Influence of physical characteristics of weak rock mass on height of caved zone over abandoned subsurface coal mines
V.Palchik
Abstract This is a study of the height of caved zones over abandoned subsurface coal mines in Donetsk,Ukraine.The heights of the caved zones in porous weak rock mass over shallow,abandoned under-ground workings (up to 80m)were detected by drilling and linked with laboratory measurements of the physical characteristics of the rock mass over-lying the underground openings.An empirical model is developed to determine the height of a caved zone in weak porous rock mass over shallow,underground workings.The model shows that the decrea in uniaxial compressive strength of the immediate roof and the increa in average porosity of rock layers over the immediate roof lead to an increa in the ratio between the height of caved zones and the height of underground coal workings.It is suggested that the bulking of the weak imme-diate roof is insufficient to arrest caving over un-derground workings and therefore the significance of the rock mass overlying the immediate roof in formation of caved zones is incread.It is argued that the caving height in weak porous rocks may reach 4.1–11.25times the height of underground coal extraction.
Keywords Caved zone ÆAbandoned underground opening ÆCompressive strength ÆPorosity
Introduction
Roof caving over abandoned subsurface mines may result in sinkhole subsidence of the ground surface (Day 1984;Culshaw and Waltham 1987;Price 1990;Miller and Stee-ples 1991;Palchik 1991;Kemmerly 1993;Zhou-Wanfang 1997;Palchik 2000).Therefore,the potential height of the caved zone over underground workings at shallow depth is one of the most important parameters to be understood for engineering constructions on the overlying ground surface (Whittaker 1985;Paganini 1988;Palchik 1991;Karfakis 1993;Waltham and Chorlton 1993).
From 1917to 1951,dozens of small coal mines operated at depths up to 80m in Donetsk,Ukraine.The rock mass overlying the underground openings in coal ams con-sists of beds of shallow-dipping (less than 12°)argillite,sandy shale and sandstone.The underground openings in the old mines were mainly drifts along short (25–28m)coalfaces (Palchik 1989).
Palchik (2000)determined the criteria for existence of cavities in the underground openings at shallow depths in Donetsk,Ukraine,bad on the lithological compositions of the overlying rock mass,thickness and quence of rock layers,and rock strength.He showed that cavities in
openings at shallow depth exist only where there are thick and relatively strong sandstone layers in the overlying rock mass.In other cas,where underground openings are collapd,it is necessary to calculate the height of the caved zone to predict whether collap pits and sinkholes will develop on the ground surface.
芝士蛋糕的做法
In conditions where the roof is bedded,the height of the caved zone over underground openings can be calculated using a bulking factor and the original working height of the openings (Belyaev 1984;Whittaker 1985;Smart and Aziz 1989;Palchik 1991;Singh and Singh 1999).For strong rock in the Donetsk area,at intermediate and deeper depths,the bulking factor can be calculated using the uniaxial compressive strength of the immediate roof (Belyaev 1984;Palchik 1991).
However,at shallow depth (up to 80m),rocks of the collapd roof are very weak (r l <11MPa)and very porous (porosity 23–47%)due to the influence of near-surface weathering.The depth border of the weathering zone ranges between 55and 85m.Palchik (2000)has shown that porous sandy shale and sandstone in such rock
mass is very weak and friable.He also showed that the layers do not restrain bedding paration and
collap
Published online:1March 2002ªSpringer-Verlag
2002
Department of Geological and Environmental Sciences,Ben-Gurion University of the Negev,P.O.Box 653,Beer-Sheva,84105,Israel E-mail:vplachek@bgumail.bgu.ac.il Tel.:+972-8-6461770Fax:
+972-8-6472997
92
Environmental Geology (2002)42:92–101DOI 10.1007/s00254-002-0542-y
upon caving of the immediate roof.Pores in such rock mass are signi ficant stress concentrators a
nd therefore strongly in fluence rock strength (Palchik 1999).The higher the porosity,the lower the rock strength.
This study develops a new model for predicting the height of a caved zone in weak porous rocks over underground openings at shallow depth.The calculations u uniaxial compressive strength of the immediate roof and the porosity of the weak friable rock layers overlying the immediate roof.Porosity is the most important parameter that controls the strength of the overlying layers.
Measured values of the height of the caved zone,deter-mined by drilling,show that the calculation can predict roof failure.
