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THE USE OF THE SPECIFIC DRILLING ENERGY FOR ROCK MASS
CHARACTERISATION AND TBM DRIVING DURING
TUNNEL CONSTRUCTION
B. Celada1, J. M. Galera2, C. Muoz3, I. Tardguila3
1Universidad Politcnica de Madrid (Spain); [email protected]
2Geocontrol Chile S.A. (Chile); [email protected]
3Geocontrol SA. (Spain); [email protected], [email protected]
Keywords: Geotechnical Site Characterization, Specific Energy, TBM.
INTRODUCTION
Recording drilling parameters is a useful and economical technique for acquiring geomechanical
information of rock mass parameters. Although it is not usually emplyed at geotechnical boreholes
it is systematically used in TBMs.
Drilling equipments at drill rigs and TBMs drilling monitoring devices provides systematically
records of the main drilling parameters (thrust, torque,) from which the specific energy (Se) can
be easily derived and expressed in terms of the energy necessary to drill a determined volume of
rock (GJ/m3).
The values of specific energy can be correlated with the main quality indexes as RMR. Also
laboratory test have been carried out measuring UCS, Vp an Young Modulus, and have beencompared with the specific energy.
This paper shows the obtained correlations using the data coming from a 80 m long pilot borehole
as well as from the data recorded while the excavation of the Guadarrama tunnel that have a length
of 28.3 km.
DRILLING SPECIFIC ENERGY
The specific energy (Se) can be defined as of the energy necessary to drill a determined volume of
rock (GJ/m3).
Several approaches can be found in the literature but the most accurate and used one is due to Teale
(1965) coming from the oil industry and derived from the main parameters that are involved in
drilling a rock mass.Parameters that appear to govern the drilling process may be grouped as follows:
Parameters related to the equipment such as drilling machine, rod or bit.
Parameters related to the drilling process: the weight on bit, rotary speed, drilling fluidproperties and circulation velocity. These are the three main elements on which the
driller can intervene within the limits of possibilities of the equipment.
Parameters related to the ground response: rate of penetration, rotation torque, drillingfluid pressure, reflected vibration through the drilling rods. For given drilling conditions,
these parameters depend on the characteristics of the ground.
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The eight parameters usually recorded by the main digital recorders are:a. Drilling fluid pressure (Pf).b. Rotation torque applied to the string of rods by the head (T).c. Thrust applied to the drilling bit (F).
d. Drilling speed (V).e. Rotation speed (N).f. Retention force (hold-back) (Fr).g. Reflected vibration.h. Drilling time for 5 mm penetration (t).
Drilling data varies with drilling equipment and the way it is used, so it is necessary to standardize
the testing procedure. While the drilling process is taking place, a relatively constant drilling fluid
pressure, rotation speed and thrust on the bit must be provided in order to obtain consistent data.
When drilling parameters are maintained constant, study of rate of penetration allows the detection
of changes in lithology and in the rock compactness or the presence of an anomaly such as a cavity
or a fracture. It is closely related to the `hardness of the strata being drilled, therefore, this
parameter is very important and needs to be recorded and interpreted carefully in order to get all
significant lithological information.
A relatively constant flow rate (fluid pressure) must be provided to the borehole by a water pump.
Ideally, pressure would be measured at the bit. However, because of the impossibility of placing a
transducer near the nozzle, the pressure is measured adjacent to the pump at the ground surface.
Thrust on the bit is the main parameter that affects the drilling speed; for a given soil formation, the
drilling speed is roughly proportional to the down-thrust.
For this reason it is recommended to keep down-thrust as constant as possible during the drilling
process in order to obtain information directly from the drilling speed.
Rotation speed is measured by an electromagnetic proximity sensor. It is usually chosen to suit the
drilling conditions. A constant and not very high rotation speed is preferred because higher rates ofpenetration could mask certain lithological variation that can be reflected by the torque parameter.
Torque is applied and measured in the drilling rod and transmitted to the drilling bit. It should vary
nearly instantaneously with rock condition; therefore, torque should be recorded continuously.
Hold-back pressure is necessary to prevent the drilling rod from penetrating too fast in soft ground
and to prevent the equipment falling into a hole when a cavity is encountered.
