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Methods to reduce floor heave and sides closure along the arched gate roads
253
Mining Science and Technology, 10 (1990) 253-263 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Methods to reduce floor heave and sides closure along the arched gate roads A. A F R O U Z Department of Mining and Metallurgical Engineering, McGill University, Montreal, Que. H3A 2A 7 (Canada) (Received June 29, 1989; accepted August 18, 1989)
Abstract Afrouz, A., 1990. Methods to reduce floor heave and sides closure along the arched gate roads. Min. Sci. Technol., 10: 253-263. Back analysis of experimental results from the laboratory and longwall gate roads have been utilized to elaborate on the causes of floor heave and sides closure along the roadways. Techniques to control and reduce the floor heave and sides closure along mine gate roads with soft floor is discussed. Appropriate. practical and theoretical suggestion is given. Application of some of the techniques, especially those utilizing resin or grout injection into the boreholes, are in the mechanized, high productive and continuous mining necessitating continuous ground reinforcement or support system.
Introduction
Closure of gate roads with soft floor is a problem encountered in the carboniferous coal measures of the North America and Europe. Parameters influencing the stability and performance of the gate roads are numerous. They are often directly or indirectly inter-related. The most important of these are: (a) Depth of cover and varying strata pressure. (b) Dimensions and shape of the gate roads. (c) Method of the gate road support and its size. (d) Direction of the roadway advancement relative to the dip of the seam. (e) Rate of advance. (f) Geological factors, such as stratigraphy, discontinuities, cementation between the dis-
continuities, washouts and existence of the tectonic actions. (g) Type of the roadside packs, its dimensions and behaviour as a support. (h) Barrier pillar, its dimensions and behaviour. (i) Panel width or the face length. (j) Extracted height of the seam. (k) Proximity of any mining activity to the gate roads. (1) Method of the roadway development and the production technique. (m) Rock/support interaction properties. (n) Non-uniform stress distribution in the support system. (o) Bearing capacity of the roadway floor. (p) Type and quality of the support or reinforcement utilized along the roadway and behind the face. (q) Support intervals along the roadways.
0167-9031/90/$03.50 © 1990 - Elsevier Science Publishers B.V.
254
(r) Influence of water on the floor material. (s) Physical, mechanical and time-dependent behaviour of the rocks surrounding the roadway. (t) Influence of any ripping or dinting on the neighbouring supports, i.e. pillar, pack and steel arches. (u) Position of the roadways relative to the faceline and its direction of advance (or retreat) e.g. advanced heading, behind the face line with ripping lip, or in-fine with the face. Assessment of all the above noted parameters is prerequisite to a sound prediction, planning and the ground control along the gate roads. There has been several attempts on the basis of theoretical analysis and on the assumptions of circular roadways to evaluate the above noted parameters [1-3]. However, these results, although useful, do not close fit the in situ performance of gate roads with arched cross-section. This paper is a result of laboratory and field data relating to the performance of arched gate roads with soft floors serving longwall faces in the Sydney Coalfield, Nova Scotia, Canada, and the South Wales Coalfield, U.K. Causes of floor heave and sides closure along the gate roads
Inherent weakness of the rock The floor beneath the coal seams is usually underclay. It consists of clay minerals with
A. AFROUZ
abundant randomly oriented stigmarian rootlets, carbonaceous layers and sometimes pyritic nodules [4]. The underclay thus formed has low uniaxial compressive strength values in the range of 10-18 MPa [5,6]. This soft rock is usually slickensided which promote flow of the floor under relatively low strata pressure [7].
Effect of water There are 5 main groups of clay minerals present in the underclay floor of the coal seams. These are: kaolinite, montmorillonite, illite, chlorite and vermiculite. These minerals, especially the first two, have great water absorbtion properties, of up to 3 times their dry volume, with resulting swelling, cracking and disintegration of the rock mass.
Ground pressure This is one of the main factors causing deformation and consequently floor heave and sides closure. This phenomenon is mostly encountered in the relatively deep gate roads, with more than 700 m cover depth. Roadways located under remnant pillars also experience substential floor lift due to presence of zones of high ground pressure projected from the perimeter of the pillars downwards with an angle of fracture expressed as follows [5,6]: /3= k[(~r/4) + (~/2)]
(1)
where fl = average fracture angle of the
Fig. 1. Fracture characteristics of coal (left) and underclay (right) samples showing influence of the bedding. Dimensions of the samples: length 100 mm, diameter 50 ram.
METHODS TO REDUCE FLOOR HEAVE AND SIDES CLOSURE ALONG THE ARCHED GATE ROADS
ground to horizontal. ( = 24-40°; see Fig. 1); = average angle of internal friction of the composite strata to horizontal ( = 1 0 - 3 0 ° ) ; k - - c o n s t a n t relating the laboratory uniaxial test results of the intact rock samples to the in situ measurements on the rock mass ( - - 0 . 4 0.8).
255
"ac"t
'-J'
Size and shape of the gate roads Wider roadways are more prone to floor lift. On the other hand, increase in the height of a roadway, relative to its width results increase in the sides closure. This is more predominant in the deep workings [5]. Mining operations necessitate a m i n i m u m cross-sectional area for the gate roads, especially for the ventilation and materials handling purposes. Therefore, knowledge of the rock types encountered in the floor and sides of the gate roads--cover d e p t h - - m a g n i t u d e , direction and ratio of the major to minor strata pressures on the gate roads can be utilized to determine the ratio of width to height of the gate roads in the planning stage.
--,.._. (a)
Penetration
Pack
of
each
leg
into
very
•
w
w
soil
I
floor
r
Pillar
(b) S o f t f l o o r lift, m a i n l y n e a r e r to t h e pillar s i d e .
Effects on the type and dimensions of the supports, packs and pillars along the gate roads These can be classified into one or a combination of the following: (1) Penetration of the arch legs into very soft floor (Fig. 2 a ) - - i n this case the floor exhibits no resistance to the load exerted on the arched support. (2) Floor heave in the soft underclay (Fig. 2b)--which increase with moisture content of the rock mass. For asymmetrical ground loading, the floor heave is higher towards the more solid rib pillar or barrier pillar than the pack side. (3) Buckling or elasto-plastic type of floor lift (Fig. 2c)--this occurs in the medium strength rocks under relatively dry mining conditions. It is generally accompanied with cracks or fractures.
i!
\-..
(c) B u c k l i n g f l o o r lift in the medium s t r e n g t h r o c k s
Fig. 2. Effects of the support, rib pillar and the gobside pack on the floor along the gate roads.
