Advanced Predicction Methods for Roadway Behaviour by Combining Numerical Simulation, Physical Modellyng and in-situ Monitoring

7/29/2019 Advanced Predicction Methods for Roadway Behaviour by Combining Numerical Simulation, Physical Modellyng an

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25th In t ernat ional Conference on Ground Cont ro l in Min ing

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ABSTRACT

For the roadway design in the German hard coal mining

industry, an advanced combined planning system has been appliedduring the past 20 years. The German mining industry is workingwith single entry roadways system. After first longwall passage theroadways will be stabilised with side wall building material

packages. Later this roadway will be used again for the second

retreating longwall.

The intention of the implementation of several different design

methods for the planning of the roadways is to enhance thereliability of the results. On one hand the planning system usesnumerical simulation and physical modelling in addition to

analytical and empirical methods. On the other hand in-situmonitoring and analysis during the roadway development will beused to verify and calibrate the numerical and physical models.

Especially before the implementation of new support elementsin underground, this system is suitable and reasonable because thenew support elements will be tested in the physical and numericalsimulation prior to cost intensive underground tests.

The interaction between the numerical and physical modellingin comparison with underground monitoring and analysis duringdevelopment and the later use of roadways will be shown with

some examples.

The first example shows a rock bolted rectangular roadwaywhich will be mined at both sides of the roadway. Different support

and roadway building material package systems are investigatedunder various stress conditions. The results are utilised to optimisethe roadway support and the roadway side package systems.

The influence of different points in time of the installation ofseveral support elements in a combined arch and bolted roadway isshown as another example. Two different types of support systems

are compared within the physical models. These models are used asa base for ongoing numerical analysis and simulation to find out theideal point of time to assemble the different support elements.

The last example handles the influence of slickensides in theroof of arch shaped roadway on their stability. Different slickenside

positions and densities are investigated and classified.

INTRODUCTION

The history of German hard coal mining goes back many

centuries into the past. The first exploitation near the surface wasmentioned in documents of the year 1300 after Christ. Since theexploitation went deeper and deeper. In the 16th century, adits weredeveloped. From the beginning of the 19th century hard coal has

been developed by shafts in underground mining (Figure 1). Today,

the deepest extractions are in the area of about 1,700 m (5,600 ft).The extraction of coal in several seam levels (multiple seammining) with different interburden thicknesses is typical for

German hard coal mining. The result is a high recovery of thereserves. The large depth and high recovery lead to high rockstresses in the area where mining is being conducted.

Figure 1. History of German hard coal mining from surface mining

to deep mining (at a depth of 1,700 m)

The seam bearing layers of the Upper Carbon arecomposed of

hard coal, sandy mudstone, siltstone and sandstone. Figure 2 showsa typical sequence from the Ruhr Carbon. Here, it is shown thatthe waste rock of the seams can be composed differently.

Depending on the lithology and the depth, the waste rocks havestrength between 20 and 200 MPa. In the roof and floor of theseam, weak siltstones or sandy mudstones occur often with strength

of 45 MPa on average. Due to the folding beginning with the UpperCarbon the rock mass is highly tectonically stressed. Today, this

Advanced Predict ion Methods for Roadway Behaviour by CombiningNumerical Simulation, Physical Modelling and In-Situ Monitoring

Andreas Hucke, Senior GeologistAxel Studeny, Geotechnical EngineerUlrich Ruppel, Senior Mining Engineer

Deutsche Montan Technologie GmbH, Rock MechanicsEssen, Germany

Holger Witthaus, Certificated Expert in Geomechanics and Support-SystemsHead of Competence Center for Roadway Support

Deutsche Steinkohle AGHerne, Germany


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becomes visible by a large number of faults, joints andslickensides.

Figure 2. Typical lithological strata sequence in German hard coalbearing carbon

CONDITIONS OF CURRENT MINING

Reserves

For the geological and geomechanical conditions of German

hard coal mining the following features of parameters exist:

High primary stresses. Additional stress by multiple seam mining.

Partially low strength of the rock mass. Large number of separation planes.

Figure 3. Convergence verse stress-strength-ratio

The rock stress is influenced by the ratio of stress to rockstrength. The resulting coefficient concerning the stress-strengthratio is defined by the quotient of the stress p in MPa and the

square root of the uniaxial compressive strength of the waste rock.

