / and second drifts along the portion of the periphery of the first 17 and second 18

can be done using a high pressure sprayer as one example.

(n) A thin plastic layer is installed over the broken rock or ore on the floor of the third drift. In the preferred embodiment the thin layer is a plastic membrane that prevents liquid cernent from draining down into the levelled broken rock or ore.

(o) Then a pattern of reinforcing steel in the form of a mesh, rebar or screen, is installed over the plastic layer to provide adéquate strength to the concrète floor to be poured over the plastic layer and broken ore on the floor of the third drift. The reinforcing steel is lifted and supported the desired height above the thin concrète impervious layer. The reinforcing steel in the third drift extends past the periphery of the third drift to overlap the ends of the adjacent reinforcing steel 27 in the first and second drifts.

(p) Concrète is then pumped or poured over the reinforcing steel to form a concrète floor in the third drift with a thickness sufficient to support cemented rock fill or the équivalent above the concrète floor when the third drift is tightly backfilled. The previous sand filled areas along the periphery of the first and second drifts, including a space under the lip 42 of the concrète floor 35 in the first and second drifts, are filled with concrète and the reinforcing steel overlap to form a continuous concrète floor in the first, second and third drifts.

(q) The third drift is tightly backfilled with cemented rock fill or the équivalent.

(r) Steps (c) to (p) are repeated across the first lift to the limit of the ore or to the design limits of that phase of excavation of ore resulting in a continuous concrète floor across the entire lift.

(s) Steps (c) to (r) are repeated for excavation of a second lift beneath the continuous concrète floor of the first lift or any extension of the first lift to a new area as shown in

FIG 9.

- 17 FIG 8 shows schematically a concrète floor 43 poured in an excavated area of a drift with the reinforcing steel 44 around the periphery of the concrète floor 43 not in proximity to the walls of the drift exposed prior to pouring a concrète floor in the area 45 to form a continuous concrète floor with concrète floor 43.

At the edge of the area to be excavated, wall pins and rebar hangers are utilized to support the perimeter of the concrète floor slab using convential civil engineering techniques and standards.

When reference is made herein to concrète posts, these include reinforced concrète posts and when reference is made to pouring a concrète floor on the ground and on the top ends of the posts, it also includes the pouring or casting of a reinforced concrète floor, i.e. a floor designed with rebar and screen éléments within the concrète, so that the posts cannot puncture the same.

the Présent Invention

DPM mining according to the présent invention provides a new mining method that has the potential to totally revolutionize underground mine planning of midsized ore bodies. The key breakthrough cornes from the small stope size - 7.5m x 7.5m x 6m - that has a reinforced concrète roof held up by four large concrète posts. The individual blocks in the initial geological block model now become the stoping plan and the o r\ d u concrète floor is held up with a grid of posts allowing mining in any direction under the concr floor .

While the original concept of DPM was developed some time ago until recently computer modeling wasn't powerful enough to calculate the redistribution of loads every time a drift round was removed in an individual DPM room. Current 3D modeling answered many of the what if questions: what is the loading on the posts? Does the loading increase with each lower lift? How strong does the backfill hâve to be? How thick do the concrète floors hâve to be?

The benefits to the mine owner of using the présent invention particularly in association with the double post mining method include:

1. DPM mine planning - The mine plan for DPM mining is the geological block model; ail that is required is access to the top 6m high mining lift and a second access for ventilation and egress. Mining and backfilling of 100% of the 6m lift proceeds in parallel. A safe planning rule of thumb is that an orebody can support a 1000 tpd mining rate per 100 ore blocks - with the number of blocks known the mining rate can be estimated and then the mine infrastructure designed. Parallel mining and backfilling plus 100% of the ore lift in production gives a much higher mining rate per million tons of orebody compared to other mining methods such as blasthole or eut and fill or underhand drift and fill mining methods.

2. Following the Ore - the normal mine planning process of designing and scheduling stopes and pillars is an itération process; planning various scénarios takes time and a change in orebody size or shape or a change in métal prices requires a complété redesign. The versatility of the présent invention means that mining can hait at any point under the concrète floor if the orebody ends or the grade diminishes. Similarly mining can continue past the concrète to follow the ore, in effect becoming a new top slice. This means that a change in the shape of the ore body or grade will not affect production or require a redesign. Also, in the future if métal prices or ore values increase, a road header can drive through the backfill to reach now profitable ore at the far end of the ore body.