Theoretical background
The bulking factor controlled caving model is widely ud in engineering practice to calculate the height of the caved zone (Turchaninov and others 1977;Kratzsch 1983;Smart and Aziz 1989;Whittaker and Reddish 1989;Palchik 1991;Karfakis 1993;Singh and Dhar 1997;Singh and Singh 1999).This model shows that the height of the caved zone (H )in bedded rocks is dependent upon the working height of underground coal extraction (h )and a bulking factor (k )and can be expresd as:
H ¼h
k À1
ð1ÞThe bulking factor (k )is the increa in volume of rock due to caving.It is the ratio between the volume of rock after caving to its initial volume.The bulking factor of rock is larger if rock strength is greater (Kratzsch 1983;Belyaev 1984;Palchik 1991).Belyaev (1984)and Palchik (1991)have shown that in the Donetsk area,with strong rock at intermediate and deeper depths of longwall mining,the bulking factor depends on the square root of uniaxial compressive strength of the immediate roof:
田朴珺床戏k ¼1þa ffiffiffiffi
r l p ð2Þwhere r l is the uniaxial compressive strength of the im-mediate roof in MPa and a is an empirical coef ficient (for carbonate rocks in Donetsk area a =0.05).
Inrtion of the strength-dependent bulking factor given in Eq.(2)in Eq.(1)yields the strength dependence of the ratio between caving height and the height of underground coal extraction:H h ¼1a ffiffiffiffir l
p ð3Þ
Dependencies k –r l [Eq.(2)]and H /h –r l [Eq.(3)]are plotted in Fig.1,where uniaxial compressive strength in-creas from 25to 150MPa,the bulking factor k increas from 1.25to 1.61and the value of H /h decreas from 4to 1.63.
Note,however,that Eq.(3)is a model of ratio H /h for rocks with uniaxial compressive strength from 25MPa (H /h =4)to 150MPa (H /h =1.63),but in this study the rocks are very weak (1MPa<r l <11MPa).It may also be noted that the rock mass over the immediate roof is friable and therefore after collap of the immediate roof,the overlying very porous rock mass los strength and
collaps under its own weight into the mined-out space.
Porosity controls the strength of the overlying rock layers.Therefore,it is appropriate to examine the applicability of model (3),which was developed for strong rocks,to the soft porous rocks,and to develop a new
蛇跟猪属相配吗equation
Environmental Geology (2002)42:92–101
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that can describe H/h and take into account not only the uniaxial compressive strength of the immediate roof,but also the porosity of the overlying weak,friable rock mass.
Description of rock layers
The deposits of the Quaternary(alluvium)and Carbona-ceous periods are part of the geological structure of the rock mass over the old mining workings at shallow depth. The alluvium consists of sands and clays and its thickness ranged from0to20m.The thickness of the Carbona-ceous layers(sandy shales and sandstones)over collapd openings are between0.9and10m.Sometimes there are thin layers of argillite above the layers of sandy shale and sandstone.
Since sandy shale and sandstone are located immediately over underground workings and,hence,play an important role in the formation of the caved zone,their uniaxial compressive strength and porosity were studied in detail (e next ction).The geological profile and caving in the weak rock mass overlying the abandoned underground opening at shallow depth in the Donetsk area are shown schematically in Fig.2.
Sandstone consists of weakly cemented quartz grains. Sandy shale consists of weakly cemented quartz grains and mica in the form of thin,horizontal scales.Sandy shale and sandstone are porous due to near-surface weathering and the small amount of cement(calcareous cementation)located at quartz grain contacts.The porosity of such rocks increas with decreasing cement content.
Porosity(n)of the cylindrical samples of sandy shale and sandstone was calculated(n=(1–q/G s)·100%)from mea-sured values of dry bulk density(q=1.43–2.1g/cm3)and specific gravity of solids(G s=2.68–2.71g/cm3)and ranged between23.1and46.9%.The precision of porosity esti-mation is0.1%.
Usually the porosity of sandstone and sandy shale tends to decrea with increasing depth below the surface.This loss of porosity is due almost entirely to pressure-solution and redeposition of quartz.
Drilling and mechanical testing
In this study only areas of construction of industrial and civil structures are considered(including Donetsk Sub-way)where abandoned underground openings have where there was roof caving.Eighty-one boreholes were drilled in31construction areas over col-lapd abandoned wo
rkings at shallow depth(<80m). Conditions of the openings and their heights were un-known due to the impossibility of getting underground. Each area has two tofive boreholes between15and80m depth.Height of the caved zone in weak rock over open-ings(H)was detected through the accelerated sinking of the drilling tool and drillingfluid loss in boreholes that were drilled from the ground surface down to the aban-doned openings.In the boreholes the openings were
filled by material of the collapd immediate roof.Height of underground coal extraction(h)was determined from the thickness of the coal am in borehole A,drilled through coal pillars(the height of underground coal ex-traction was equal to the thickness of coal am;e Fig.2).