There are also two main causes of sides closure due to the effects of the ground pressure on the supports, pillars and packs. These are: (a) Buckling of the arch leg (Fig. 3a)--this occurs generally in the more solid pillar side than the weaker pack side.
256
A. A F R O U Z
Provisions for minimum utilization of water on the soft floors ..o.
(a) B u c k l i n g of the a r c h leg in the pillar s i d e of a w e a k r o a d s i d e p a c k
Pack
#~
Pillar
(b) C o n v e r g e n c e of the arch legs along the hard p a c k e d g a t e r o a d s having s t r o n g e r floor
In situ measurements on coal and underclay floor [8] indicated that introduction of water to the dry floor along the gate roads increased their floor heave by an average of 18-30%, respectively. The effect was even greater where the floor was water saturated or when clay bands were present in the underclay. Where reduction of water on the floor is not possible (e.g. seepage of the groundwater to the gate road), spraying chemicals or organic additives which will increase surface tension of the floor material can be helpful. This will make the floor water repellent.
Fig. 3. Main types of sides closure along gate roads.
Installation of steel base plates (b) Convergence of the arch legs (Fig. 3b) - - t h i s occurs usually where the horizontal ground pressure is high, there is no or little difference between the magnitude of the horizontal ground pressure exerted to the sides of the same arches or the strengths of the pillar and the pack are designed to be close to each other.
Methods of reducing floor heave and sides closure Under the tremendous ground pressure around longwall faces. It is not economically possible to construct and support the gate roads so that they stay intact over their utilization period. Therefore, certain amount of deformation should be planned for when designing shape, dimensions and type of the support requirements. Hence, the purpose is not to eliminate the floor lift and sides closure completely, but to minimize them to acceptable limits. This can be achieved by the following techniques:
To reduce penetration of the arch legs into the soft floor. The steel base plates should have rounded corners and edges, secured under the arch legs. Dimensions of the base plates can be matched to the bearing capacity of the particular material and the prevailing strata load. L o a d / d e f o r m a t i o n relationship of the floor beneath the base plates can be "expressed as follows [5,6]: a = Iq . P"=
I. P/A
.E
(2)
where 8 = deformation or settlement of the floor, mm; P = strata vertical load applied on the base plate, MN; l-- thickness of the floor layer affected by the strata loading, m; A = area of contact between the base plate and the floor, m2; and E - - average Youngs' modulus of the floor material, GPa. K 1 and n are proportionality constants varying between zero to one; depending upon: the area (A) and perimeter ( p ) of the base plates, strength and Youngs' modulus of the floor material. The ultimate bearing capacity of the floor
M E T H O D S TO R E D U C E F L O O R HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
beneath the base plate (q) is given empirically [61:
q= -K + m(p/A) n
(3)
where K = compressive stress in the floor directly under the base plate (-- - 2 . 6 to - 2 . 8 MPa; the negative sign indicates the compression); and m = proportionality constant, dependent on the perimeter of the base plates ( = - 0.2 to + 0.2 MPa). Base plates with bearing area of A -- 0.1 m 2 and 5 m m thickness are satisfactory under the ground pressure of up to 25 MPa, if the gate road is used once over a planned life of 1.5-2 years [8]. To prevent slippage, the plates should be provided with bolting facilities to bottom of the arch legs. The wooden chocks conventionally utilized beneath the arch legs tend to deteriorate or deform after which they will not be functional.
Elimination of advanced headings wherever abnormally soft floor is encountered This will reduce the period for which the gate road is needed to stay open, thereby reducing the total floor lift along the roadway. In general, floor lift along the gate roads with advanced heading and soft floor are expected to be 33% higher than those without the advanced heading, under similar ground conditions [6].
Elimination of either main or tail stables This measure reduces the total time and manpower utilized per face advance. Higher rate of advance results in shorter life requirement for the production panel, which subsequently leads to shorter life requirement for the gate roads. This measure does not stop the floor lift, but indirectly reduces its total heave.
257
Reduce asymmetrical floor lift along gate roads Consideration may be given to provide yield facilities for the pillar side of the roadways to be equal to that of the pack side. One method of achieving this is by cutting a slot of 0.5-1 m deep and 5-20 cm high in the solid rib along the roadways before the arches are installed. Then pack the slots with wood chocks or concrete blocks. This action will transfer the peak stress deeper into the coal rib and away from the immediate vicinity of the gate road. In doing so, it will yield and equalize the ground pressure on both sides of the gate roads, thereby providing a more even stress distribution across the roadways. To facilitate the even stress distribution is site specific. It can only be done by trial and error, by changing the dimensions of the slot cut, type and quality of packing the slot to provide the same yield characteristics as that of the opposite side of the gate r o a d .
Facilitate symmetrical yielding on both sides of the gate roads This can also be achieved by designing the pack, to be place in the gob side of the roadway, so that it will provide a similar overall yielding (or stiffness) properties to the opposite side. Various commercially available monolithic and non-monolithic pack materials such as: Shotcrete, Aqualit, Aquapak, Astrapak, Tekpak, Anpak, Anti and Antail can be used.
Complete removal of the soft floor Generally, gate roads with soft floor require at least one and upto 3 times dinting, or back ripping, to keep the road open during the production life of their corresponding faces. The total thickness of dinting in such cases may well exceed 1.5-2.0 m of soft floor during the initial 6-8 months of the production period. Two situations may arise, requir-
258 ing different technique to be used. These are: (1) If the gate road has competent roof and soft floor, it is advantageous to leave the roof undisturbed a n d switch from ripping to dinting the soft floor down to a more stable bed rock. It is suggested to plan to remove the soft floor in one operation, during the roadway drivage. This technique will eliminate the conventional and hazardous ripping operation and lend itself to mechanized dinting of soft floor. Therefore, the overall efficiency of the roadway development, maintenance and safety will increase. To counter sides closure along the deepened gate roads, splayed leg arches can be used with lagging board or concrete blocks superimposed with wire mesh at the interface between the arches and the rock. The initial cost of the extra wire mesh is less than the outlay for conventional dinting or ripping operations. Moreover, the harder floor of a deepened gate road is more adaptable to reinforcement than a soft and weak floor, if it becomes necessary. (2) If competency of the floor is the same or higher than that of the roof along the gate roads, it is advisable to resort to the ripping operation and only remove the obstructing bulges due to the floor heave.
Utilization of inverted arches This technique is applicable where no dinting is anticipated through the life of the gate road. It will increase the total bearing of arch on the floor and reduce the sides closure or distortion of the arch legs. Splayed leg arches may also be used to counter the sides closure. They are less expensive than the inverted arches; but they have adverse effect on the function of stilts, if they are used, and are less effective then the inverted arches.