Through underground observations, rock stress analysis and asystematic rock evaluation, a systematic relationship between the

rock stress and the roadway convergence could be determinedempirically for the German hard coal mining. Figure 3 shows therelationship between the stress and convergence after roadwaydevelopment with yielding arch support for a waste rock strength of

45 MPa. The roadway has only minor deformations up to a stress-strength ratio of 3 (this corresponds to a depth of 800 m). If thisvalue is exceeded, the convergence of the roadway increases

significantly.

pH = 1 x pV

P H=2 - 3.5 X P V

260m

pH = 25 MPa

pV

= 25 MPa

1000m

pH = 13 MPa

pV = 6,5 MPa

p H = 2 x pV

Figure 4. Rock stress in Germany and Australia

The parameter stress-strength ratio is useful for comparingdifferent reserve conditions. Figure 4 shows a comparison between

the primary stress situation of a typical German and a typicalAustralian mining situation. Although in Australian mines a higherratio of horizontal to vertical stress occurs, the absolute values areclearly below the stresses in German hard coal mines. By

comparing the rock stresses (Table 1), a larger stress occurs inGerman hard coal mines, compared to those, in particular, inAustralia and, the USA.

Table 1 Comparison of vertical and horizontal stress-strength ratio

Stress-strength

ratio

Germany UK Australia USA

Vertical

pV/

D

25/45 =

3,7

15/45 =

2,2

6,5/40 =

1

9/40

= 1,4HorizontalpH/D

25/45 =3,7

22,5/45 =3,4

13/40 =2,1

18/40= 2,8

Due to larger depths and additional mining-induced stresses, thestress-strength ratio in German hard coal mining is nowadays often

higher than the values stated in Table 1. This results in highroadway deformations which have to be controlled by high-capacity support systems.

Stress-strength ratio

Depth 0 400 800 1200 1600 2000 mVertical rock stres s 0 10 20 30 40 50 Mpa

Rock stres s 0 1,5 3 4,5 6 7,5 p/D

headingconvergence[%]

1

2

3

4

5

low mean high

D = 45 MPa

50

40

30

20

10

stablescaling,brittle

heavy stressed/squeezed


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Roadway Usage

Additional parameters influencing the roadway behaviour andthe requirements for thesupport systems are:

Roadway orientation and usage. Roadway cross section and size.

Even here there are obvious differences between the German

and the international hard coal mining. The complex multiple seammining in large depths in Germany requires the repeated use of

roadways after the first passage of the face.Partially the roadwayshave to be prepared for a second use.

The reasons are:

High rock temperature and gas content of the seams requiresventilation behind the face.

Obligatory prevention of stable not mined pillarsbecauseadditional stress as a result of multiple seam mining leads tohigher rock stresses. Additionally, an irregular subsidence

leads to higher mining damages on the surface.

Figure 5 shows the different stages of a double-used roadway.

Figure 5. Different stages of roadway usage

In German mining, roadways have normally an arch-shaped or arectangular cross section. Due to coal seam thickness an overcut orundercut is required. Technical requirements demand an openingwidth at the face of at least 5 to 5.50 m. This results in roadway

widths for arch-shaped roadways of 7.5 m after development.

Compared to these details, the U.S. mining system uses

rectangularroof bolted roadwaysof approx. 4.5 to 6 m wide, whichare mostly developed in-seam with stable or yielding pillars byutilising a 2 to 4 roadway system. The roadways are only used up

to the first passage of a face. Large additional mining-inducedstresses by multiple seam mining and subsidence of roof strata

behind the face are not normally present in U.S. mining system.

Support Systems

The factors stated above lead to a high load of roadways inGerman hard coal mining. This results in high roadway

deformations. Due to concerns over the safety of equipment andventilation it is necessary to limit these deformations. ThereforeDSK uses different support systems today:

Roof bolting in rectangular roadways. Roof bolting in archshaped roadways.

Yielding arch support with concrete backfill in archshapedroadways.

Combined support i.e. bolt and arch support with concretebackfill in archshaped roadways.

Figure 6 shows the support systems used in hard coal miningdepending on the stress-strength ratio and use of roadways. It isobvious that a major part of roadways has to be kept in use afterfirst face passage (OA, TR). It is also obvious that these roadways

are in the area of medium or high stress-strength ratio most of thetime. Therefore, the support systems used have to be at a high

standard. For this reason, combined support systems with roofbolting and backfilled yielding arch support or standing support areoften used. According to the roadway deformation, further safetymeasures such as installation of additional roof bolts may be

necessary.

Typeof use

Stress-strength-ratio< 3.0

low< 4.5medium

> 4.5highA B C

D

1

2

3

4

OR

OA

TR

Typeof use < 3.0

low< 4.5medium

> 4.5highA B C

D

1

2

3

4

OR

OA

TR

Boltsupport

Combined

support

FF

Figure 6. Support systems used in hard coal mining depending onthe stress-strength ratio and the use of roadways. BF =with backfilling; D = Development; OR = One-sided

retreat; OA = One-sided advance; TR = Two-sided retreatworking

Compared to this fact, roadways of the U.S. mining system are

normally in the area of minor or medium rock stress and are notused after first face passage. Therefore, only roof bolting is often

sufficient for roadway control.