The samples for mechanical tests to determine the r l value were derived from cores taken from the immediate roof of the collapd openings in boreholes from31construction areas.Drilling showed that the maximum number of rock layers,which were collapd over the immediate roof,is generally less than four.Therefore,samples for determi-nation of porosity were extracted only from four rock layers over the immediate roof.Sampling was difficult from porous sandy shales and sandstones becau of their friable nature and weathering cracks.Some samples were split during sampling.The uniaxial compressive tests were conducted in the Test Laboratory of Donetsk Geological and Prospecting Institute(Ukraine).Cylindrical samples of rock ud in tests had a diameter d=54mm and a ratio L/d=
2(L is here a height of sample),as suggested by ISRM standards.Loading experiments were performed with a 50MPa hydraulic load frame.The applied load and the deformations were respectively measured by means of load cells and strain gauges.
Three tofive samples were ud for determination of r l in the immediate roof in each borehole B,drilled in
collapd
Fig.2皮肤红肿痒
Schematic profile of caving over
厦门冬季旅游攻略underground opening.H w is
mining depth,H is height of
caved zone and h is height of
underground coal extraction
(or thickness of coal
am)
94Environmental Geology(2002)42:92–101
underground openings.Standard deviation of mean r l in the immediate roof in each area of construction is different and ranges from 0.15to 0.8MPa.In areas with relatively high strength (9–11MPa),the standard deviation is 0.8MPa,whereas at low strength (r l =1–1.5MPa)devia-tion is 0.15
MPa.Three to five samples were also ud for determination of n in each of four rock layers over the immediate roof in each of the boreholes.Standard devia-tion of mean porosity within the same rock layer within the same area of construction is 0.1–0.5%.Standard de-viation of the mean has been calculated as follows:
D ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðx 1Àx mean Þ2þðx 2Àx mean Þ2þÁÁÁÁþðx n Àx mean Þ2
n À1
s ð4Þ
where x 1,x 2,...x n is value of obrved parameter (strength or porosity)in 1th,2th ...and n th sample respectively,n is the number of obrved samples,and x mean is the arith-metic mean of parameters which were obrved in n samples.
Results of drilling and uniaxial compressive tests are summarized in Table 1,where thickness of coal strata have a range of 0.6<h <1.6m and the height of the caved zone (H )varies from 4to 12.5m,
New model of ratio H /h in weak rock mass
Obrved (actual)values of ratio (H /h )detected through u of borehole tests in 31areas of construction are plotted
Table 1Results of drilling and mechanical tests.A borehole to coal pillar;B borehole to collapd opening;h obrved thickness of coal am
(height of underground coal extraction);H obrved height of caved zone;H /h ratio between height of caved zone and height of under-ground coal extraction;r l mean uniaxial compressive strength of
immediate roof Area no.Borehole no.h (m)H (m)H /h r l (MPa)12(A)
1.5–6 3.22-1(A) 1.48–2(B)–925(A) 1.4– 6.1155(B)–8.555-1(B)–8.5310(A) 1.12–7.46 1.410(B)–8.35414(A)0.6– 5.08 3.714(B)– 3.05519(A) 1.6–7.81
2.319(B)–12.56
20(A)0.8–5
8.1
20(B)–420-1(B)– 4.120-2(B)– 3.85721(A)0.9– 6.83 2.221(B)– 6.15827(A)0.65– 6.46 1.327-1(A)0.68–27(B)– 4.2928(A) 1.2– 5.46 3.628(B)– 6.551031(A)0.86– 5.81 5.131-1(A)0.85–31(B)–51132(A)0.75– 6.67 3.432(B)–512
33(A) 1.35– 4.37
7.1
33-1(B)– 5.933–2(B)–633–3(B)– 5.81334(A)0.8– 4.63 3.634-1(A)0.8–34(B)– 3.71436(A)0.8–5 4.736(B)–41537(A) 1.2– 6.58237-1(B)–7.837-2(B)–81638(A)0.75–10.67 1.638-1(A)0.73–38(B)–81739(A) 1.1–7.27 1.739(B)–81840(A) 1.6–7.28 2.340(B)–11.651942(A) 1.6– 5.66742(B)–9.052043(A) 1.3– 6.31 2.743(B)–8.22144(A)0.95– 6.79344(B)– 6.452245(A)0.8–10.94 1.145(B)–8.752346(A) 1.25– 4.56 4.746(B)– 5.946-1(B)– 5.82447(A) 1.35– 5.56 3.247(B)–7.525
48(A) 1.12– 6.7
5
48(B)
–
7.5
Table 1
(Contd.)Area no.Borehole no.h (m)H (m)H /h r l (MPa)2649(A) 1.05–9.76 1.2549(B)–10.252750(A) 1.55–7.81 2.150(B)–12.350–1(B)–11.928
51(A) 1.15– 4.7
10
51(B)– 5.551-1(B)– 5.851-2(B)–551-3(B)– 5.12952(A)0.8– 5.51152-1(A)– 4.252(B)– 4.63053(A) 1.05–7.19 2.753(B)–7.553154(A) 1.2– 6.63
2.1
54-1(A) 1.2–54(B)–7.9554-1(B)
公证处公证流程
–