The hold-back pressure has to be subtracted from the down-thrust, in order to obtain the effective
net weight on the bit.
Variations in drilling parameters are related to the ground properties. In a given type of soil or rock,
the variations of one of the recorded parameters are predominant. However, though this is of great
help in the interpretation, it may happen that two different soils have the same dominant parameter.
For this reason, it is absolutely necessary to do an initial calibration with the execution of at leastone logged destructive borehole near to a cored one, and then compare the parameter values with
the lithology obtained in the cored holes. In the absence of the calibration cored borehole it will be
more difficult to define the nature of the formation.
Under particularly favourable conditions, it is possible to do a satisfactory soil description with a
precision of less than 0.10 m on the depth or thickness of a layer.
This is the first level of interpretation, which is possible from both analogue and digital recorders.
However, the main interest for numerical data is that it can be used in computer operations and for
combined parameters which are purely empirical or may have a physical meaning.
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Several combination of drilling parameters have been used. The most known ones are:
1. Alteration index (Pfister, 1985)A = 1 + (W / Wmax) (V / Vmax)
W = weight on the bit (thrust retention force + weights of rods and bit) (kN).
Wmax = it is the theoretical maximum value of W (kN).V = it is the instantaneous penetration rate (with maximum value Vmax) (m/s).
The alteration index, indicative of relative hardness varies from 0 in the sorter soils to 2 in the
harder ones on a given site. It is very sensitive in medium to low strength soils.
2. Energy used for drilling (Pfister, 1985)The energy parameter is calculated from the equation:
W = T N / V
T = is the value of the rotation torque (kN m).
N = rotation speed (rps).
V = instantaneous penetration rate (m/s).
The drilling energy is very useful in the analysis of hard soils and soft rocks.
3. Resistance to drilling (Somerton, 1959)Sd = W (N / V)
1/2
W = weight on bit (thrust retention force + weight of rods and bit) (kN).
N = rotation speed (rps).
V = instantaneous penetration rate (m/s).
4. - hardness parameter (Bingham, 1965) -hard = N F D
2/ V T
N = rotation speed (rps).F = thrust applied on the drilling bit (kN).
D = bit diameter (m).
V = penetration rate (m/s).
T = rotation torque (kN m).
hard can be thought of as hard to drill and could relate to the difficulty of eroding and
transporting soil particles away from the drill bit. For example, a clay might tend to clog the
bit and therefore be hard to drill, whereas sands may be easy to evacuate and quick to
drill.
5. Exponent method (Jorden and Shirley, 1966)
E = log (V / N D) / log ( F D / T)V =drilling speed (m/s).
N = rotation speed (rps).
D = bit diameter (m).
F = thrust on bit (kN).
T = rotation torque (kN m).
Exponent method is related with d-exponent which it is an empirical parameter to track the profile
of rock strength in shale.
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6. Specific energy (Teale, 1965)E = F / A + 2NT / A V
F = thrust on bit (kN).
A = area removed by drill bit (m2).
N = rotation speed (rps).T = rotation torque (kN m).
V = drilling speed (m/s).
This parameter is employed to obtain geological and geotechnical information. A more detailed
description is given below.
Specific energy is defined as the energy required for excavating unit volume of rock. It is a useful
parameter that may also be taken as an index of the mechanical efficiency of a rock-working
process.
The drilling specific energy is expressed by an equation that calculates the energy as a function of
parameters recorded at the selected frequency. It can be expressed as follows:
Es = F/A + 2NT / AV = et + er
Where:
F = thrust on the bit (kN).
A = hole section (m2).
N = rotation speed (rps).
T = rotation torque (kN m).
V = rate of penetration (m/s).
The first member of the equation represents the contribution of the thrust (thrust component). It isequivalent to the pressure acting over the cross sectional area of the hole.
The second member is the rotary component of energy.
Specific energy has the same dimensions as pressure or stress. This is because of the fact that if a
force F acting on a normal surface (A) moves it through distance ds, the increment of work done,
dW, is equal to Fds. The change in volume effected by the movement, dV, is Ads. If Es is the
specific energy at any point, then e = dW/dV = F/A = p, the pressure at that point.
For a given excavation, A is constant, so et is directly proportional to F. For given A and N, er is
proportional to T/V.
