National Institute for Occupational Safety and Health,Pittsburgh Mining Research Division,Pittsburgh,PA 15236,USA

Keywords:Coal rib stability Rib support design Coal pillar rib rating Rib quality mapping CPRR Coal classification

ABSTRACT Researchers from the National Institute for Occupational Safety and Health(NIOSH)are developing a coal pillar rib rating (CPRR) technique to measure the integrity of coal ribs.The CPRR characterizes the rib composition and evaluates its impact on the inherent stability of the coal ribs.The CPRR utilizes four parameters:rib homogeneity,bedding condition,face cleat orientation with respect to entry direction,and rib height.All these parameters are measurable in the field.A rib data collecting procedure and a simple sheet to calculate the CPRR were developed.The developed CPRR can be used as a rib quality mapping tool in underground coal mines and to determine the potential of local rib instabilities and support requirements associated with overburden depth.CPRR calculations were conducted for 22 surveyed solid coal ribs,mainly composed of coal units.Based on this study,the rib performance was classified into four categories.A preliminary minimum primary rib support density(PRSD)line was obtained from these surveyed cases.Two sample cases are presented that illustrate the data collection form and CPRR calculations.

1.Introduction

Over the past decade,falls of rib or face have resulted in 16 fatalities,representing over 50% of the ground-fall fatalities in U.S.underground coal mines.More recently,within the past 5 years,this proportion has increased to 80% of the ground-fall related fatalities in these mines.In 2018 and 2019,100% of the three ground-fall related fatalities in U.S.underground coal mines were attributed to falls of rib or face[1].These statistics clearly indicate that,over time,falls of rib or face are contributing more to mineworker fatalities than any other ground fall fatality cause.It is commonly understood that coal mines today are facing ever increasing challenges in the form of increased overburden depth,multipleseam interactions,thinner coal beds,and thicker in-seam partings.While it is difficult to quantify how much each of these factors contribute to the relative stability of the roof and ribs,the consensus is that these are factors so important that they must be considered in modern mine design.To eliminate injuries and fatalities due to falls of rib in underground coal mines,NIOSH researchers are currently working on the development of engineering-based rib control methods.

Currently,mine operators’decisions concerning rib control and support guidelines are based on the Roof Control Plan Approval Handbook[2].Appendix O in the Roof Control Plan Approval Handbook provided information on the conditions that contribute to rib fall hazards and the available methods for protecting miners from rib falls.Two significant geologic conditions that contribute to hazards related to falls of ribs were identified:(1)seam height and(2)depth of cover.The appendix contains statistics indicating that,of 23 rib-fall fatalities that occurred between 1996 and 2010,22(96%) occurred where the mining height was at least 2.1 m (7 ft),and 18 (76%) occurred where the depth of cover was at least 213 m (700 ft).

In coal pillar design,the intact uniaxial compressive strength(UCS) of coal may not be important [3],but in coal rib design,the UCS of coal ribs was found to be a controlling parameter for rib performance[4,5].Colwell[4]found that for development loading conditions,the most important parameters controlling the rib performance are the overburden depth,cleat orientation with respect to the entry direction,the uniaxial compressive strength of coal,and the density of rib support.Seedsman et al.[5]provided a framework for coal pillar rib support and found that a pattern of rib support is determined by the coal strength index(UCS of coal to the virgin stress ratio).Stone[6]utilized an empirical database for rib support to determine the minimum rib support density required during roadway development.Initially,Stone assumed that the main factors controlling the rib support densities are the depth of cover and the mining height.Stone later updated the rib support database by including the strength of coal material as one of the parameters controlling the density of rib support.Stone found that the regression analysis for the rib support database significantly improved by including the uniaxial compressive strength of coal.

The lithologic strength of coal units can be determined directly from the unconfined compressive strength (UCS) test or indirectly from the Schmitt hammer test,point load test,and lithotype identification (or cleat density if it is coal).If UCS testing is not available,testing with a low energy Schmidt hammer using a largearea plunger can provide reasonable estimates on lithology strength.Each lithologic unit of coal and rock should be tested at least 10 times and averaged for the rebound number obtained on the Schmidt hammer test for each individual lithologic unit,excluding the lowest and highest set of numbers.

Rashed et al.[7]compared the point load tests and the in-situ Schmidt hammer test with the unconfined compressive strength of coal,and the results are expressed in Eqs.(1) and (2).

where UCS is the unconfined compressive strength of coal;PLI(50)the point load strength index for standard size sample (50 mm);and nSchmidtthe Schmidt hammer rebound number.