Floor reinforcement Floor reinforcement will increase the inherent strength of the floor material, improving its stability. Boltingl dowelling and injection
A.AFROUZ are the main techniques utilized. Rock bolts and especially cable bolts are suitable for hard and relatively competent floor layers having weak bonding between the layers or joints. So the bolts knit the strata together. Soft or weak floors, especially in the coal mines do not efficiently respond to the steel bolting. Dowelling is modified version of bolting, where the bolts are wooden (or plastic) and the column is filled with cement grout or resin to hold the wooden bolts to the rock. To obtain m a x i m u m benefit from this type of reinforcement, the dowel should be installed as soon as the floor of the gate road is exposed and before any major floor movement or fracture is developed. The advantage of the wood dowelling over the conventional steel bolting is less cost and more compatibility to mechanized or continuous dinting, if it becomes necessary. This technique can also be utilized to support roof and wails on a continuous or mechanized basis. Alternatively, all major existing fractures or open joints in the soft floor can be reinforced, as soon as possible, by the injection of cement, resin or other adhesive materials into boreholes. The injection pressure can be 2060 kPa. This technique is more costly than the dowelling, but provides a more uniform reinforcement throughout the floor. It is adoptable to continuous mining systems and for roof or wall reinforcement. Araldite resin reinforcement with a volumetric ratio of Resin/Hardner/Plasticizer of 100"5"1 was applied to the underclay samples of 100 m m length and 50 m m diameter, under laboratory conditions, and cured in a well ventilated room temperature of 1 9 - 2 1 ° C for 24 hours. Comparison of the averaged unconfined compressive strength test results have shown: as -- U.C.S. of the untreated underclay sample = 10.0-18.5 MPa ot = U.C.S of the reinforced underclay sample = 12.8-20.1 MPa
259
METHODS TO REDUCE FLOOR HEAVE AND SIDES CLOSURE ALONG THE ARCHED GATE ROADS
% = U.C.S. of the resin sample cured for 24 hours = 75.0 M P a The following empirical expression holds within + 17% error for the resin injected underclay samples: Vt'o t = B(K'O
s -}- Vr • Or)
(4)
where Vt = total volume of the reinforced samples, m 3 ( = V ~ - Vb + Vr = V~); Vs = volume of the untreated underclay sample, naB; Vb = volume of the borehole for the resin injection, naB; V, = volume of the resin used for reinforcement, m3; ( = 0.1 V~); and B = constant related to the efficiency of bonding between the rock and the resin ( = 0.5-1). Determination of the strength and a m o u n t of the reinforcement needed to the soft floor under the in situ conditions can be m a d e as follows: Considering the most critical condition in the weak f l o o r / s u p p o r t stability, as shown in Fig, 4. This condition occur when the floor under the support is failing and there is no considerable shear on the vertical plane AB extended from the support base perimeter to the floor. The vertical stress of an element of the floor at depth z beneath the support (Ovs) can be expressed as:
Gs=Q+y.z
(5)
where Q = load intensity of the strata on the floor beneath the support, which is higher
Support I I I ~ ~L . . . . . . . . . .
.. Unsupported floor
I
[
i
z
"
than the bearing capacity of the soft floor, MPa; and ~, = weight per unit volume of the floor material, M N / m 3. The corresponding horizontal stress (%~) using Terzaghi's [9] expression is: Ohs = Ovs tan2(45 -- 0.5q5) --
2S 0 tan(45 - 0.5q,)
= ( Q - v ' z ) tan2(45 - 0 . 5 0 ) - 2S 0 tan(45 - 0.50)
(6)
where q~ = average internal friction angle of the floor rock, degree; and S o = Cohesion of the floor rock, MPa. If the floor along the gate roads is reinforced uniformly, with an injection material of unconfined compressive strength (o r), then on the unsupported side of the reinforced floor, the vertical stress (ovf) at a depth z using eqn. (4) can be given as: Ovr = ot - 3' "z = [B(Vs'osq-
Vr'or)/Vt]
-~/-z
The corresponding horizontal stress ( % f ) is: %f = Ovf tan2(45 - 0.50) + 2S 0 tan(45 - 0.50) = (o~ - V" z) tanZ (45 - 0.50) + 2S 0 tan(45 - 0.50)
Ohs = Ohf
(9)
Substituting eqns. (6) and (8) into eqn. (9) gives:
- 2S 0 tan(45 - 0.50) [
(8)
For the floor to remain stable, the induced horizontal stresses on both sides within the floor should be equal, i.e.:
( Q - v ' z ) tan2(45 - 0.50)
Fig. 4. Schematic of the support base and soft floor interaction at a depth z below the support perimeter.
(7)
= (o t + y - z ) tan2(45 - 0.5q~) + 2So tan(45 - 0.5q~)
260
A. AFROUZ
~
;;;,,,-,
•~oo .!o ~.~. ~ ~..~
-~o ~..~ 0 >
~ ~N ~,~ .~ ~
bl
222
ooo
XXX
XXX
XXX
XXX
ooo
.,o
~
'~
0
~
m
O
.~ 5 g 0
N
~
~
0
)
"
~
r,.)
XX XXX
X X X X X
X X X X X X X X
X
X X X X X
X X X X X X X X
XX
X
261
M E T H O D S TO R E D U C E F L O O R HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
Therefore, the minimum required strength of the resin reinforced floor ((It) is: qt = Q - [4S0/tan(45 - 0.5~)]
(10)
Substituting eqn. (4) into eqn. (10) for the required U.C.S. of the resin (Or): (i~ = { Vs- { Q - [4S0/tan(45 - 0.5,1 }
-B.Vs.(I~)/V~ (11) Comparison of the required strength of the resin for the rock reinforcement along the gate roads under various conditions is given in Table 1 and illustrated in Fig. 5. Compressive strength ((i~) and elastic moduli (E~) for
TABLE 2 Compressive strength (Or) and elastic moduli (E~) for some commercial polymers at 19-21° C Type of polymer
or (MPa)
PVC
3o
Hard rubber Polymer-cement Epoxy Resin Polyester
40 45
E~ (GPa) 2.1 2.8 3.2 3.2
52 93
CR39
160
1.9 1.7
Makolon Plexyglass Prespec
270
2.6
Cataline61-893 AralditeMY753
380 380 470 490
Castolite Araldite CT 200
510 570
3.5 3.3 4.2
4.5 4.8 4.8
500400R---- 0 . 0 5 b~-
~'~..
3 00-
~
~ " v R---- 0.10
200-
d
~-vR
z0o15
loo. R----O°IO R z0.15
0 5
10
15
20
U.C.S. o f R o c k ( (Is , MPa )
50Q400.