ROADWAY PL ANNING

Due to the conditions for German hard coal mining described

above, it is necessary to follow a strict roadway planning system. Astandard planning procedure has been used since 2004 in DSK.

Aft er developm ent

Behind the 1st face Intersection with 2nd face

Intersection with 1st face

BF BF

BF


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Special tools are used for the support dimensioning. These toolsallow a technical planning, regarding the available information andthe known rock mechanics process. Planning in terms of rockmechanics begins long time before roadway development and

depends on exterior influences and requirements of mining

operation, rock management as well as ventilation technology andair conditioning.

Special planning tools have proved to be useful concerning thesolution for the planned tasks. These tools are described in thefollowing.

Basically, there is a difference between methods based onanalysis of underground measurements (empirical) or numerical

simulations and physical models as a method to determine theroadway and support behaviour during development and use of aroadway. Concerning open questions and safety of assumptionsconcerning planning, measurement methods are used which allow

precision of rock mechanic planning by means of an iterativecheck. New geological and geomechanical knowledge during

roadway development are included in this planning.

The technology of measurement is highly relevant with regardto these facts. Measurements can detect critical states by measuringdeformations. A control of important planning parameters is

possible. Figure 7 shows the applied methods schematically.

In the following, the numerical and the physical modelling as apossible part of the planning process will be described. Numericand physical models are used for planning when evaluating newsituations (e.g., larger depths, double roadway systems), new

support systems or to analyse the risks by variations of different

rock mechanic parameters. The combination of these modellingtechniques is to calibrate numerical models by use of physicalmodels. These calibrated models are used for additional variationcalculations. Vice versa numeric simulations provide boundary

parameters for physical models. By means of underground

measurements reliable models can be created, providing importantinformation for the roadway planning process, in particular, apartfrom standard planning. In the following both model techniques are

described. After that some examples will follow.

Numerical Modelling

DMT uses mainly the software FLAC provided by ITASCA.FLAC calculates iteratively the mechanical interaction between

stresses and deformations with the result of a stable or unstablefinal state. The algorithm for calculations is as follows: Aftercreating a model, grid generation and implementation of thegeomechanical parameters, the model is exposed to a stress

situation leading to deformations in the model and ends depending on the situation - in a state of equilibrium or does not

lead to a reduction of movements and, therefore, to a stable finalstate. By including several material laws concerning rock mass,

separation plane and support modelling, the program is suitable forsimulating in-situ situations with large deformations. This fact has

been proved by successful roadway planning.

FLAC is used 2- or 3-dimensionally, depending on the

underground situation. The 2-dimension model is especially forroadway analysis. For analysis of intersections, e.g., face-to-roadway transition, a 3-dimensional model is used.

Dimensioningof the support

Mine layout

Model techniques

.

K= -32+11*p/

Realization

RockClassification

Supportand

material testing

Geological and tectonicalcore analysis

Numerical stress forecast

Geomechanical test

Numerical

Physical

Empiricalequation

Figure 7. Schematic representation of applied methods for supports design in German hard coal mining


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Through many years of experience and calibration, modules forsimulation of different states of roadway use are developed by

DMT. These modules allow a simulation of these states very closeto reality. They include:

Calibrated rock mechanics parameters for typical strata ofGerman coal seams.

Calibrated rock mechanics parameters for modelling differenttypes of separation planes (e. g. slickenside, polished

slickenside).

Simulation of different support elements, e.g. bolts, yieldingarch support, roadside packages and their connection to therock.

Different states of use.

Figure 8. Shows a 2- and a 3dimensional model with differentgeomechanical and support components.

Figure 8. Numerical Modelling: 3D- and 2D-Model with differentgeomechanical and support components

The results of numerical calculations are such as roadwaydeformations, fractured zones in the rock, support load and stress

distributions.

The advantages of numerical modelling are:

Investigation of complex geotechnical systems is possible. Realisation of variation calculation with relative low

expenditure.

Calculation of small sized and large sized stressredistributions.

Detail analysis (e.g. observation of the bolt load during thecalculation).

Physical Modelling

Physical modelling is a classical instrument of rock mechanicaland support analysis for planning in German hard coal mining. For

several decades, model tests have been carried out in German hardcoal mining. In the first model tests, special photoelastic materialshave been exposed in polarised light and electrical analogue models

for the simulation of stress distributions. Due to these exposedmaterials the realistic process of fracturing could not be simulated.After development of physical models, constructed layer by layerand made of a material corresponding to the in-situ dimensions it

became possible to simulate realistic fractures.