Like any static structure analysis,rib stability can be determined by the load applied on the rib and the strength of the rib.The depth of cover is a good indication of the load on the rib for primary support design.On the other hand,the rib(seam)height may affect rib performance,but there are other important parameters that affect rib strength,such as rib homogeneity,bedding condition,and face cleat orientation with respect to entry direction.The coal pillar rib rating (CPRR) was developed to help quantify the integrity of coal pillar ribs.All these parameters are measurable in the field.The CPRR was generated because none of the current rock-mass classification systems can be readily adaptable to coal ribs for at least one of the following reasons:(1)coal-mass properties,such as cleat density and orientation in addition to coal pillar rib heterogeneity,were not incorporated in any rock-mass classification system,and (2) almost all classification systems are originally based on case histories not relevant to coal-pillar rib performance.Consequently,the direct application of a current classification system would,generally,yield an unrealistic result.

The goal of the CPRR is to aid in the early selection of coal pillar rib support based on the rib composition,mining height,and the expected loading conditions.The CPRR was developed using a hybrid numerical-empirical approach.Underground site investigations,calibrated rib models,and the findings presented in published papers pertaining to geology and underground coal rib performance were used to develop the CPRR.Through underground coal mines investigations,three main rib categories were observed:(1)solid coal ribs with or without thin partings of thicknesses under 0.05 m (2 in),(2) coal ribs with in-seam partings(greater than 0.15 m or 6 in),and(3)coal rib with a roof brow.Only the solid coal rib category is presented in this paper.The other rib categories are still under investigations.Future research is proposed to expand the applicability of the CPRR for conditions where in-seam parting and strong coal/rock brows are encountered.This will extend the potential for usage of the CPRR to most coal ribs.

A data collection procedure was developed to collect the rib properties required to estimate coal pillar rib rating (CPRR).Detailed explanation of this data collection protocol is shown in the following section.

2.Proposed procedure for collecting rib data

When conducting a rib performance survey,it is vitally important to check the stability of the roof and ribs at the site before work is to be conducted.If loose rock or coal cannot be scaled safely,then choose an alternate location for accurate data collection.The location should have lithology members in the rib without cracks or separations in the rock or coal with minimal rock dust so that the lithology is easily discernible.

Fig.1 shows a sample of data collection sheets for the proposed data collection procedure.The first sheet of the collecting rib data procedure includes general data about the mine and mining and rib geological conditions,such as entry dimensions and the rib composition.In the rib profile section,enter the average block size of the coal that spalls into the entry and note the nature of the coal blocks.The coal brightness profile [8]is a quick determination of the brightness and the relative strength of the individual coal units within a rib profile (see Fig.2).Rusnak [8]provided a correlation between the UCS and the brightness profile of coal.The megascopic lithotype of coal was divided into four groups:bright coal,banded bright coal,banded dull coal,and dull coal.The brighter the coal,the higher the cleat density and,hence,the weaker the coal.In contrast,dull coal typically has a lower cleat density,resulting in a higher strength.Hence,from visual observation an estimate of the intact strength could be obtained.Determination of the intact strength of coal based on coal lithotype is the least accurate method and should be used only if the strength of coal is not available from either direct or indirect methods.

Most coals will fall under the banded bright or banded dull categories,and the corresponding category is the code that should be written on the sketch of the rib profile.The rib profile is the area to label the type of lithologies present in the rib.Identifying lithologies other than coal,such as sandstone,limestone,shale,and clay,is very important for understanding the strength of the rib and getting a more realistic CPRR value.The Geologic Features section of Fig.1 is used for highlighting any geologic feature that is not part of the typical rib section.These anomalies include,but are not limited to,unusually thick parting,sandstone or shale channels,and slips or slicks.

Fig.3.Data collection sheets sample (sheet 2).

The second sheet (Fig.3) is used to characterize the properties of the lithologic units composing the rib.Depending on the number of units in the rib,additional pages could be required.Lithologic units are only considered if their thickness is greater than 0.15 m(6 in).If the lithologic unit is less than 0.15 m (6 in),then it will be included with the next lithologic unit that is greater than 0.15 m (6 in).

After identifying the lithologic rib units (coal or rock),determine the joint spacing of the rock units as well as the cleat spacing and orientation for coal units.Additional measurements will be needed for coal units,including cleat persistence,strike and dip of the cleat,if water is present,and if the cleating is closed or open.Indicate if samples were collected for UCS testing or if testing has been done.The final step for lithologic unit characterization is to determine the uniaxial compressive strength of the rib units,which can be determined by direct or indirect methods or the coal lithotypes.