Now, the torque/penetration rate curves approximate to a straight line through the origin. The slope
of this line is T/V and it is approximately constant.
It follows that for given A, and N, er and therefore e itself should keep a constant value.Another approach to the above is to put in the equation the term p as the penetration per
revolution (p=V/N). Then, the equation of rotary component of specific energy can be written as
follows:
Er = 2T / Ap (kN/m2)
T is the torque required to remove a layer of rock of depth p in one revolution. Since the amount of
energy required for brittle materials like rock is not much affected by the rate at which it is applied,
the relationship between T and p may not be significantly affected by changes in rotation speed.
The ratio T/p can therefore be a useful index of specific energy.
In Figure 1 it can be seen the relationship between penetration per revolution and specific energyfor claystones.
In the er equation it can be observed that specific energy will reach very high values al low thrust.
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Below a certain value, the thrust will be inadequate to effect penetration of the bit.
As the thrust increases, the value of specific energy falls until it reaches a value beyond which it
continue to decrease so slowly as to remain virtually constant. This can be seen in Figure 2.argilitas
0
1000000
2000000
3000000
4000000
0 0,001 0,002 0,003 0,004 0,005 0,006 0,007 0,008 0,009 0,01
penetration per revolution (m/rev)
specificenergy(KJ/m3)
argilitas
0
100000
200000
300000
400000
500000
0 5 10 15 20 25
drilling thrust (KN)
rotaryspecificenergy(KN/m
2)
Figure 1 - Relationship between penetration per
revolution and specific energyfor claystones.
Figure 2 - Relationship between rotation
specific energy and drilling thrust in claystones.
Specific energy has a high value at any change in lithology but it falls after that and remains in a
constant value.
The lowest value attained is a measure of the maximum mechanical efficiency of the particular tool
in the particular operating conditions.
However the fall in specific energy does not continue indefinitely: a stage may be reached when the
tool is pushed so heavily into the rock that it becomes overloaded and clogs. The reduction in
efficiency will cause the specific energy to rise again.
THE USE OF SPECIFIC ENERGY FOR GEOTECHNICAL SITE CHARACTERIZATIONThe first use that can be derived from the drilling specific energy is to correlate it with the main
geomechanical rock mass parameters such as RMR (Bieniawski, 1989), UCS, etc. This use provides
a cost effective geotechnical tool and can be used either in standard geotechnical boreholes or in
open boreholes.
The drilling parameters systematically recorded were depth, rate of penetration (V), weight on bit
(F), fluid pressure, torque (T) and rotation speed (N); allowing to obtain the drilling specific energy.
Also a set of lab test were carried out providing the opportunity to make correlations with the basic
intact rock parameters.
Drilling parameters and geophysical logging tools were measured at the following boreholes:
PFM-5, 6, 7 and 8 (20, 11, 13.95, 13 m), for set up purposes.
BH0 (117 m) at a clayly flysch formation, for set up purposes. BH-1 (80 m) at a carboniferous formation.
BH-2 (227.5 m) at a metamorphic formation.
BH-3 (210 m) at marls, schist and vulcanites.
BH-4 (64.2 m) at sandstones, shales and coal seams.
Correlations with rock mass parameters
With the data coming from all the mentioned boreholes, a correlation between specific energy and
different rock mass parameters have been investigated.
In the Figure 3 it can be observed the different drilling parameters measured and the specific energy
obtained with depth.
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0
10
20
30
40
50
60
70
0 100 200
RQD (%)
0
10
20
30
40
50
60
70
0 20 40
J oints /m
0
10
20
30
40
50
60
70
0 20 40 60
RM R
Figure 3 - Rock mass parameters and specific energy obtained with depth (BH-4).
The parameters used for geotechnical purposes are:
Rock Mass Rating (RMR).
Number of Joints per metre.
Rock Mass Uniaxial Compressive Strenght (cm
).
Specific Energy (Esp) is the main index related with all drilling parameters. It has been analyzed
respect to RMR, Number Joints per metre, Young Modulus (Edyn) and Rock Mass Uniaxial
Compressive Strength (cm
), as defined in Kalamaras and Bieniawski (1995). These correlations can
be observed below.