Several test rigs for physical modelling have been developed atDMT during the last decades to analyse complete or parts of the

systems concerning rock mechanics and support behaviour. Thesetest rigs are as follows:

3-dimensional test rig Roadway test rig (Figure 9) Face test rig Part system test rig

Figure 9. Physical modelling: roadway test rig

The theory of similarity is the basis of physical modelling. Thismeans that all parameters being relevant for a test have been

reduced to a prescribed scale (Figure 10). Equivalent material hasto be found with the scaled parameters (e.g. strength, elastic). Thismaterial will then be used in the model. Nowadays roadway test rig

and part system test rig are used primarily. Similar to numericalmodelling, complex geotechnical systems including the rock massand the support elements are reproduced in the roadway test rig

(roadway during different stages of usage, cross section in Figure

5). The load is applied by props to simulate different stresssituations. The results are deformation and failure processes at thesupport, fracture processes in the rock, and resulting roadway

deformations.

Figure 10. Physical modelling: scaled support elements

In the part system test rig, small-scaled models are reproducedrepresenting parts of the roadway (e.g. roof of a roadway), enabling

Replication of bolts

Lagging

Bolts

Lagging reinforcement

Numerical FD-Modell: FLAC3D

Numerical FD-Modell: FLAC2D


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detailed analysis to be carried out. The results are primarily thedetection of fracture zones.

The advantages of physical modelling are:

Clearness. Realistic failure structures. Every state of roadway usage can be simulated.

Stopping of the test and changing of the conditions is possibleat any time. Immediate damage analysis, visible damages can be

interpreted.

Dissection of the model.

EXAMPLES

Three different actual case examples are presented in thefollowing. They provide a survey of the different fields ofapplication of numerical and physical modelling. The first example

is about planning a rectangular roadway; one with a new supportsystem and the other hand with a new roadside pack system. Thesecond example treats the influence of difference in building acombined support system with roof bolts and backfilled yielding

arch support. The third example investigates the effect ofslickensides parallel to the layers in the roof of an arch-shapedroadway.

First Example: double usage of a rectangular roadway.

In a roadway supported only by roof bolting, it had to be testedif failures in the roof are controllable for every stage of usage of the

roadway by the newly developed support systems. Figure 11 showsthe roadway after development. A fracture can be clearly seen on

the side walls. The coal moves some distance into the roadway. Ifthere is additional stress in front of the face, roof buckling occursnear the left side wall. This could be observed in-situ later on in

parts of the roadway (Figure 12).

Figure 11. Roadway after development. black arrows: fractures;

white arrows: movement on slickenside

By means of numerical models different geological conditionssuch as strength of the roof or weakening zones in the roof have

been varied to analyse the possible causes for the buckled roof.This is helpful to prepare measures for changing geologicalsituations. In this case, it became clear that slickensides in the roof

cause such a folding in particular (Figure 12).

The used support systems (Adjustable Dual Roof Truss) can

prevent rock from breaking into the roadway during all stages ofuse. After the first face passage the horizontal stresses werereduced so that the buckling did not propagate more.

Figure 12. Influence of slickensides in the roof on the roadwaybehaviour - in-situ, physical model and numericalmodel

Secondly, the physical and numeric modelling helped to develop

a new system of roadside package. The rock mechanics suitabilityof this system was investigated before developing the hardware.This new method was a roadside package constructed by textilecoated concrete pillars (Figure 13). Tests proved that this method is

suitable for practical application, which confirmed the results of themodelling.

Figure 13 The newly developed roadside package system. Left:

after construction; Right: with shear fractures (blackarrows) after subsidence of the roof

For quality assurance different numerical situations were carried

out to analyse the effect of pillar parameters, including strength,elasticity, and residual strength, and their effect on the stability ofroadway and the expected cross section after deformation. For

example, it became visible that solid pillars lead to new fractures inthe floor and roof (Figure 14). For this reason, the strength of theconcrete has to be limited.

Left sidewall Right sidewall


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Figure 14 . Influence of the strength of roadside packs on roadway

behaviour

Second example: arch shaped roadway with different backfill

properties

Minor difference in executing the support can lead to extreme

effects in a mining operation in case of a large rock stress.Numerical and physical models proved this. Physical tests wereperformed to compare a model with an optimal support system to

one with a worse system. Principally, both models used the samesupport system, (i.e., combined support type A). However, oneapplied optimally and the other applied with delay and poor

backfill quality. Figure 15 shows the situation after first facepassage. An explicit higher roadway and support deformation

concerning the badly realised support can be verified. Thisdeteriorates extremely the stability of the roadway at the 2nd face

passage. Also a breaking of the roadside package after face passagecan be seen in this case.