The third sheet (Fig.4) is used to report the applicable rib support:type,pattern,and bolt specifications(size,length,and grade).It also contains notes about causes of rib instability and the face advance method.

Fig.4.Data collection sheets sample (sheet 3).

Fig.5.Data collection sheets sample (sheet 4).

The fourth sheet(Fig.5)is used to classify the observed rib condition by rib deterioration index (RDI).The RDI classifies the observable rib condition into six categories ranked from 1 to 6.The RDI of 1 represents ribs of less than 0.05 m(2 in)of skin spalling,which have vertical rib profiles with intact rock dust.The RDI of 2 represents ribs of 0.05-0.15 m(2-6 in)of skin spalling,hence,small blocks of sloughed ribs will be observable next to the rib.The RDI of 3 represents ribs of 0.15-0.30 m (6-12 in) spalling,hence,large blocks of sloughed ribs will be observable next to the rib.Obvious rib damage characterizes ribs of an RDI of 4,in which damage to the pillar could extend from 0.3 to 0.6 m(1 to 2 ft).Positioning next to ribs of an RDI of 5 could be difficult where significant pillar damage could extend from 0.6 to 1.2 m (2 to 4 ft).The last rib category has the highest RDI of 6,in which positioning next to the rib could be impossible.

3.Coal pillar rib rating (CPRR)

The CPRR is a technique used for measuring the integrity or stability of coal ribs.The CPRR is calculated from the rib factor of safety (RibFOS) [9]and the ratio of in-situ minimum horizontal stress to vertical stress(Sh/Sv).The CPRR can be represented as the scaled area under the relationship between RibFOS and Sh/Sv.The calculated area under this relationship is scaled between 1 and 100.A CPRR of 1 designates the weakest coal rib,and a CPRR of 100 designates the strongest rib.The CPRR measures the quality of coal ribs but not the effect of mining-induced stresses,such as development,abutment,or multiple-seam interaction loads on the rib.

The coal mass constitutive model developed by Mohamed et al.[10]was used to simulate the loading and deformation behaviors of different coal rib lithotypes.In the coal mass model,the response of each coal lithotype is controlled by multiple input parameters.In this paper,each lithotype is referenced by its UCS,e.g.a UCS of 8.57 MPa(1244 psi)refers to the behaviors of a bright coal(BC)lithotype while a UCS of 34.99 MPa(5075 psi)refers to a dull coal(DC)lithotype.Plane strain models[11]were used to calculate the CPRR of 3.3-m (11-ft) height solid coal ribs with different coal rib lithotypes (see Fig.6).

The gateroad in these models was oriented parallel to the maximum horizontal stress.The face cleat orientation was assumed to be parallel to the gateroad orientation.The in-situ stresses in roof and floor strata were estimated by the model developed by Esterhuizen[12],and the in-situ stresses in coal were estimated by the model developed by Liu et al.[13].Mohamed et al.[9,10]proposed rational mechanical properties for the roof-to-pillar interface and floor-to-pillar interface by calibrating the rib models with a field instrumented case study.The in-situ minimum horizontal stressto-vertical stress ratio in these models ranges from 0.06 to 0.53,which corresponds to an overburden depth range of 91-320 m(300-1050 ft).The end-constrain effects provided by roof and floor strata on the calculated CPRR was minimized by assuming tall ribs of 3.3-m(11-ft)high.Fig.6 shows that the BC rib of highly cleated coal mass resulted in the lowest CPRR of 17 while DC resulted in the highest CPRR of 90.

4.Parametric study to develop the CPRR calculation sheet for solid coal ribs

Calculating the CPRR using the numerical model approach is time consuming and requires expertise in numerical modeling,making it unfavorable in practical application.A more practical alternative is to develop a simplified approach using a system of simple linear equations correlating the CPRR of solid coal ribs with their geological and geometrical parameters.

Fig.6.Calculated CPRRs for solid coal ribs with different uniaxial compressive strengths (UCS).