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Environmental Geology (2002)42:92–101
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against the model of H /h [Eq.(3)]in Fig.3.It is clear that the value of H /h in weak rock cannot be described by Eq.(3),which was developed for strong rocks,since: 1.The linear squared regression coef ficient R 2=0.6339is relatively small.2.There is discrepancy between the obrved and calcu-lated values of H /h .While the obrved H /h is between 4.37and 10.93,the calculated H /h is 1.1–2.71times higher.Figure 4shows the relationship between uniaxial com-pressive strength of the immediate roof and the obrved value of H /h in all areas of construction.This figure clearly indicates that the value of H /h increas with decreasing uniaxial compressive strength of the immediate roof.Us-ing the least squared fit method it was found that depen-dence H /h –r l best follows the power law (squared
regression coef ficients R 2=0.6069).The trendline of the power law is plotted in Fig.4.The mechanical strength of the immediate roof (r l ),however,in fluences H /h only partly (R 2is relatively small).This can be explained by the partial in fluence of the rock mass over the immediate roof on ratio H /h .Debris of rock layers overlying the imme-diate roof also take part in formation of the caved zone.The dominant parameter controlling the strength of rock mass over the immediate roof and hence the ratio H /h ,is the porosity of weak sandy shales and sandstones in the rock mass.Porosity creates some zones of weakness along which failure could propagate easily and roof caving could occur.
In Fig.5the in fluence of mean porosity of four rock layers over the immediate roof in areas of construction (e Table 2)on ratio H /h were compared.The lines in Fig.5are linear fits for the relationship between ratio H /h and mean porosity in each of the four layers.It is evident that there is a difference between the in fluence of porosity on different layers:values of R 2are 0.634,0.3021,0.3271and 0.1409for the correlation of obrved value of H /h and the mean porosity of the first,cond,third and fourth layer
Table 2
Porosity of rock layers over immediate roof.n 1,n 2,n 3and n 4are
mean porosities of first,cond,third and fourth layers over the
immediate roof respectively;n aver is average porosity of the four
layers overlying the immediate roof according to Eq.(5)Area no.n 1(%)n 2(%)n 3(%)n 4(%)n aver (%)13536.94240372323236.934.63433338.8394439428.93135.63331530.13037.63636629323029317293034.236.13282930.53535.4339283233.7343010313637.841.135113435.43938.836122334.335.4362913263333.9322914273036.938311529.43331.937.233164141.7454745173235.835.634341829.9293739341934373744.335203434343434213436.937.843.835223843.44245422327.633.537.5473324283231.435.229253443.93837.738264240.544.741.1432734343434.5342831312833.43129303845433530313536.24434.231
28.4
29.7安卓手机怎么截屏
34
36
32
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Environmental Geology (2002)42:92–101
respectively.The individual in fluence of mean porosity in each layer on the value of H /h is not very pronounced (regression coef ficients are relatively small).This can be explained by the combined in fluence of porosity in veral layers overlying the immediate roof since the veral layers can also be collapd.
Using a method of successive approximation and the squared fit method,the ratio H /h with respect to values of average porosity of the different layers over the immediate roof (geometric mean,arithmetic mean and also weighted mean porosity which is calculated taking into account the thickness of rock layers)were generalized.The average porosity of the first and cond layers;average porosity of
the first,cond and third layers;and the average porosity of all four layers were examined.As a resul
t,the average porosity (n aver )at which dependence H /h –n aver gives the largest squared regression coef ficient (R 2=0.72–0.74)was determined.This average porosity re flects the combined effect of porosity in layers overlying the immediate roof and has the following form:
n aver ¼
P
k ¼4i ¼1n i h i
P k ¼4i ¼1
h i
小辣妹
ð5
Þ
Environmental Geology (2002)42:92–101
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