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Shales
y = 0,0006x + 0,0136
R2 = 0,3248
0,020
0,025
0,030
0,035
0,040
0,045
0,050
20 25 30 35 40 45 50 55 60
RMR
Esp(GJ/m
3)
Sandstones
y = 0,0002x + 0,0088
R2 = 0,2313
0,000
0,005
0,010
0,015
0,020
0,025
20 25 30 35 40 45 50 55 60
RMR
Esp(GJ
/m3)
Schists
y = 0,0243x + 0,6298
R2 = 0,3045
0,0000
0,5000
1,0000
1,5000
2,0000
2,5000
0 10 20 30 40 50 60
RMR
Esp(GJ/m3)
Coal
y = 0,0002x - 0,0044
R2 = 0,9944
0,0006
0,0010
0,0014
0,0018
0,0022
29 30 31 32 33 34 35 36
RMR
Esp(G
J/m3)
Massive Sulphide
y = 0,0004x + 0,0488
R2 = 0,6103
0,04
0,05
0,06
0,07
0,08
0,09
0,1
0,11
0,12
0 20 40 60 80 100RMR
Esp
(GJ/m3)
Figure 4 - Correlation between RMR and Esp.
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Shales
y = - 0,0002x + 0,0396
R2 = 0,4831
0,020
0,025
0,030
0,035
0,040
0,045
0,050
0 5 10 15 20 25 30
N Joints/m
Esp(GJ/m3)
Sandstones
y = -0,0002x + 0,0182
R2 = 0,3527
0,000
0,005
0,010
0,015
0,020
0,025
0 5 10 15 20 25 30 35N Joints/m
Esp(GJ/m
3)
Schists
y = -0,0014x + 1,0433
R2 = 0,0501
0,5000
0,7500
1,0000
1,2500
1,5000
0 5 10 15 20 25 30 35
NJoints/m
Esp(
GJ/m3)
Massive Sulphide
y = -0,0004x + 0,0754
R2 = 0,52
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,10
0 10 20 30 40 50
N Joints/m
Esp(GJ/m3)
Figure 5 - Correlation between N Joints/m and Esp.
In both cases the Specific Energy shows a relationship respect to the rock mass quality, to greater
values of Specific Energy the rock mass quality will be better. It is important to take into account
that the number of joints per metre is an important part of the value of RMR, and according the
expected results was obtained a logical trend that show the correlations carried out.Following correlations show Specific Energy (Esp) versus Dynamic Elastic Modulus (Edyn).
Figure 6 - Correlation between Edyn and Esp.
From Full Wave form Sonic it is obtained P (Vp) and S (Vs) wave velocities, that can be related to
elastic deformational parameters of the rock, in this case it is used to obtain the Young Modulus,
Sandstones
y = 3E-07x + 0,005
R2
= 0,4303
0,0000
0,0050
0,0100
0,0150
0,0200
0,0250
0,0300
0,0350
0,0400
0 20000 40000 60000 80000 100000
Edyn (MPa)
Esp(GJ/m3)
Shales
y = 3E-07x + 0,0146
R2 = 0,3489
0,0100
0,0150
0,0200
0,0250
0,0300
0,0350
0,0400
0,0450
0,0500
10000 20000 30000 40000 50000 60000 70000 80000 90000
Edyn (MPa)
Esp(GJ/m3)
Coal
y = 3E-07x + 0,0066
R2 = 0,3718
0,0000
0,0050
0,0100
0,0150
0,0200
0,0250
0,0300
0,0350
0 20000 40000 60000 80000 100000
Edyn (MPa)
Esp(GJ/m3)
Marls
y = 9E-07x + 0,0069
R2 = 0,8582
0,010
0,015
0,020
0,025
0,030
0 5000 10000 15000 20000
Edyn (MPa)
Esp(GJ/m3)
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and in a logical trend, to greater values of this modulus, it is obtain greater values of Specific energy
(Esp).
And the last correlation with drilling parameters is between Specific Energy and Rock Mass
Uniaxial Compressive Strength (cm
).
Fig. 2.6.2.d Correlation betweencm
and Esp.
Figure 7 - Correlation between cm
and Esp.