3. Elimination of Work - The présent invention éliminâtes most ground control functions such as rock bolting, cable bolting and shotcreting (except for the top slicing). Other mining functions like eut lose raises, long hole drilling and the equipment to carry out the functions are reduced. The présent invention also éliminâtes a lot of higher cost mining functions - primary, secondary and sill pillar recoveries, fill fences or bulkheads etc. Most mines spend 30% of their labor and material on ground control. Ground control work also reduces development advance rates by 30 to 50% - more development footage or headings, more delays. By eliminating development work, both productivity and safety statistics improve by that percentage.

4. Ore Recovery - The initial geological block model with conventional mining methods is usually chopped by 20% or so by the mining engineers as the size of stopes and pillars don't necessarily follow the orebody. Room and pillar or post pillar mining methods leave 20 to 30% of the orebody behind as non-recovered support pillars. The présent invention recovers 100% of the ore identified by the geological block modelling. The présent invention can also remove internai dilution (low grade ore blocks that hâve insufficient value to be milled) as well, thus the mining grade can be higher than the original block model average geological grade. Room grades are confirmed by mapping, face sampling and post hole chip sampling. The orebody can be mined selectively with minimum of internai and wall dilution.

5. Capital Development Cost - The présent invention mines the orebody from the top down; pre production waste development is limited to providing access to the top 6m lift or multiple locations depending on the size or shape of the orebody. Two other factors corne into play - less development leads to quicker ore production plus a higher mining rate is achieved earlier. Operating revenue reduces the capital cost dollar for dollar thus the ROI of the project is substantially increased.

6. Mechanized Mining - The présent invention provides room to maneuver large road headers and the concrète roof éliminâtes falls of ground. Ground that is soft enough to eut with a road header usually limits the safe size of openings. The présent invention concrète roofs and posts eliminate most ground imperfections. If there is a combination of weak and hard ore the hard sections can be drilled and blasted.

7. Cemented Tailings Fill - Future development of The présent invention will examine other opportunities for improvement, such as using paste fill to replace CRF. Using paste fill the posts may hâve to compress 250mm and post spacing may hâve to be reduced to 6m x 6m. Once the 3D model is calibrated by mining with stiff fill, weaker fills can be modeled. For rooms with one post in the centre, they can be test mined to allow different fills to be evaluated and post loading, thickness of concrète floor etc can ail be monitored by instrumentation.

8. Safety - Reducing accidents is a complex operation; the largest source of accidents is development work, sealing, rock bolting and other ground control functions. Fails of ground, fails of backfill or unexpected pillar or back failures, working on broken ore, runs of fill, driving raises etc are ail source of injuries. In base métal mines large stope blasts often cause dust explosions. The présent invention créâtes a shop like work environment that can be monitored, uses large equipment with high productivity and reduces the number of miners underground. New hazards such as tripping on rebar or chemical burns from working with concrète will hâve to be identified and managed.

Test Mine

DPM mining according to the présent invention was designed and is currently used in a test mine in Mexico. The test mine design is based on mining 6m lifts of 1000 ton blocks of ore generated by a 3D geological block modal. Each DPM room is mined by 2 drift rounds or a combination of drift rounds and slashes that dimensionally match the geological block model; the model beçomes the stoping plan for the orebodies with 100% ore recovery.

DPM mines the orebody from the top down. The initial lift utilizes standard drift and fill mining except a grid preferably of 7.5m concrète posts and a continuous concrète floor is installed prior to backfilling with cemented rock fill (CRF). Lower lifts are similar to room and pillar mining but carried out under a concrète roof temporarily supported by a grid of concrète posts. As with any new technology there are a few new terms that hâve been developed to explain the system e.g. DPM top slicing, DPM rooms, double posting, pre breaking around posts and filler posts.

DPM is a very flexible mining method that can use drill blast muck techniques for hard ore and roadheaders for softer ores. Mining can be done in any direction under the concrète floor and it can extend out past the concrète to follow the ore this new area then becomes a top slice. Every DPM room within the orebody will hâve exactly the same standard design. The outer perimeter rooms hâve the addition of wall pins and rebar hangers to support the perimeter of the concrète floor slab.