Q=(It(FS)
300i =0,05
200~4
~j
some commercially available resins and polymers is given in Table 2. These can be prepared to have different mechanical properties, to be cuttable by rock cutting machines in the field. They can be cost effective where high production and continuous reinforcement or support system is sought. For the equilibrium to be reached throughout the floor along the gate roads, it is necessary that the total strength of the reinforced floor ((It) be greater than the strata pressure (Q) expressed as:
100-
---0.10 7- OG15
0
J 5
0
= 10
i 15
• 20
U.C.S. o f R o c k ( (Is , MPa )
(12)
where (FS)--the minimum required factor of safety for the floor ( = 1.05-1.5). Substitution of eqn. (12) into eqn. (11) provides the required volume of the resin as follows:
l/r= { gs{ ot( FS ) -[4S0/tan(45
- 0.5q,)] }
- B - V ~ . o s } / o ~ (13) KEY:.
'~
Case n
rlos:l-3
&16-21;
Q=20MPa,w=4m.
Case
nos:7-8
•21-25;
Q=50MPa,w_--5m.
Case
nos .* 4 - 6
&10-15;
Q----aoMPa, w ~ s m .
Fig. 5. Charts for selecting the U.C.S. of resin reinforcement under various ground conditions.
Equations (11) and (13) provide an approximate guidance on the quality and quantity of resin to be used to reinforce a floor of known physical and mechanical characteris-
262 tics and dimensions, under known ground pressure. In practice, if the result of calculation from the eqn. (13) is a negative value, it indicates the actual volume of the resin to be utilized, for a given volume of the floor to be reinforced. If the result of eqn. (13) is positive, then there is a surplus strength in the floor to maintain its equilibrium without any reinforcement. In the latter case, there is no need for the floor reinforcement, unless the ground conditions is changed so that a negative result is obtained.
Practical example
A.AFROUZ
Solution (a) Utilizing eqn. (1): /9= k [ ( r r / 4 ) + (ga/2)] = 0.5[0.785 + 12.51 = 28.8 ° (b) F r o m eqn. (5): Ovs = Q - y . z = 36 - (23 × 0.9) = 15.3 MPa Since the U.C.S. of the underclay ( o s ) = 15 MPa, the floor will fail under the strata pressure and it requires reinforcement. (c) Using eqn. (10): (lt
Floor of a gate road at depth of 650 m. Consists of 90 cm thick underclay, on average. Following averaged data is available: Weight per unit volume of the underclay (V) = 23 M N / m 3 Unconfined Compressive Strength of the underclay ((is) = 15 MPa Constant relating the U.C.S. of underclay to that of the in situ rock mass (k) = 0.5 Internal friction angle of the underclay to horizontal (0) = 25 ° Cohesion of the underclay ( S o ) = 0.3 MPa Strata load intensity on the floor beneath the support (Q) = 36 MPa Efficiency of bonding between the underclay floor and the resin reinforcement ( B ) = 0.9 Determine: (a) The expected fracture angle of the ground to horizontal (fi). (b) The vertical stress in the untreated floor at a depth z = 0.9 m beneath the support
(%). (c) The m i n i m u m required strength of the resin reinforced floor ((it)(d) The m i n i m u m factor of safety (FS). (e) The required unconfined compressive strength of the resin, if the underclay floor is to be reinforced with a 10% volumetric ratio of the underclay: resin.
:
Q - [4S0/tan(45 - 0.5q,)]
= 36 - [4 × 0 . 3 / t a n ( 4 5 - 12.5)] = 34.1 MPa (d) F r o m eqn. (12):
( F S ) = Q/(it = 36/34.1 = 1.06 --- 1.1 (e) Given Vr = 0.1V~ and using eqn. (11): o r = { V~{ Q - [4S0/tan(45 - 0.50] } - 0.9V~ • (iS}/0.1V~ -- (36 - 1.9 - 13.5)/0.1 = 146 MPa.
Conclusions (1) Water increases the coal and underclay floor heave by 18-30%, respectively. (2) Solid pillars left on one side of the gate roads contributes towards the asymmetrical floor heave, if the yielding characteristics of the gobside packs is not similar to that of the pillar. (3) Pillars left in the overworked area tend to transfer the strata pressure towards the floor of the underlaying roadways at an angle between 50 ° and 66 o to the horizontal. (4) Gobside packs have no appreciable effect on the floor heave unless they are packed
M E T H O D S TO R E D U C E FLOOR HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
very tightly a n d their compressive strength surpass that of the floor. (5) Installation of steel base plates increases stability of the arched support a n d reduces p e n e t r a t i o n of the s u p p o r t legs into soft floor. (6) F l o o r r e i n f o r c e m e n t b y injection is promising a n d lends itself to the theoretical a n d in situ analysis. This technique can also be utilize to reinforce the r o o f or sides. It is feasible in mechanized, high p r o d u c t i v e a n d c o n t i n u o u s m i n i n g systems necessitating continuous g r o u n d r e i n f o r c e m e n t or support. The injection can be d o n e either as pre-development, or p r e - p r o d u c t i o n reinforcement, or as a p o s t - p r o d u c t i o n system. The reinforced sections are cuttable b y rock cutting machines, if it becomes necessary.
References 1 Sheorey, P.R. and Dunham, L.K., An approximate analysis of floor heave occurring in roadways behind
2
3 4 5 6
7
8 9
263
advancing longwall faces. Int. J. Rock Mech. Min. Sci., 15 (1978): 227-288. Whittaker, B.N. and Bonsall, C.J., Design aspects relating to the stability of coal mining tunnels. In: Stability in Underground Mining. Pergamon, Oxford (1981), Chapter 24, pp. 519-533. Goetz, W., Planning roadways from the point of strata mechanics. Gli~ckauf, 118 (1982): 13-23. Afrouz, A., The Lower Four Feet floor rocks of South Wales. Colliery Guardian, 224 (12) (1976): 656-660. Afrouz, A., Floor behaviour along longwall roadways. Int. J. Rock Mech. Min. Sci., 12 (1975): 229-240. Afrouz, A., Yield and bearing capacity of coal mine floor. Int. J. Rock Mech. Min. Sci., 12 (1975): 241253. Afrouz, A. and Harvey, J., Rheology of rocks within soft to medium strength range. Int. J. Rock Mech. Min. Sci., 11 (1974): 281-290. Afrouz, A., Mechanical Behaviour of Mine Floor. Ph.D. Thesis, Univ. Wales, Cardiff (1973). Terzaghi, K. and Peck, R.B., Soil Mechanics in Engineering Practice. J. Wiley, New York, N.Y. (1967) 2nd ed.