Correlation with intact rock parameters
The laboratory measurements are useful to study the intact rock, the correlations carried out
between laboratory tests (UCS and PLT) and field measurements (mainly obtained from full wave
form sonic (Vp) and drilling parameters (Esp)) indicate the relation between these parameters.
The results obtained from different lithologies are showed below.
Shales
y = 0,0032x + 0,0338
R2 = 0,4107
0,020
0,025
0,030
0,035
0,040
0,045
0,050
0 0,5 1 1,5 2 2,5
c
m
Esp(GJ/m3)
Schists
y = 0,121x + 0,8479
R2
= 0,2183
0,25
0,50
0,75
1,00
1,25
1,50
1,75
0,00 1,00 2,00 3,00 4,00c
m(Mpa)
Esp(GJ/m3)
Sandstones
y = 0,0035x + 0,0132
R2 = 0,7033
0
0,005
0,01
0,015
0,02
0,025
0,03
0,035
0,04
0 2 4 6 8
cm (Mpa)
Esp(GJ/m3)
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Sandstones
y = 27,635x + 5872,5
R2 = 0,5356
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 20 40 60 80
UCS (MPa)
Vp(m/s
)
Sandstones
y = 0,0003x + 0,0123
R2 = 0,6608
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
0,040
0 10 20 30 40 50 60 70 80
UCS (Mpa)
Esp(GJ/m3)
Shales
y = 33,033x + 5383,8R2 = 0,3365
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 20 40 60 80
UCS (MPa)
Vp(m/s)
Marls
y = 0,0597x + 0,0151
R2
= 0,3777
0,005
0,010
0,015
0,020
0,025
0,030
0,035
0,040
0,00 0,05 0,10 0,15 0,20 0,25
UCS (MPa)
Esp(GJ/m3)
Granites
y = 61,909x + 3306,3
R2 = 0,5152
0
1000
2000
3000
4000
5000
6000
7000
0 10 20 30 40 50 60
UCS (Mpa)
Vp(m/s)
Schists
y = 0,0178x + 0,6071
R2 = 0,8162
0,25
0,50
0,75
1,00
1,25
1,50
1,75
0 10 20 30 40
UCS (MPa)
Esp(GJ/m3)
Figure 8 - Correlation between
UCS and Vp.
Figure 9 - Correlation between
UCS and Esp
In both cases, when UCS increases, Vp and Esp increase in the same way, it is indicative of the rock
quality, its geomechanical propierties. With the data obtained form Point Load Test (PLT), early
studies (Bieniawski, 1979; Broch and Franklin, 1972) found that relationship between UCS and
PLT could be expressed as:
UCS = (K)Is50 = 24Is50
Where K is the conversion factor, this relationship has been applied to the data obtained with the
Point Load Test for the sandstones and shales, with the objective to obtain more data for the
correlations.
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THE USE OF SPECIFIC ENERGY FOR TBMs DURING TUNNEL CONSTRUCTIONThe following TBM drilling parameters are usually sistematically recorded:
Advance rate (ARA)
Time of excavation
Weigh of the debris in the belt Thrust (total/contact) (F)
Rotation speed (N)
Torque (T)
From these data two different interpretations can be done:
Qualitative
Quantitative
In the first type the following circumstances have been noticed:
A significant increase in the rate of advance with a decrease in the geomechanical groundquality.
An increase in the debris weight with face instability.
Instantaneous torque increase with face instability.
The difference between the applied and the contact thrust is equivalent to the TBMfriction. If this value increases the TBM can get stocked.
In relation with a quantitative interpretation, the following values have been considered: a) Penetration rate (p)
)(
)/()/(
rpmN
mmmVrmmp = (1)
that gives the ground resistance to be excavated.
b) Penetration index (Ip)
p
kNFI cp
)(= (2)
that proportionate the thrust per cutter to penetrate 1 mm per revolution.
c) Specific energy of excavation (Es)
As defined by Teale (1965)
ARAA
TN
A
FmkJEs
+=
2)/( 3 (Teale, 1965) (3)
where for TBM machines, Es = specific energy of excavation (kJ/m3), F = total cutterhead thrust
(kN), A = excavated face area (m2), N = cutterhead rotation speed (rps), T = applied torque (kNm)
and ARA = average rate of advance (m/s).
As it can be observed there are two addends, the first one corresponds to the thrust energy (Est)
while the second one corresponds to the rotation energy (Esr).
