Baseline Approach

The baseline approach refers to the standardization of geological assessment and geotechnical analysis in Canadian underground coal mines. Geological assessment aims at interpreting geological structures, formations and their histories using established methods. Geotechnical assessment studies the engineering properties of roof strata, their effects on mine openings and interaction with ground supports.

Every mine is different in its geology, roof strata, rock properties, mining method etc. With the standardized method, the baseline approach can be applied to different conditions and is expected to yield results which are useful for mine design. In this approach, parameters, which are significant for mine design are recognized and given due considerations. In the following, the principles to be followed are given based on the work of Steve Forgeron and Christopher Mark and their recent reports.

Geological assessment is a continuous process throughout the mine life. Its goal is to characterize its rockmass by using appropriate techniques and to provide information for a safe and efficient mining practice.

The maximum possible information must be recovered from the exploratory drilling in consultation with mining and geotechnical staff at the earliest stage. Drilling should be followed by geophysical logs, geodip and directional logs. Seismic surveys require sonic logs. Local conditions may call for other types of logging. The details of the following parameters should be logged because of their importance in rockmass rating:

- Point load strength index of all intervals within 8 m (26.2 feet) of the immediate roof.

Laboratory testing should be conducted on rock samples representing the lithology.

The depositional environment and tectonic history of coal measure rocks must be investigated to

gain knowledge of its nature and extent and assess the impact of discontinuities on mining.

In a high-energy fluvial deposit, sharp contacts between lithological units are common and lateral change can be extreme. In bay-fill lacustrine deposits, coarsening of rockmass in upward sequence produce more gradual vertical change and less horizontal change.

To optimize the use of geological data, all information should be kept up to date on geological drawings. An operator should have the following drawings readily available:

(a) Cross sectional maps parallel to dip and strike of the deposit to show the interval from topographic surface to at least 50 m (164 feet) below the seam with major sedimentary units and faults.

· Rosette or Schmitt net diagrams indicating the strike and dip of major fault planes, joint systems and coal cleat,

(c) Isopach maps to show the interval between the seam roof and the base or top of any strong or weak layers in the roof and floor of the seam.

(d) Isopach maps to reveal thickness of strong and weak sedimentary units in roof and floor. A sandstone layer more than 5 m (16.4 feet) thick can adversely affect the roof cavability.

All underground workers should be trained in basic geology and ground control and should receive regular update on geological conditions and supports used. They can provide information, which is not readily available to technical staff and can react fast to any unpleasant event and take remedial measure.

Quantitative description of rockmass is an essential part of geological assessment. The detail is included in geotechnical assessment given below. Geotechnical assessment takes into consideration of all major factors: strata, geology, effects of excavation and ground supports. It gives more quantified engineering analysis for the purposes of mine design and ground stability control. In geotechnical assessment, there are 4 principles to follow:

The rockmass properties must be quantified using engineering classification systems to facilitate mine design. Two commonly used classification systems in underground coal mines are RMR (CSIR rockmass rating) and CMRR (Coal Mine Roof Rating).

RMR was initially developed for geotechnical engineering and has now been widely adopted in the mining industry. CMRR was specifically developed for coal mines and has been widely used in US to quantify the stratified roof strata, predict the roof stability, design the entries and determine the feasibility of extended cut. Such classification systems should be adopted to assist mine design.

Horizontal stress plays an important role in coal mine ground failure. Its magnitude and orientations relative to mine workings must be considered in mine design. The following factors contribute to the possible structural instability due to the in situ horizontal stress:

(a) Roof type - Weak roof is more susceptible to damage out of horizontal stress. With more lamination it becomes more incompetent to withstand the horizontal stress effect.

(b) Entry orientation - Entries, driven at 20° in the stress shadow (nearly parallel to the maximum horizontal stress), seem to be more stable than those driven perpendicular to the maximum horizontal stress.

(c) Longwall orientation -The in situ horizontal stress must bypass the gob area, ahead of the longwall resulting in stress concentration zones near the headgate and/or tailgate. Its location depends on the panel orientation, the direction of retreat and the sequence of longwall panel extraction.

(d) Special surface features - Topography variation can change the orientation of the maximum stress at shallow depth.

In longwall mining, the barrier pillar between adjacent panels is critical to the roof stability. For an optimal longwall pillar design, the Analysis of Longwall Pillar stability (ALPS) can be used. The Pillar Stability Factor (PSF), given by ALPS, should be utilized to design the primary support, the secondary support and the entry width.

Pillar-squeezing, -failure or -bump can jeopardize safety and productivity of a mine. Since site-specific characteristics can not be considered in a closed for solution, numerical modeling technique can be used. The Analysis of Retreat Mining Pillar Stability (ARMPS) is a modeling software to determine PSF of the retreating longwall face at a depth of less than 228 m (750 feet).