The backfill cycle is very standardized; install the posts, préparé and pour the concrète floors, then fill with CRF. Posting starts with drilling a grid of post holes surveyed to match the corner location of each ore block from the 3D location of the geological block model as shown in FIO 1. A precast concrète post is than installed into each hole, followed by drilling pre-shearing holes around the post.

Préparation for installing the concrète floor starts with spreading a layer broken followed by a layer of plastic; the ore acts as a cushion to prevent blast damage to the concrète roof while the layer of plastic keeps wet concrète from leaking into the cushion material. At this time filler posts are installée! in the DPM lifts - they are bolted to the bottom flange of the post from the previous lift forming the double posting system.

Rebar and welded concrète mesh can now be installed, followed by spécial concrète forms that are backfilled with sand. Removing the sand after the adjacent room is mined allows the rebar to be over lapped, thus forming a continuous concrète floor. Standard 3000psi concrète is pumped to complété the reinforced slab. Once the concrète floor sets the CRF is tight filled using a push blade on an LHD plus a Paus Slinger truck for the nooks and crannies.

The DPM mining and backfill cycles use only standard mine proven equipment, concrète and CRF. Subséquent DPM mining is then carried out under the pre-posted composite roof beam comprised of reinforced concrète plus tightly-packed CRF.

The test mining area was computer modeled using FLAC 3D. Based on previous 2D modeling 0.4m diameter concrète posts and a 7.5m x 7.5m x 6m room size was fixed. An 8 room wide x 12 room long by 5 lift high (or 400,000t) area was selected to allow for maximum load development within the backfill; excavation is via primary and secondary panels 2 rooms (15m) wide accessed from a central entry drift. The concrète floor was modeled only as a tension member as the concrète floor plus cemented rock fill act as a composite beam.

A total of 10 computer runs were performed using various stiffness' for the backfill, posts and floors; each run taking about 120 to 150 hours to completely mine the 480 blocks.

Snapshots of data results were captured every 15 minutes for analysis.

Some of the results were:

1. Normal 6% cemented rock fill generated post loading mainly between lOOt and 250t and the loads stabilized after 4 lifts. Posts were designed for 400t thus post loading is about 50% of the design strength of the posts in compression.

2. To mobilize the backfill strength of typical 6% CRF the posts had to be compressible; weaker fills hâve to move further to arch loads to the walls thus causing more post compression. DPM has designed 400t capacity compression springs that can be adjusted to match the required movement.

3. The concrète floors act only as a tensile member to confine the CRF and the loads arched as predicated. Backfill arching is seen on 2 scales - initially it remains within the DPM rooms; as additional lifts are mined it expands to cover the lift.

4. Surprisingly with weaker fills the tensile loads on the posts in the backfill increased to 300t. The concrète posts in effect become large friction rockbolts in the composite CRF beam. To take advantage of this anchoring phenomenon the posts were redesigned with flanges to attain a continuous 150t tensile strength for individual posts and 300t for double posting.

Instrumentation

Through the years many attempts hâve been made to fully instrument a mine to provide useful, real-time feedback with regards to loads, stresses, etc. The présent invention provides the framework for this type of instrumentation coverage.

The main item to be instrumented is the concrète post loading as one goes through the mining and backfill cycle. However this alone will not provide a snapshot of what is happening within the backfill and concrète floors - for example is the fill separating from the stope back while the backfill arches? This type of technical questioning soon lead to list of the various items that had to be monitored with unique instrumentation to provide the necessary answers.

A summary of the instrumentation installed in a quadrant of the test mine area or 9 sets of posts is as follows:

1. Instrumented cable bolts installed in the back above 9 post locations to measure the movement of the hanging wall or the convergence of the hanging wall (HW) into the backfill thus loading the backfill. Similarly cables could be installed from the roof through the CRF and bolted to the top of the 9 posts supporting the top concrète floor will measure the élévation of the concrète floor vs. the back to see if there is any séparation of fill from the back. This will also see how far the concrète floor has moved down relative to the back of the stope.

3. Instrumented cables will measure a range of tensile loads in key areas of floor slab loading to monitor the tension in the rebar. Cables can also be installed around the perimeter of the floor slab to see what stresses are encountered near the edge of the floor. Similarly by draping instrumentation cables over a 2 inch diameter wall pin with the ends anchored in the floor slab the loading along edge of the floor slab along the walls can be measured.