A parametric study for calculating CPRR based on practical ranges of rib height,uniaxial compressive strength of rib units,number of rib units,and the bedding condition,which affect the CPRR,was conducted.A total of 1206 FLAC3Dmodels were conducted to calculate the CPRRs of 201 different rib compositions.The levels and corresponding values of each parameter are shown in Fig.7.Four levels of rib height were considered in the parametric study:1.5,2.1,2.7,and 3.3 m(5,7,9,and 11 ft).Except for the 3.3-m(11-ft)high ribs consisting of a single-unit,three levels of uniaxial compressive strength for the rib units were considered:8.57 MPa (1244 psi) for bright coal,19.69 MPa (2857 psi) for banded bright coal,and 34.99 MPa (5075 psi) for dull coal.For the 3.3-m (11-ft) high ribs consisting of a single-unit,13 levels of intact uniaxial compressive strength were considered:8.57 MPa(1244 psi) for bright coal,11.39,14.09,15.19,16.89,17.69,and 19.69 MPa (1653,2045,2204,2451,2567,2857 psi) for banded bright coal,and 21.79,23.49,27.39,28.39,31.19,and 34.99 MPa(3161,3408,3973,4118,4524,5075 psi) for dull coal.Two levels of bedding conditions,clay-free and soft-clay beddings,were considered for ribs consisting of two units.Three levels of bedding position were considered at 1/4-height,mid-height,and 3/4-height of the rib.To ensure a good resolution for calculating the CPRR,six levels of overburden depth were considered:91.4,137.2,182.9,228.6,274.3,and 320.0 m (300,450,600,750,900,and 1050 ft).The face cleat orientation was assumed to be parallel to the entry orientation in the CPRR calculations.The effect of face cleat orientation with respect to entry orientation on the CPRR was determined empirically.The roof and floor were assumed to be elastic in all models.

A single-page calculation sheet for the calculations of the CPRR of solid coal ribs was developed(see Fig.8)based on the proposed parametric study shown in Fig.7.The top portion of the CPRR calculation sheet contains the basic rib information such as mine name,site identification,and description of the rib lithotypes(coal brightness,thickness,and uniaxial compressive strength).The uniaxial compressive strength of rib units can be measured by a direct method or estimated from indirect methods as explained previously.The weighted average compressive strength and the total rib height are recorded in the top portion of the CPRR calculation sheet.The rib homogeneity index is calculated and recorded.The orientation of the face cleat with respect to entry direction and bedding condition(no-bedding,bedding with no-clay,and bedding filled with soft-clay)are also recorded.The calculations of the CPRR are conducted in two steps as shown in Section 4.1 and 4.2,respectively.

4.1.Step 1:Calculate the basic coal pillar rib rating (CPRRBasic)

The basic coal pillar rib rating(CPRRBasic)for a solid rib is calculated by adding the rib homogeneity (αHomo) and bedding condition (αbedding) ratings,these two parameters reflect the actual geological condition of the rib (see Eq.(3)).The CPRRBasicis calculated assuming the rib height is 3.3-m (11-ft) height and the face cleat orientation is parallel to the entry direction.An adjustment for the rib height and the cleat orientation is conducted to reflect the actual height and cleat orientation.

The rib homogeneity rating is calculated using Eqs.(4) and (5).

where σcis the weighted average compressive strength;σBCthe uniaxial compressive strength of the bright coal lithotype;r the rib homogeneity index;and σk,σk+1,and σk-1the uniaxial compressive strengths of weakest coal unit (k),coal unit (k+1),and unit(k-1)in the rib,respectively.The minimum rib homogeneity index(r) is 0.25.

Fig.7.Flow chart for parametric study of calculating CPRR for solid coal ribs.

Fig.8.CPRR calculation sheet and adjustment factors for solid coal ribs.

The homogeneity rating for solid coal ribs ranges between 0 and 45 (Part-A,Fig.8).Despite the coal lithotype,heterogenous ribs(r ≤0.25) have a zero-rib homogeneity rating.The highest rib homogeneity rating of 45 is assigned for homogeneous coal ribs(r=1) dominated by strong dull coal units.

There are three bedding conditions that could be encountered in solid coal ribs:(1) no bedding,(2) clay-free bedding,and (3)bedding filled with soft-clay.The rating of bedding condition is calculated using Eq.(6).

The rating of the bedding conditions in solid coal ribs ranges between 4 and 46(Part-A,Fig.8).The minimum rating of bedding condition was assigned for ribs dominated by weak bright coal and contain bedding(s) filled with soft-clay.The maximum rating of bedding condition was assigned for single-unit rib of strong dull coal.If the rib contains more than one type of bedding condition,then the rating of bedding condition will be assessed based on the minimum rating of those types.