Continuous monitoring of the performance of the ground and the support system is an essential part of ground stability control in underground mining. The ground movement can be monitored using telltales and convergence poles. Detailed geological mapping of roadways and mining areas should be undertaken to establish and validate geological and geotechnical models. A well-planned monitoring program should be in place in an operating mine.

In this section, the concept of baseline approach is further explained with examples of application to the four Canadian coal mines.

A mine can only be designed based on available information. At the initial stage, information provided by exploration is very limited. As mine development continues, there is more and more exposure to the real ground condition. Any information of geological structures, rockmass etc. must be collected and analyzed. The geological mode must be updated and the mine design is reassessed. In general, the following information, (but not limited to the listed) needs to be updated periodically.

The continuous geological assessment played a significant role in Prince Colliery. It involved the following activities:

a. A series of 15.2 m (50 feet) long boreholes were drilled, 304.8 m (1000 feet) apart, up into the roof strata along the North levels to retrieve cores. This was aimed at assessing certain roof features, i.e. the Hub-river sandstone channel, the overlying Rider stream and rockmass ratings (RMR and CMRR).

b. The floor strata were investigated using several cored boreholes for a Rider Seam.

c. Horizontal holes, 570 ~ 762 m (1870.1 ~ 2500 feet) long, were drilled in the Northern reserve to look into specific geological targets: Sandstone channels, Rider Seam and other structures. Horizontal drilling at strategically planned interval together with geophysical logging (Natural gamma) proved to be an excellent tool for exploration beyond mine workings.

d. In seam reflection seismic survey was carried out, which located and tracked down the scour zone East of the #1 North panel.

e. Petrographic analysis was done on the Hub Seam coal, which defined the feasible limits of the property to the East and the North.

In Quinsam Coal, continuous geological assessment became fruitful to modify the initial design:

The Comox coalfield suffered from numerous faults. As part of the regular geological assessment, many small displacement faults were delineated in the immediate roof of the #1 Seam within the 2 North Mine. Entries of this mine was initially designed 6 m (19.7 feet) wide. When local faults were detected in the immediate roof, the entry width was locally reduced to 5.2m (17.1 feet).

Quinsam Coal had an exploration program. The last reported activities included the recovery in the coal seam and down-hole geophysical surveys over the Summer, 1996. In the 2 North Mine, there were 10 exploratory holes for a cumulative length of 1647 m (5403.5 feet) from the main #1 heading. It enhanced the proven reserve estimate by 0.8 million tonnes of coal. In the 4 South Mine, there were 6 exploratory holes drilled for a total length of 551 m (1809 feet). It confirmed the continuity of the #3 Seam and delineated faults in advance of driving the main heading.

In general, geological features vary at random in spatial distribution. Hence, maximum information must be retrieved from any exploratory drilling in order to better understand the true geology. In addition, to quantify the rockmass properties for the purpose of mine design, all major parameters as listed in 2) of section 8.1 must be available. These data can be retrieved from continuous exploratory drilling, core logging and field mapping of newly exposed areas. Detailed geological information is significant in terms of ground stability control, particularly in preventing structurally controlled ground failure and reducing the related ground damages. The actual means of information retrieval is site-dependent.

The geological history played an important role in the distribution of coal deposits and the character of their roofs. Awareness of the geological history will benefit mine design and ground support. It requires an extensive program to get this information. Field stress measurement is often needed to determine the in situ stress condition.

The retrieved information in Smoky River Coal indicated that this coalfield underwent sediment compression and shortening leaving a legacy of intense folding and thrusting of rockmass. The excess horizontal stress has possibly been consumed either in deforming or shed by shearing rockmass along the large thrusting sheets.

The retrieved information in Sydney Coalfield did not show any tectonic disturbance but gentle folding due to North-East South-West compression. The latter led to the unrelieved excess horizontal stresses, which are shown by in situ stress measurement.

When underground workers are trained in basic geology and ground control, their first hand information can be a reliable source for assessing the ground conditions and decision making. An operating mine should have a regular training program to update the ground conditions and support systems. Every personnel working underground should have such training.

To quantify the rockmass properties for the purpose of mine design and support, a number of methods are available. Engineering classification of rockmass is a practical and empirical method. RMR (CSIR rockmass rating) and CMRR are the most commonly used systems in coal mines. These are well documented with practical applications.

In Quinsam Coal, the immediate roof of the #1 Seam consists of 0.6 m (2 feet) grey shale and siltstone in the upward sequence. The immediate roof of the #3 Seam within the same mining property contains predominantly medium grained sandstone. The rockmass ratings were determined by the mine personnel as follows:

Classification system

#1 Seam

#3 Seam

RMR

18

69

Q

0.1

16.3

CMRR

24

78

The low rockmass ratings in the immediate roof of the #1 Seam indicate poor roof quality. The #3 Seam had a much stronger roof with much higher ratings. Difficulty was experienced in roof caving in the seam. To avoid this problem, the method was changed to partial extraction of coal. This resulted in the yield pillars left in situ in a checkerboard pattern. The less competent immediate roof over the #1 Seam required longer roof bolts than in the #3 Seam and needed occasional welded wire-meshes.