4. The concrète post compression movement and post loading will be measured by the réduction in height of the compression members below the posts. The concrète posts hâve been designed with a conduit pipe to allow instrumentation wires to run though the post and through conduit imbedded in the concrète floor slabs. Post compression pads boit to the post bottom flange and are reusable.

5. The tensile loading of the post can be measured in several ways, instrumented cable boits cast in the concrète parallel to the rebar or a standard mine extensometer could be installed into a conduit in the post and anchored to the top and bottom steel flanges.

6. Instrumented 3/4inch dia. flange bolts will be used between the instrumented posts to monitor tensile loads from one post to the next.

The computer 3D model shows the backfill loads arching to the walls. Custom instrument packs are being developed to monitor the loads within the backfill to ensure the arching is developing as predicted, to check if the backfill is separating from the floor or back, and to monitor in real-time what is happening as the backfill is being compressed (packed) into place.

Tilt meters will be located in various areas of the concrète floor to see how the floor is bending near the concrète posts or how the floor edges bend as one goes through the mining or backfill cycle.

Ail of the instrumentation that leaves the Yield Point factory is calibrated with it⁷ s own on board computer and battery power supply. Each instrument has its own custom data file thus downloading data from a number of instruments automatically feeds into the proper data file. Data files can be updated at regular intervals as each lift is mined and at regular intervals i.e. every three months, the 3D model can be re-run.

It should be understood that the invention is not limited to the above described preferred embodiments, but that various modification obvious to those skilled in the art can be made without departing from the spirit of the invention and the scope of the following claims.

Claims (5)

1. A method of forming a continuous concrète floor in undercut excavation comprising once a first drift having a floor and side walls has been excavated along its length, installing a pattern of reinforcing steel in the form of mesh, rebar or screen to provide adéquate strength to a concrète floor to be poured over the reinforcing steel, installing forms around a perimeter of the floor of the first drift, wherein said forms installed against the walls of said first drift are a length equal to a length of any overlapping reinforcing steel to be installed in an adjoining drift when excavated, filling said forms with sand so the reinforcing steel is covered, then pouring or pumping concrète over the reinforcing steel and sand to form a concrète floor in the drift with a thickness sufficient to support cemented rock fill or the équivalent above the concrète floor when the drift is tightly backfilled and removing the forms.

2. A method of forming a continuous concrète floor in undercut excavation according to claim 1 wherein once a second drift having a floor and side walls has been excavated along its length where the second drift is separated from the first drift by a third drift of unexcavated ore, forming a concrète floor on the floor of the second drift following the method of claim 1.

3. A method of forming a continuous concrète floor in undercut excavation according to claim 2 wherein once the first drift and second drift hâve been backfilled with cemented rock fill and a third drift, between said first and second drifts and having a floor and side walls has been excavated along its length forming a concrète floor on the floor of the third drift by removing the sand covering the ends of the reinforcing steel from under the concrète floor of the first and second drifts along the portion of the periphery of the first and second drifts adjoining the periphery of the third drift; providing reinforcing steel in the third drift extending to overlap the ends of the reinforcing steel in the first and second drifts; pour or pump concrète over the reinforcing steel to form a concrète floor in the third drift with a thickness sufficient to support cemented rock fill or the équivalent above the concrète floor when the third drift is tightly backfilled and the previous sand filled areas along the periphery of the first and second drifts are filled with concrète and the reinforcing steel overlap to form a continuous concrète floor in the first, second and third drifts.

4. A method of forming a continuous concrète floor in undercut excavation in accordance with any one of claims 1 to 3 wherein once the drifts hâve been excavated along their length, the floor of the drifts is backfilled with broken ore and graded, then a thin plastic layer is provided over the broken ore before installing a pattern of reinforcing steel.

5. A method of forming a continuous concrète floor in undercut excavation according to any one of claims 1 to 4 wherein after forming the concrète floor in the first or second drift, tightly backfilling the first or second drift and the second drift with cemented rock fill or the équivalent before excavating a third drift between the first and second

- 30 drifts up to the edge of the concrète floor in the first drift and the second drift.