Joint Generation

The JSET command is used to make additional cuts in the solids, created with the POLY command, to define joints, faults, and holes or excavations. The JSET command can be used to make single cuts or multiple parallel cuts. Statistical parameters may be used to vary orientation, spacing and persistence to match logged jointing data.

The JSET command is first demonstrated in this section for making single cuts. Some planning should be made to optimize the sequence in which joints are created. Joints which define the geometry of excavations are usually cut first (seeSection 3.2.3), followed by the minor joints or joint sets. Through-going faults are usually defined last.

The primary keywords for the JSET command are dip, dd (dip direction), and origin (origin). Unless other keywords are used, the JSET command will create a single plane cutting through the model in the orientation specified. The origin point may be any point on the plane. Figure 3.5 shows how the orientations for dip and dip direction relate to the coordinate axes in 3DEC. The dip range is from 0 to 90^◦. The dip direction range is from 0 to 360^◦.

North (z) Up (y)

East (x)

Joint plane

= Dip direction

= Dip

Strike line

Figure 3.5 Terms describing the attitude of an inclined plane:

dip angle,α, is positive measured downward from the horizontal (xz) plane; dip direction,β, is positive measured clockwise from north (z)

Control of the continuity of cuts made using the JSET command is accomplished with the HIDE and SEEK commands. The JSET command will only cut blocks that are currently visible.

Example 3.5illustrates the creation of a noncontinuous joint.

Example 3.5 Creation of a noncontinuous vertical joint

new

poly brick 0,1 0,1 0,1 ; create a block

jset dip 0 dd 0 or 0,.5,0 ; make a horizontal cut hide dip 0 dd 0 or 0,.5,0 below ; hide the bottom block

jset dip 90 dd 90 or .5,0,0 ; vertical cut through top block only

seek ; make all blocks visible

ret

Figure 3.6shows the full model and the joint structure plot for this example. Note that the vertical joint does not penetrate the bottom block. 3DEC automatically assigns a joint ID number = 2 to the horizontal joint and a joint ID number = 3 to the vertical joint. If desired, the joint ID number can be controlled with the JSET command. For example,

jset dip 0 dd 0 or 0,.5,0 id = 1000

will create a horizontal joint with an ID number of 1000.

The ID numbers for joint faces and contacts are given sequentially as they are created. Therefore, if two faults are defined that intersect, the edge to edge contacts at the line of intersection will have the joint IDs of the second fault defined. This order becomes important if different properties are to be assigned to the different faults. Property numbers of the face-to-face contacts that comprise most of the area of joints can be assigned using the CHANGE command, in which the particular joint to be changed is identified using either its joint ID number or orientation. The difficulty comes in assigning property numbers to the edge-to-edge contacts that are created at joint and fault intersections. It is easy to assign property numbers to edge-to-edge contacts by use of the joint ID number. It is difficult to assign property numbers to edge-to-edge contacts by orientation because they have a different orientation than either of the two intersecting planes that created them. There is a PLOT option that allows plotting of joint material properties (thejoint materialitem in the

options menu). This plot is useful in checking that the edge-to-edge contacts are assigned the correct properties.

3DEC (Version 3.00)

Itasca Consulting Group, Inc.

x Y z

dip= 70.00 above dd = 210.00 center 5.000E-01 5.000E-01 5.000E-01 cut-pl. 0.000E+00 mag = 1.00 cycle 0

27-Aug-02 10:45

(a) full-solid view

3DEC (Version 3.00)

Itasca Consulting Group, Inc.

x Y z

dip= 70.00 above dd = 210.00 center 5.000E-01 5.000E-01 5.000E-01 cut-pl. 0.000E+00 mag = 1.00 cycle 0

27-Aug-02 10:45

(b) joint-structure view

Figure 3.6 Model created with the JSET and HIDE commands

Concave blocks can be made by use of the JOIN command. The blocks that have been joined are still convex, but the join logic locks the interface between them. For example, add the following commands at the end ofExample 3.5.

hide (0.5,1.0) (0.5,1.0) (0,1) join on

Figure 3.7shows the concave block that is created. Note that only visible blocks can be joined.

3DEC (Version 3.00)

Itasca Consulting Group, Inc.

X Y

z

dip= 79.00 above dd = 160.00 center 5.000E-01 5.000E-01 5.000E-01 cut-pl. 0.000E+00 mag = 1.00 cycle 0

27-Aug-02 10:47

Figure 3.7 Concave block created with the JOIN command

Joined blocks are plotted in the same color on the graphics screen. Also, contacts between joined blocks are identified as master-slave (m-s) contacts. Type PRINT contact to check the contact type.

Note that “slaved” blocks will be automatically joined if they are connected to the same “master”

block. For example, if block A and block B are joined, and block A and block C are joined, then block B will be joined automatically to block C.

The JSET command can also be used to generate a set of joints automatically based upon physically measured parameters (i.e., joint dip, dip direction, spacing and persistence). By hiding selected blocks, a set of noncontinuous joints can be generated. In Example 3.6, a jointed rock slope is created containing both shallow and deeply dipping joint sets. Two noncontinuous fractures are also created to define a rock wedge in the slope; seeFigure 3.8.

Example 3.6 Rock slope containing continuous and noncontinuous joints

new

poly brick 0 80 0 50 -30 80

; shallow-dipping fracture planes (continuous) jset dip 2.45 dd 235 org 30 12.5 0

jset dip 2.45 dd 315 org 35 30 0

; high angle foliation planes (continuous) jset dip 76 dd 270 spac 16 num 3 org 30,12.5,0

; intersecting discontinuities (non-continuous) hide 0 80 0 10 0 50

hide 55 80 0 50 0 50

jset dip 70 dd 200 org 0 0 35 jset dip 60 dd 330 org 50 50 15 seek

hide 0,30 13,50 -30,80 ret

3DEC (Version 3.00)

Itasca Consulting Group, Inc.

x Y z

dip= 70.00 above dd = 210.00 center 4.000E+01 2.500E+01 2.500E+01 cut-pl. 0.000E+00 mag = 1.00 cycle 0

27-Aug-02 10:49

Figure 3.8 Rock slope containing continuous and noncontinuous joints

Bear in mind that joints are displayed as straight-line segments in the 3DEC model; many segments may be required to fit an irregular joint structure. The modeler must decide the level at which the 3DEC joint geometry will match the physical jointing pattern. The effect of geometric irregularity

on the response of a joint can also be taken into account via the joint material model — e.g., by varying properties along the joint.

One final point is made concerning joint generation. When using continuum programs, it is usually appropriate to take advantage of symmetry conditions with excavation shapes in order to reduce the size of the model. Symmetry conditions cannot be imposed as easily with discontinuum programs because the presence of discontinuous features precludes symmetry except for special cases. For example, it is not possible to impose a vertical plane of symmetry through the model shown in Figure 3.8because the joints in the model are not aligned with the vertical axis.