The 30 m (98.4 feet) roof strata over the Phalen Seam contained 3 stratigraphic Facies: Flood Plain Facies, Backswamp Facies and Channel Facies in addition to mudstone and siltstone. Sandstone occasionally replaced the siltstone when it became very thick. Rockmass ratings based on the RMR system indicated the unsupported standup time for each stratum as:

Formation

Unsecured stand-up time

tratigraphic Facies:

Less than an hour

Shale:

few minutes and

Sandstone:

1000 hours.

As can be seen, the sandstone could stand up unsupported for a long time while the other strata essentially required much more support.

In U.S. coal mines, CMRR is widely used as a roof rating system. One of its applications is to assess the feasibility of use of extended cut. The following data are empirical correlation of the immediate roof and the extended cut practice.

CMRR

Extended cut feasibility

/ 54

Successful

Between 37 and 55

varies

<= 37

Unsuccessful

The immediate roof of the Phalen Seam had an overall CMRR value = 35. From the U.S. experience, the practice of extended cut would not be feasible in Phalen Colliery. The immediate roof of the #3 Seam in Quinsam Coal has a CMRR value above 54. The U.S. experience shows the possibility of use of extended cut. In fact, the "two cut" sequence of extended cut is practiced selectively under the competent roof.

At depth less than 1000 m (3280.8 feet), the horizontal stress is much higher than the vertical stress. This is particularly evident in Canadian shield. High horizontal stress often causes difficulty in ground control in coal mines due to the laminated strata and the wide range of extraction in nearly 2-dimensions. To reduce this type of problem, a long drift should be oriented parallel to the maximum stress. Thus, it is important to understand the in situ stress field.

The in situ stress can be determined by field measurement either in 2-dimension or 3-dimension. A number of stress measurement methods are available and one should be chosen to meet the site-specific needs. Alternatively, field mapping and observation of ground failure can provide some in-sight to the in situ stress but these can not quantify it.

In Phalen Colliery, frequent core disking occurring in the thin, intercalated sandstone beds during drilling was an indication of high horizontal stress concentration. The latter caused severe "cutter roof" failure along one of the ribs. The in situ stress measurements revealed that the major principal horizontal stress was in the North-East South-West direction (at 240° azimuth according to Dr. S. Zou) and its magnitude was about twice the vertical stress. This matched the field observations.

In recognition of the regional stress field, the development of #1 North panel and subsequent panels in Prince Colliery has been reoriented to align the entries in the direction of the high horizontal stress.

- Development load, which is uniformly distributed over the pillar prior to retreat,

The peak abutment load is usually manifold higher than the development load and hence the former dictates the design.

The laboratory compressive strength is not adequate to estimate pillar strength in the field and needs some interpretations. Different formulae are used in different countries to determine the pillar strength. However, the pillar strength is not the same as pillar-load-bearing capacity. Even if the stress acting on a pillar in situ exceeds its calculated strength, the pillar may still be able to stand up during its post-failure stage. It is a safer practice to consider the calculated pillar strength for pillar design. The pillar strength formula developed by Bieniawski is used in ALPS.

PSF is the ratio of the pillar strength to the pillar load. The calculated value of PSF must be above unity and meet the pillar design requirements.

For a complex underground mine layout, numerical modeling is a useful tool for determining the stress distribution pattern and mine design. Using ARMPS, different features of a mine's layout can be modeled. It is capable of considering features like angled crosscuts, varied spacings between entries, the width of barrier pillar and the slab cut in the barrier pillar during retreat. It can consider 4 loading conditions in determining the pillar load. The pillar strength is also determined using the Mark-Bieniawski formula. The ratio of the pillar strength to the pillar load (or PSF) is eventually calculated.

Retreat longwall pillars in both Phalen Colliery and Prince Colliery were originally designed using the sea bed strain overlap between walls. The design has later been checked using ALPS and found adequate.

ARMPS has been validated using 140 cases from US coal mines. PSF from ARMPS varies from 0.75 to 1.5 for all the safe cases considered.

Field monitoring can take different forms from local spots using simple devices such as taletell to sophisticated mine-wide monitoring systems such as micro-seismic monitoring. Every mine should have a monitoring program in place to suit its needs.

In Phalen Colliery, the strong sandstone bed in the roof strata varies in its thickness. As a result, the easily cavable immediate roof becomes very thin in some areas. This affects the caving behavior of the roof strata in the gob. Telltale extensometers were used in entries to monitor the roof movement. Convergence and pressure measurement was undertaken in its shield supports to monitor the behavior of roof caving.

Formation	Unsecured stand-up time
tratigraphic Facies:	Less than an hour
Shale:	few minutes and
Sandstone:	1000 hours.

CMRR	Extended cut feasibility
/ 54	Successful
Between 37 and 55	varies
<= 37	Unsuccessful