Mining Science and Technology, 10 (1990) 253-263 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Methods to reduce floor heave and sides closure along the arched gate roads A. A F R O U Z Department of Mining and Metallurgical Engineering, McGill University, Montreal, Que. H3A 2A 7 (Canada) (Received June 29, 1989; accepted August 18, 1989)
Abstract Afrouz, A., 1990. Methods to reduce floor heave and sides closure along the arched gate roads. Min. Sci. Technol., 10: 253-263. Back analysis of experimental results from the laboratory and longwall gate roads have been utilized to elaborate on the causes of floor heave and sides closure along the roadways. Techniques to control and reduce the floor heave and sides closure along mine gate roads with soft floor is discussed. Appropriate. practical and theoretical suggestion is given. Application of some of the techniques, especially those utilizing resin or grout injection into the boreholes, are in the mechanized, high productive and continuous mining necessitating continuous ground reinforcement or support system.
Introduction
Closure of gate roads with soft floor is a problem encountered in the carboniferous coal measures of the North America and Europe. Parameters influencing the stability and performance of the gate roads are numerous. They are often directly or indirectly inter-related. The most important of these are: (a) Depth of cover and varying strata pressure. (b) Dimensions and shape of the gate roads. (c) Method of the gate road support and its size. (d) Direction of the roadway advancement relative to the dip of the seam. (e) Rate of advance. (f) Geological factors, such as stratigraphy, discontinuities, cementation between the dis-
continuities, washouts and existence of the tectonic actions. (g) Type of the roadside packs, its dimensions and behaviour as a support. (h) Barrier pillar, its dimensions and behaviour. (i) Panel width or the face length. (j) Extracted height of the seam. (k) Proximity of any mining activity to the gate roads. (1) Method of the roadway development and the production technique. (m) Rock/support interaction properties. (n) Non-uniform stress distribution in the support system. (o) Bearing capacity of the roadway floor. (p) Type and quality of the support or reinforcement utilized along the roadway and behind the face. (q) Support intervals along the roadways.
0167-9031/90/$03.50 © 1990 - Elsevier Science Publishers B.V.
254
(r) Influence of water on the floor material. (s) Physical, mechanical and time-dependent behaviour of the rocks surrounding the roadway. (t) Influence of any ripping or dinting on the neighbouring supports, i.e. pillar, pack and steel arches. (u) Position of the roadways relative to the faceline and its direction of advance (or retreat) e.g. advanced heading, behind the face line with ripping lip, or in-fine with the face. Assessment of all the above noted parameters is prerequisite to a sound prediction, planning and the ground control along the gate roads. There has been several attempts on the basis of theoretical analysis and on the assumptions of circular roadways to evaluate the above noted parameters [1-3]. However, these results, although useful, do not close fit the in situ performance of gate roads with arched cross-section. This paper is a result of laboratory and field data relating to the performance of arched gate roads with soft floors serving longwall faces in the Sydney Coalfield, Nova Scotia, Canada, and the South Wales Coalfield, U.K. Causes of floor heave and sides closure along the gate roads
Inherent weakness of the rock The floor beneath the coal seams is usually underclay. It consists of clay minerals with
A. AFROUZ
abundant randomly oriented stigmarian rootlets, carbonaceous layers and sometimes pyritic nodules [4]. The underclay thus formed has low uniaxial compressive strength values in the range of 10-18 MPa [5,6]. This soft rock is usually slickensided which promote flow of the floor under relatively low strata pressure [7].
Effect of water There are 5 main groups of clay minerals present in the underclay floor of the coal seams. These are: kaolinite, montmorillonite, illite, chlorite and vermiculite. These minerals, especially the first two, have great water absorbtion properties, of up to 3 times their dry volume, with resulting swelling, cracking and disintegration of the rock mass.
Ground pressure This is one of the main factors causing deformation and consequently floor heave and sides closure. This phenomenon is mostly encountered in the relatively deep gate roads, with more than 700 m cover depth. Roadways located under remnant pillars also experience substential floor lift due to presence of zones of high ground pressure projected from the perimeter of the pillars downwards with an angle of fracture expressed as follows [5,6]: /3= k[(~r/4) + (~/2)]
(1)
where fl = average fracture angle of the
Fig. 1. Fracture characteristics of coal (left) and underclay (right) samples showing influence of the bedding. Dimensions of the samples: length 100 mm, diameter 50 ram.
METHODS TO REDUCE FLOOR HEAVE AND SIDES CLOSURE ALONG THE ARCHED GATE ROADS
ground to horizontal. ( = 24-40°; see Fig. 1); = average angle of internal friction of the composite strata to horizontal ( = 1 0 - 3 0 ° ) ; k - - c o n s t a n t relating the laboratory uniaxial test results of the intact rock samples to the in situ measurements on the rock mass ( - - 0 . 4 0.8).
255
"ac"t
'-J'
Size and shape of the gate roads Wider roadways are more prone to floor lift. On the other hand, increase in the height of a roadway, relative to its width results increase in the sides closure. This is more predominant in the deep workings [5]. Mining operations necessitate a m i n i m u m cross-sectional area for the gate roads, especially for the ventilation and materials handling purposes. Therefore, knowledge of the rock types encountered in the floor and sides of the gate roads--cover d e p t h - - m a g n i t u d e , direction and ratio of the major to minor strata pressures on the gate roads can be utilized to determine the ratio of width to height of the gate roads in the planning stage.
--,.._. (a)
Penetration
Pack
of
each
leg
into
very
•
w
w
soil
I
floor
r
Pillar
(b) S o f t f l o o r lift, m a i n l y n e a r e r to t h e pillar s i d e .
Effects on the type and dimensions of the supports, packs and pillars along the gate roads These can be classified into one or a combination of the following: (1) Penetration of the arch legs into very soft floor (Fig. 2 a ) - - i n this case the floor exhibits no resistance to the load exerted on the arched support. (2) Floor heave in the soft underclay (Fig. 2b)--which increase with moisture content of the rock mass. For asymmetrical ground loading, the floor heave is higher towards the more solid rib pillar or barrier pillar than the pack side. (3) Buckling or elasto-plastic type of floor lift (Fig. 2c)--this occurs in the medium strength rocks under relatively dry mining conditions. It is generally accompanied with cracks or fractures.
i!
\-..
(c) B u c k l i n g f l o o r lift in the medium s t r e n g t h r o c k s
Fig. 2. Effects of the support, rib pillar and the gobside pack on the floor along the gate roads.
There are also two main causes of sides closure due to the effects of the ground pressure on the supports, pillars and packs. These are: (a) Buckling of the arch leg (Fig. 3a)--this occurs generally in the more solid pillar side than the weaker pack side.