4.2.Step 2:Calculate the CPRR adjustments

The basic coal pillar rib rating is adjusted for cleat angle and rib height as follows:

(1) Adjustment for face cleat angle (αcleat)—documented research and field observations concluded that face cleat adjustment is only applicable for face cleat angles equal to or greater than 20° with respect to entry direction [14].Similar to the rock mass rating [15],a maximum adjustment of 7 was assigned to the face cleat orientation in ribs dominated by strong dull coal units.On the other hand,no adjustment was assigned for ribs dominated by weak bright coal.The CPRR adjustment for cleat orientation is calculated as follows.

(2) Adjustment for rib height (αH)—rib height adjustment for solid ribs of less than 2.7 m (9 ft) is calculated using Eq.(8).

where H is the rib height in m.

The CPRR of shorter ribs (less than 2.7 m (9 ft)) was adjusted depending on the rib homogeneity,bedding condition,and the calculated weighted average uniaxial compressive strength.The adjustment of the CPRR for the rib height in solid coal ribs ranges between 0 and 12 (Part-B,Fig.8).A maximum adjustment of 12 was assigned for 1.5-m (5-ft) high heterogenous ribs,while an adjustment of 4 was assigned for the same rib height of homogeneous ribs with a weighted average uniaxial strength of less than or equal to 17.24 MPa (2500 psi).

Finally,the coal pillar rib rating for solid coal ribs (CPRR) is obtained by the summation of the adjustment factors to the basic CPRRBasicas follows.

The validity of the simplified CPRR calculation method was proven by testing it against the complex numerical modeling approach.Fig.9 shows the CPRR calculated by the simplified method in comparison with the CPRRModelcalculated by numerical modeling.A good correlation exists between the simplified method and the numerical modeling method,where the R2is 0.90 (Fig.9).Therefore,the simplified method can substitute the complicated numerical modeling approach for calculating the CPRR of solid coal ribs.

5.Classification of expected rib performance for solid coal ribs

A regression model for correlating the RibFOS with CPRR and the overburden depth was developed(Eq.(10))using the results of the numerical model parametric studies.

where dois the overburden depth in m.

The R2of regression of the RibFOS model is 0.963.

Fig.10 shows the graphical representation of the RibFOS model.The RibFOS model was extrapolated to the depth of cover of 457 m(1500 ft),beyond the range of the parametric study(320 m or 1050 ft),to make it applicable to the surveyed rib cases.

CPRR calculations were conducted for 22 surveyed solid coal ribs using the CPRR calculation sheet.The surveyed ribs were collected from room-and-pillar and longwall mines.All surveyed ribs were subjected to the development conditions with no multipleseam interactions.The 22 cases were classified into four categories based upon in-mine observation and data collection:(1) unsupported ribs with spalling,(2) unsupported ribs without spalling,(3) supported ribs with spalling,and (4) supported ribs without spalling.Field observations have shown that the unsupported ribs without spalling have the best rib performance as described in Fig.10.

Four performance categories of solid ribs were identified as follows:

(1) Category I (RibFOS ≤0.90)—All cases in this category are identified as supported ribs with spalling.All cases recorded in this category have very low CPRR (less than 30) and the majority have high overburden depth (457 m or 1500 ft).Despite rib support applied in this category,the ribs showed some level of spalling.

Fig.9.CPRR calculated by simplified method in comparison with CPRR calculated by numerical modeling.

Fig.10.Classification of solid coal ribs based on CPRR and overburden depth.Boundaries between rib classes are defined by the rib factor of safety (RibFOS).

(2) Category II(0.90 ≤RibFOS less than 1.50)—Most of the cases in this category are identified as supported ribs without spalling.The decision of rib support in these cases was adequate to eliminate rib spalling,except for two cases that showed rib spalling despite rib support.

(3) Category III (1.50 ≤RibFOS less than 4.50)—All cases in this category are identified as unsupported ribs with spalling.

(4) Category IV(RibFOS ≥4.50)—The two cases in this category are identified as unsupported ribs without spalling and with shallow depths.

6.Applying RibFOS to assess expected rib support requirements

Rib support requirements can be calculated empirically by establishing a relationship between the RibFOS of surveyed ribs and the applied rib support densities in these surveyed cases.Table 1 summarizes the bolt specifications of the rib support cases.Mechanical bolts and fully grouted resin bolts were used in these cases.The anchor capacity of mechanical bolts was measured in the filed using pull out tests [16].The primary rib support density index (PRSD) in ton/m or ton/ft was calculated using Eq.(11).

where Cap is the anchorage capacity in tons;L the length of the bolt in m(ft);N the number of bolts per vertical rib bolt row;and S the bolt spacing in m (ft).