Following it is presented the main results obtained during the excavation of Guadarrama Tunnels
and San Pedro Tunnels.
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Correlation between Ip vs. Esr.
These data have been systematically recorded during the excavation of Guadarrama Tunnels that
consists in two twin tunnels each one of 28.3 km of length, constructed in gneissic and granitic
rocks. This results were firstly showed at Tardguila and Surez (2005).
Figures 10 and 11 show the existing relation between the penetration index and the specific rotationenergy of excavation.
TNEL 4
0
10
20
30
40
50
60
70
80
90
100
5006007008009001000
Anillo N
Energaespecficaderotacin(MJ/m3)
Indicedepenetracin
Energa especfica de rotacin
Indice de penetracin
Figure 10. Relationship between
Ip and Esr for 500 segments rings
Figure 11. Correlation between
Esrand Ip (Esr= 8 Ip0.52
).
In the first one it can be observed the direct relation between both parameters considering 500
segment units. From this relation it can be concluded that the rotation specific energy depends on
the geomechanical quality of the rock mass as the penetration index does.
Correlation between Specific Energy and RMRThe following figure shows the correlation between the Specific Energy and RMR obtained from
the excavation of San Pedro tunnels constructed using an open TBM in gneissic rocks.
Tnel de San Pedro Tubo Este Boca Sur
y = 23,584x0,3587
R2
= 0,6238
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20
Energia Especfica de rotacin (Kwh/m3)
RMR
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CONCLUSIONS
The use of the specific energy in the geotechnical site investigation as well as during tunnel
excavation with TBM provides a very interesting geotechnical tool for site characterization.
In relation with geotechnical boreholes, it has been observed that:
Esp depends strongly on the geomechanical rock mass quality expressed by the RMR andthe Number of Joints per metre:
- As higher the RMR is, higher are the values of Esp.
- RQD gives high scatter in relation with Specific Energy.
- Its a good tool for the rock mass characterization. As higher the number of
Joints/m are, lower are the values of Esp.
A clear influence of the lithology in the values of Esp is observed.
The Dynamic Young Modulus (Edyn), in relation with Esp and RMR, shows good resultswith the lithologies analyzed.
The obtained correlation between Vp and specific energy gives an interesting tool forcharacterization purposes.
In relation with TBM tunnel excavation it can be concluded:
The rotational specific energy is a very useful tool for the geotechnical control of a tunnelexcavated using a TBM.
The rotation specific energy depends on the geomechanical quality of the rock mass as thepenetration index does
The thrust specific energy represents only 2% of the total specific energy needed to excavatea tunnel with a TBM.
ACKNOWLEDGEMENTS
Part of the work included in this paper has been done while the contract RFCR-CT-205-00001
(ADEMA Advanced in Exploration Methods and Applications) included in the VII R+D
Framework of the European Commission.
REFERENCES
Bingham, M. G. (1965). A new approach to interpreting rock drillability. Oil & Gas Jouurnal.
Bieniawski, Z. T. (1979). The geomechanics classification in rock engineering applications. In: Proc. 4th
International Conference on Rock Mechanics. Montreaux. Balkema. Vol. 2.
Bieniawski, Z.T. (1989). Engineering classification of jointed rock masses. Transactions, South African Inst. Of CivilEngineers, vol. 15, n 12, pp. 335 - 344
Jorden J.R., Shirley O.J. (1966). Application of drilling performance data to overpressure detection. J. Pet. Technol.,
7, 987 - 991
Kalamaras, G. S. and Bieniawski, Z. T. (1995). A rock mass strength concept. In: ISRM International Congress of
Rock Mechanics. Tokyo. Japan.
Somerton, W.H.; (1959) A laboratory study of rock breakage by rotary drilling. Society of Petroleum Engineers. Vol.
216 pp. 92 -97.
Tardguila, I.; Suarez, J.L. (2005). Metodologa para el seguimiento y control del terreno en el interior de los tneles
de Guadarrama. In: Tnel de Guadarrama. Ed. Entorno Grfico, pp. 479-501.
Teale, R. (1964) The concept of specific energy in rock drilling. Rock Mechanics Mining Science, vol. 2, pp 57 73.