256
A. A F R O U Z
Provisions for minimum utilization of water on the soft floors ..o.
(a) B u c k l i n g of the a r c h leg in the pillar s i d e of a w e a k r o a d s i d e p a c k
Pack
#~
Pillar
(b) C o n v e r g e n c e of the arch legs along the hard p a c k e d g a t e r o a d s having s t r o n g e r floor
In situ measurements on coal and underclay floor [8] indicated that introduction of water to the dry floor along the gate roads increased their floor heave by an average of 18-30%, respectively. The effect was even greater where the floor was water saturated or when clay bands were present in the underclay. Where reduction of water on the floor is not possible (e.g. seepage of the groundwater to the gate road), spraying chemicals or organic additives which will increase surface tension of the floor material can be helpful. This will make the floor water repellent.
Fig. 3. Main types of sides closure along gate roads.
Installation of steel base plates (b) Convergence of the arch legs (Fig. 3b) - - t h i s occurs usually where the horizontal ground pressure is high, there is no or little difference between the magnitude of the horizontal ground pressure exerted to the sides of the same arches or the strengths of the pillar and the pack are designed to be close to each other.
Methods of reducing floor heave and sides closure Under the tremendous ground pressure around longwall faces. It is not economically possible to construct and support the gate roads so that they stay intact over their utilization period. Therefore, certain amount of deformation should be planned for when designing shape, dimensions and type of the support requirements. Hence, the purpose is not to eliminate the floor lift and sides closure completely, but to minimize them to acceptable limits. This can be achieved by the following techniques:
To reduce penetration of the arch legs into the soft floor. The steel base plates should have rounded corners and edges, secured under the arch legs. Dimensions of the base plates can be matched to the bearing capacity of the particular material and the prevailing strata load. L o a d / d e f o r m a t i o n relationship of the floor beneath the base plates can be "expressed as follows [5,6]: a = Iq . P"=
I. P/A
.E
(2)
where 8 = deformation or settlement of the floor, mm; P = strata vertical load applied on the base plate, MN; l-- thickness of the floor layer affected by the strata loading, m; A = area of contact between the base plate and the floor, m2; and E - - average Youngs' modulus of the floor material, GPa. K 1 and n are proportionality constants varying between zero to one; depending upon: the area (A) and perimeter ( p ) of the base plates, strength and Youngs' modulus of the floor material. The ultimate bearing capacity of the floor
M E T H O D S TO R E D U C E F L O O R HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
beneath the base plate (q) is given empirically [61:
q= -K + m(p/A) n
(3)
where K = compressive stress in the floor directly under the base plate (-- - 2 . 6 to - 2 . 8 MPa; the negative sign indicates the compression); and m = proportionality constant, dependent on the perimeter of the base plates ( = - 0.2 to + 0.2 MPa). Base plates with bearing area of A -- 0.1 m 2 and 5 m m thickness are satisfactory under the ground pressure of up to 25 MPa, if the gate road is used once over a planned life of 1.5-2 years [8]. To prevent slippage, the plates should be provided with bolting facilities to bottom of the arch legs. The wooden chocks conventionally utilized beneath the arch legs tend to deteriorate or deform after which they will not be functional.
Elimination of advanced headings wherever abnormally soft floor is encountered This will reduce the period for which the gate road is needed to stay open, thereby reducing the total floor lift along the roadway. In general, floor lift along the gate roads with advanced heading and soft floor are expected to be 33% higher than those without the advanced heading, under similar ground conditions [6].
Elimination of either main or tail stables This measure reduces the total time and manpower utilized per face advance. Higher rate of advance results in shorter life requirement for the production panel, which subsequently leads to shorter life requirement for the gate roads. This measure does not stop the floor lift, but indirectly reduces its total heave.
257
Reduce asymmetrical floor lift along gate roads Consideration may be given to provide yield facilities for the pillar side of the roadways to be equal to that of the pack side. One method of achieving this is by cutting a slot of 0.5-1 m deep and 5-20 cm high in the solid rib along the roadways before the arches are installed. Then pack the slots with wood chocks or concrete blocks. This action will transfer the peak stress deeper into the coal rib and away from the immediate vicinity of the gate road. In doing so, it will yield and equalize the ground pressure on both sides of the gate roads, thereby providing a more even stress distribution across the roadways. To facilitate the even stress distribution is site specific. It can only be done by trial and error, by changing the dimensions of the slot cut, type and quality of packing the slot to provide the same yield characteristics as that of the opposite side of the gate r o a d .
Facilitate symmetrical yielding on both sides of the gate roads This can also be achieved by designing the pack, to be place in the gob side of the roadway, so that it will provide a similar overall yielding (or stiffness) properties to the opposite side. Various commercially available monolithic and non-monolithic pack materials such as: Shotcrete, Aqualit, Aquapak, Astrapak, Tekpak, Anpak, Anti and Antail can be used.
Complete removal of the soft floor Generally, gate roads with soft floor require at least one and upto 3 times dinting, or back ripping, to keep the road open during the production life of their corresponding faces. The total thickness of dinting in such cases may well exceed 1.5-2.0 m of soft floor during the initial 6-8 months of the production period. Two situations may arise, requir-
258 ing different technique to be used. These are: (1) If the gate road has competent roof and soft floor, it is advantageous to leave the roof undisturbed a n d switch from ripping to dinting the soft floor down to a more stable bed rock. It is suggested to plan to remove the soft floor in one operation, during the roadway drivage. This technique will eliminate the conventional and hazardous ripping operation and lend itself to mechanized dinting of soft floor. Therefore, the overall efficiency of the roadway development, maintenance and safety will increase. To counter sides closure along the deepened gate roads, splayed leg arches can be used with lagging board or concrete blocks superimposed with wire mesh at the interface between the arches and the rock. The initial cost of the extra wire mesh is less than the outlay for conventional dinting or ripping operations. Moreover, the harder floor of a deepened gate road is more adaptable to reinforcement than a soft and weak floor, if it becomes necessary. (2) If competency of the floor is the same or higher than that of the roof along the gate roads, it is advisable to resort to the ripping operation and only remove the obstructing bulges due to the floor heave.
Utilization of inverted arches This technique is applicable where no dinting is anticipated through the life of the gate road. It will increase the total bearing of arch on the floor and reduce the sides closure or distortion of the arch legs. Splayed leg arches may also be used to counter the sides closure. They are less expensive than the inverted arches; but they have adverse effect on the function of stilts, if they are used, and are less effective then the inverted arches.