Figure 15. Roadway with high quality (left) and low quality (right)support after first face passage

Numerical models analysed the influence of the deviation of

individual parameters on the support behaviour. Here, for the notcompletely backfilled roof and for a reduced backfill strength(Figure 16), an explicit larger load on the roadway support withrock bolt breaking and weakening of backfill is visible.

Figure 16. Roadway with different support quality after heading

(left: good quality; centre: not completely backfilled roof(with detail); right: lower backfill strength)

These explanations emphasise the importance to realise thesupport optimally. It also illustrates the reasons for unexpecteddeformations of roadways and support which were observed in-situ.

Third example: influence of slickensides parallel to the layer on theroadway behaviour

In the Carbon, weakening planes parallel to the layers occur asslickensides or polished slickensides. Due to low friction on theseseparation planes higher movements can take place. In case of theoccurrence of slickensides in the roof they are often cut by the

roadway cross section or situated in the roof.

In physical models the influence of slickensides in the roof onthe deformation behaviour of a roadway supported with combined

support Type B was investigated. Figure 17 shows a model of aroadway with slickensides compared with a roadway withoutslickensides in the roof after development. It is visible that the

roadway without slickensides does not show any fractures in theroof.

Figure 17. Arch-shaped roadway without slickensides (left) and

with slickensides (right) in the roof after development


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slickenside

On the contrary the model with slickensides in the roof showsan explicit loosening especially in the roof, in form of movements

on the slickensides and fractures in the rock. This leads failures ofthe roof bolts as well as to damages of the backfill, resulting inlarger roadway deformations and less stable roadway in thefollowing stages of usage.

To analyse the influence of a single slickenside in the roof,numerical calculations were carried out. The distance of the

slickenside to the roof was varied to determine the criticaldistances. Figure 18 shows 2 different situations with a slickensidedistance of approximately 1 m on the left and 2.50 m on the right. Itis visible that at a distance of 2.50 m, the slickenside was notactivated and did not lead to additional strain in the roof. The

reverse was true for the other slickenside with a smaller distance.Here, there are visible movements of the slickenside, which resultsin roof bolt failure, roof buckling, and deformations of the

backfilled yielding arch support. The result is clearly a less stable

roadway in subsequent stages of usage.

Figure 18. Arch-shaped roadway with 2 different distances ofslickensides in the roof (left: approx. 1m; right: approx.

2.50 m)

The conclusion of this analysis for the planning roadways is thatgeotechnical rock mass analysis should be carried out before

planning to detect the occurrence and position of weakening planesin the roof. With this information suitable measures for increasing

the roadway stability can be realised (e.g. additional roof bolting)in the case of the occurrence of slickensides within the criticalranges.

SUMMARY

Due to demanding reserve conditions in German hard coalmining a combined system for roadway planning is used. Here

empirical, static, numerical and physical methods are applied. Inaddition a recalibration of the planning by undergroundmeasurements during roadway development is carried out.

The benefits by combination of the different modellingtechniques, empirical investigations and underground

measurements are:

Decrease of specific uncertainties of the different modellingtechniques.

Enhancement of the reliability of the results of the differentmodelling methods due to recalibration of models by means of

underground measurements.

Failures, which could not be detected with classicalmethods, can be discovered.

Redundancy results in better projections and higher planningreliability.

Newly developed support elements can be verified beforeapplication in underground roadways.

Increasing of planning reliability for novel undergroundprojects.

REFERENCES

Griesenbrock, Hucke, Studeny & Witthaus (2002): Numerical and

Physical Modelling as Planning Tools for Rockbolted Roadways. -

Proceedings 21st Int. Conference on Ground Control in Mining.

Opolony, Witthaus, Hucke & Studeny (2004): Ergebnisse vonnumerischen Berechnungen und physikalischen Modellversuchen

als Planungshilfe fr eine Rechteckankerstrecke in Flz D2/C. -Ankerausbau im Bergbau. Roofbolting in Mining. AachenInternational Mining Symposium; Band 3.

Ruppel, Opolony (2000): Einsatz von Ankerausbau inHochleistungsstrebbetrieben im internationalen Vergleich Glckauf 136 (2000) Nr. 9, S. 508-514.

Documents

Advanced Predicction Methods for Roadway Behaviour by Combining Numerical Simulation, Physical Modellyng and in-situ Monitoring