Fig.11 shows the relationship between the PRSD and RibFOS for all surveyed cases (supported and unsupported).The unsupported cases have zero PRSD and no rib support was required in the surveyed cases for solid coal ribs of RibFOS greater than 1.5 (rib categories III and IV).The scatter of the calculated PRSD among the surveyed cases is attributed to the lack of standard rib support requirements.All the adopted rib support designs of the surveyed cases were assumed successful even for cases with rib spalls because it was not necessary for the coal operator to change/modify the rib support plans.A preliminary minimum PRSD line is proposed in Fig.11 by fitting all cases with minimum PRSD.Additional mine observations and data collection would enhance and refine the understanding of the rib performance categories and rib support design line.

Fig.11.Calculated primary rib support density (PRSD) versus the RibFOS of surveyed ribs showing preliminary design curves.

7.Sample cases for the application of the coal pillar rib rating

Two sample cases in a room-and-pillar mine were used to demonstrate the applicability of the proposed rib data collection procedure and the calculations of the CPRR.The mined coal is 1.39-1.52-m (55-60-in) thick at depths ranging from 76 to 106 m (250 to 350 ft).For a few cases,the coal seam could be as high as 3.0-m (10-ft) thick.The coal seam is immediately overlain by black shale with intermittent thin layers of sandy streaks.The immediate floor consists of approximately 0.7 m (2.5 ft) of claystone.The selected sample cases in the surveyed mine were at an overburden depth of 91 m (300 ft).

Sample case 1 (site ID M1-S4) was for a 3.0-m (10-ft) high rib.Three coal units were identified in this rib.The top unit is defined as banded bright coal,and the middle and bottom units are bright coal separated by a clay band.Two types of rib bolts were used at this site:a Hilti screw anchor type and fully grouted 0.6-to 0.9-m(2-to 3-ft)long,#5,Grade 55 rebar.All rib bolts were anchored in coal and have 0.43-m (17-in) square pizza pans incorporated for additional skin control.The data collection sheet for sample case 1 is illustrated in Figs.7,9-11.The calculation of the CPRR for sample case 1 is provided in Fig.12.The calculated CPRR of sample case 1 is 9.Despite the shallow overburden depth at sample case 1,it was located at the border between Categories I and II (see Fig.11).The rib classification agrees with the observed rib deterioration index of 3 in Fig.11.

Sample case 2 (site ID M1-S1) was for a 1.8-m (6-ft) high rib.The rib is composed of a single banded bright unit.Rib bolting was not in use at sample case 2 and no rib sloughing was observed.Therefore,a rib deterioration index of 1 was selected for sample case 2.The calculation of the CPRR for sample case 2 was illustrated in Fig.13.The calculated CPRR for sample case 2 is 52.This corresponds to a rib category III which agrees with the observed rib deterioration index.

Fig.12.CPRR calculation sheets for sample case 1.

8.Conclusions

This paper presents the development of a coal pillar rib rating(CPRR)for solid coal ribs.CPRR is calculated based on in-mine testing,rib surveys,and observations.It provides a useful and practical approach for classifying the performance of solid ribs.The parameters controlling the CPRR are rib homogeneity,bedding condition,face cleat orientation with respect to entry direction,and rib height.Given this new method of classification,mine operators and engineers will be better positioned to make engineeringbased decisions concerning coal mine rib control.

The performance of solid coal ribs based on the calculated CPRR and depth of cover was classified into four categories.A survey of solid coal ribs showed that Categories III and IV ribs do not require primary rib support.

Fig.13.CPRR calculation sheets for sample case 2.

Preliminary rib support design lines to estimate the primary rib support density for rib categories I and II were deduced from the surveyed cases.Additional mine observations and data collection would enhance and refine the understanding of the rib performance categories and rib support design lines.

Future research is proposed to expand the applicability of the CPRR for conditions where thick in-seam parting and solid rock brows are encountered.This will extend the potential for usage of the CPRR to most coal ribs.Application of this analysis technique,in combination with sound engineering-based rib control strategies,should reduce the potential for rib-fall-related injuries and fatalities.

Disclaimer

The findings and conclusions in this paper are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health,Centers for Disease Control and Prevention.Mention of any company or product does not constitute endorsement by NIOSH.