Floor reinforcement Floor reinforcement will increase the inherent strength of the floor material, improving its stability. Boltingl dowelling and injection
A.AFROUZ are the main techniques utilized. Rock bolts and especially cable bolts are suitable for hard and relatively competent floor layers having weak bonding between the layers or joints. So the bolts knit the strata together. Soft or weak floors, especially in the coal mines do not efficiently respond to the steel bolting. Dowelling is modified version of bolting, where the bolts are wooden (or plastic) and the column is filled with cement grout or resin to hold the wooden bolts to the rock. To obtain m a x i m u m benefit from this type of reinforcement, the dowel should be installed as soon as the floor of the gate road is exposed and before any major floor movement or fracture is developed. The advantage of the wood dowelling over the conventional steel bolting is less cost and more compatibility to mechanized or continuous dinting, if it becomes necessary. This technique can also be utilized to support roof and wails on a continuous or mechanized basis. Alternatively, all major existing fractures or open joints in the soft floor can be reinforced, as soon as possible, by the injection of cement, resin or other adhesive materials into boreholes. The injection pressure can be 2060 kPa. This technique is more costly than the dowelling, but provides a more uniform reinforcement throughout the floor. It is adoptable to continuous mining systems and for roof or wall reinforcement. Araldite resin reinforcement with a volumetric ratio of Resin/Hardner/Plasticizer of 100"5"1 was applied to the underclay samples of 100 m m length and 50 m m diameter, under laboratory conditions, and cured in a well ventilated room temperature of 1 9 - 2 1 ° C for 24 hours. Comparison of the averaged unconfined compressive strength test results have shown: as -- U.C.S. of the untreated underclay sample = 10.0-18.5 MPa ot = U.C.S of the reinforced underclay sample = 12.8-20.1 MPa
259
METHODS TO REDUCE FLOOR HEAVE AND SIDES CLOSURE ALONG THE ARCHED GATE ROADS
% = U.C.S. of the resin sample cured for 24 hours = 75.0 M P a The following empirical expression holds within + 17% error for the resin injected underclay samples: Vt'o t = B(K'O
s -}- Vr • Or)
(4)
where Vt = total volume of the reinforced samples, m 3 ( = V ~ - Vb + Vr = V~); Vs = volume of the untreated underclay sample, naB; Vb = volume of the borehole for the resin injection, naB; V, = volume of the resin used for reinforcement, m3; ( = 0.1 V~); and B = constant related to the efficiency of bonding between the rock and the resin ( = 0.5-1). Determination of the strength and a m o u n t of the reinforcement needed to the soft floor under the in situ conditions can be m a d e as follows: Considering the most critical condition in the weak f l o o r / s u p p o r t stability, as shown in Fig, 4. This condition occur when the floor under the support is failing and there is no considerable shear on the vertical plane AB extended from the support base perimeter to the floor. The vertical stress of an element of the floor at depth z beneath the support (Ovs) can be expressed as:
Gs=Q+y.z
(5)
where Q = load intensity of the strata on the floor beneath the support, which is higher
Support I I I ~ ~L . . . . . . . . . .
.. Unsupported floor
I
[
i
z
"
than the bearing capacity of the soft floor, MPa; and ~, = weight per unit volume of the floor material, M N / m 3. The corresponding horizontal stress (%~) using Terzaghi's [9] expression is: Ohs = Ovs tan2(45 -- 0.5q5) --
2S 0 tan(45 - 0.5q,)
= ( Q - v ' z ) tan2(45 - 0 . 5 0 ) - 2S 0 tan(45 - 0.50)
(6)
where q~ = average internal friction angle of the floor rock, degree; and S o = Cohesion of the floor rock, MPa. If the floor along the gate roads is reinforced uniformly, with an injection material of unconfined compressive strength (o r), then on the unsupported side of the reinforced floor, the vertical stress (ovf) at a depth z using eqn. (4) can be given as: Ovr = ot - 3' "z = [B(Vs'osq-
Vr'or)/Vt]
-~/-z
The corresponding horizontal stress ( % f ) is: %f = Ovf tan2(45 - 0.50) + 2S 0 tan(45 - 0.50) = (o~ - V" z) tanZ (45 - 0.50) + 2S 0 tan(45 - 0.50)
Ohs = Ohf
(9)
Substituting eqns. (6) and (8) into eqn. (9) gives:
- 2S 0 tan(45 - 0.50) [
(8)
For the floor to remain stable, the induced horizontal stresses on both sides within the floor should be equal, i.e.:
( Q - v ' z ) tan2(45 - 0.50)
Fig. 4. Schematic of the support base and soft floor interaction at a depth z below the support perimeter.
(7)
= (o t + y - z ) tan2(45 - 0.5q~) + 2So tan(45 - 0.5q~)
260
A. AFROUZ
~
;;;,,,-,
•~oo .!o ~.~. ~ ~..~
-~o ~..~ 0 >
~ ~N ~,~ .~ ~
bl
222
ooo
XXX
XXX
XXX
XXX
ooo
.,o
~
'~
0
~
m
O
.~ 5 g 0
N
~
~
0
)
"
~
r,.)
XX XXX
X X X X X
X X X X X X X X
X
X X X X X
X X X X X X X X
XX
X
261
M E T H O D S TO R E D U C E F L O O R HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
Therefore, the minimum required strength of the resin reinforced floor ((It) is: qt = Q - [4S0/tan(45 - 0.5~)]
(10)
Substituting eqn. (4) into eqn. (10) for the required U.C.S. of the resin (Or): (i~ = { Vs- { Q - [4S0/tan(45 - 0.5,1 }
-B.Vs.(I~)/V~ (11) Comparison of the required strength of the resin for the rock reinforcement along the gate roads under various conditions is given in Table 1 and illustrated in Fig. 5. Compressive strength ((i~) and elastic moduli (E~) for
TABLE 2 Compressive strength (Or) and elastic moduli (E~) for some commercial polymers at 19-21° C Type of polymer
or (MPa)
PVC
3o
Hard rubber Polymer-cement Epoxy Resin Polyester
40 45
E~ (GPa) 2.1 2.8 3.2 3.2
52 93
CR39
160
1.9 1.7
Makolon Plexyglass Prespec
270
2.6
Cataline61-893 AralditeMY753
380 380 470 490
Castolite Araldite CT 200
510 570
3.5 3.3 4.2
4.5 4.8 4.8
500400R---- 0 . 0 5 b~-
~'~..
3 00-
~
~ " v R---- 0.10
200-
d
~-vR
z0o15
loo. R----O°IO R z0.15
0 5
10
15
20
U.C.S. o f R o c k ( (Is , MPa )
50Q400.
Q=(It(FS)
300i =0,05
200~4
~j
some commercially available resins and polymers is given in Table 2. These can be prepared to have different mechanical properties, to be cuttable by rock cutting machines in the field. They can be cost effective where high production and continuous reinforcement or support system is sought. For the equilibrium to be reached throughout the floor along the gate roads, it is necessary that the total strength of the reinforced floor ((It) be greater than the strata pressure (Q) expressed as:
100-
---0.10 7- OG15
0
J 5
0
= 10
i 15
• 20
U.C.S. o f R o c k ( (Is , MPa )
(12)
where (FS)--the minimum required factor of safety for the floor ( = 1.05-1.5). Substitution of eqn. (12) into eqn. (11) provides the required volume of the resin as follows:
l/r= { gs{ ot( FS ) -[4S0/tan(45
- 0.5q,)] }
- B - V ~ . o s } / o ~ (13) KEY:.
'~
Case n
rlos:l-3
&16-21;
Q=20MPa,w=4m.
Case
nos:7-8
•21-25;
Q=50MPa,w_--5m.
Case
nos .* 4 - 6
&10-15;
Q----aoMPa, w ~ s m .
Fig. 5. Charts for selecting the U.C.S. of resin reinforcement under various ground conditions.
Equations (11) and (13) provide an approximate guidance on the quality and quantity of resin to be used to reinforce a floor of known physical and mechanical characteris-
262 tics and dimensions, under known ground pressure. In practice, if the result of calculation from the eqn. (13) is a negative value, it indicates the actual volume of the resin to be utilized, for a given volume of the floor to be reinforced. If the result of eqn. (13) is positive, then there is a surplus strength in the floor to maintain its equilibrium without any reinforcement. In the latter case, there is no need for the floor reinforcement, unless the ground conditions is changed so that a negative result is obtained.
Practical example
A.AFROUZ
Solution (a) Utilizing eqn. (1): /9= k [ ( r r / 4 ) + (ga/2)] = 0.5[0.785 + 12.51 = 28.8 ° (b) F r o m eqn. (5): Ovs = Q - y . z = 36 - (23 × 0.9) = 15.3 MPa Since the U.C.S. of the underclay ( o s ) = 15 MPa, the floor will fail under the strata pressure and it requires reinforcement. (c) Using eqn. (10): (lt
Floor of a gate road at depth of 650 m. Consists of 90 cm thick underclay, on average. Following averaged data is available: Weight per unit volume of the underclay (V) = 23 M N / m 3 Unconfined Compressive Strength of the underclay ((is) = 15 MPa Constant relating the U.C.S. of underclay to that of the in situ rock mass (k) = 0.5 Internal friction angle of the underclay to horizontal (0) = 25 ° Cohesion of the underclay ( S o ) = 0.3 MPa Strata load intensity on the floor beneath the support (Q) = 36 MPa Efficiency of bonding between the underclay floor and the resin reinforcement ( B ) = 0.9 Determine: (a) The expected fracture angle of the ground to horizontal (fi). (b) The vertical stress in the untreated floor at a depth z = 0.9 m beneath the support
(%). (c) The m i n i m u m required strength of the resin reinforced floor ((it)(d) The m i n i m u m factor of safety (FS). (e) The required unconfined compressive strength of the resin, if the underclay floor is to be reinforced with a 10% volumetric ratio of the underclay: resin.
:
Q - [4S0/tan(45 - 0.5q,)]
= 36 - [4 × 0 . 3 / t a n ( 4 5 - 12.5)] = 34.1 MPa (d) F r o m eqn. (12):
( F S ) = Q/(it = 36/34.1 = 1.06 --- 1.1 (e) Given Vr = 0.1V~ and using eqn. (11): o r = { V~{ Q - [4S0/tan(45 - 0.50] } - 0.9V~ • (iS}/0.1V~ -- (36 - 1.9 - 13.5)/0.1 = 146 MPa.
Conclusions (1) Water increases the coal and underclay floor heave by 18-30%, respectively. (2) Solid pillars left on one side of the gate roads contributes towards the asymmetrical floor heave, if the yielding characteristics of the gobside packs is not similar to that of the pillar. (3) Pillars left in the overworked area tend to transfer the strata pressure towards the floor of the underlaying roadways at an angle between 50 ° and 66 o to the horizontal. (4) Gobside packs have no appreciable effect on the floor heave unless they are packed
M E T H O D S TO R E D U C E FLOOR HEAVE A N D SIDES C L O S U R E A L O N G T H E A R C H E D G A T E ROADS
very tightly a n d their compressive strength surpass that of the floor. (5) Installation of steel base plates increases stability of the arched support a n d reduces p e n e t r a t i o n of the s u p p o r t legs into soft floor. (6) F l o o r r e i n f o r c e m e n t b y injection is promising a n d lends itself to the theoretical a n d in situ analysis. This technique can also be utilize to reinforce the r o o f or sides. It is feasible in mechanized, high p r o d u c t i v e a n d c o n t i n u o u s m i n i n g systems necessitating continuous g r o u n d r e i n f o r c e m e n t or support. The injection can be d o n e either as pre-development, or p r e - p r o d u c t i o n reinforcement, or as a p o s t - p r o d u c t i o n system. The reinforced sections are cuttable b y rock cutting machines, if it becomes necessary.
References 1 Sheorey, P.R. and Dunham, L.K., An approximate analysis of floor heave occurring in roadways behind
2
3 4 5 6
7
8 9
263
advancing longwall faces. Int. J. Rock Mech. Min. Sci., 15 (1978): 227-288. Whittaker, B.N. and Bonsall, C.J., Design aspects relating to the stability of coal mining tunnels. In: Stability in Underground Mining. Pergamon, Oxford (1981), Chapter 24, pp. 519-533. Goetz, W., Planning roadways from the point of strata mechanics. Gli~ckauf, 118 (1982): 13-23. Afrouz, A., The Lower Four Feet floor rocks of South Wales. Colliery Guardian, 224 (12) (1976): 656-660. Afrouz, A., Floor behaviour along longwall roadways. Int. J. Rock Mech. Min. Sci., 12 (1975): 229-240. Afrouz, A., Yield and bearing capacity of coal mine floor. Int. J. Rock Mech. Min. Sci., 12 (1975): 241253. Afrouz, A. and Harvey, J., Rheology of rocks within soft to medium strength range. Int. J. Rock Mech. Min. Sci., 11 (1974): 281-290. Afrouz, A., Mechanical Behaviour of Mine Floor. Ph.D. Thesis, Univ. Wales, Cardiff (1973). Terzaghi, K. and Peck, R.B., Soil Mechanics in Engineering Practice. J. Wiley, New York, N.Y. (1967) 2nd ed.