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Far Field Stress
The Field Stress option allows the user to define the in-situ field stress conditions prior to excavation. Two options are available for defining field stress in Phase2 - Constant or Gravity field stress.
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Field Stress: Constant
The Constant field stress option can be used for the modeling of deep excavations. In cases where the gravitational stress gradient is negligible across the height of your model, compared to the overall stress magnitude, a constant stress field is preferable. A Constant field stress is defined by two in-plane principal stresses (Sigma 1 and Sigma 3), and an angle which defines the in-plane stress orientation. The out-of-plane principal stress (Sigma Z) is also required.
Field Stress dialog (Constant)
Underground mine model (constant field stress)
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Field Stress: Gravity
For surface or near-surface excavations, it is appropriate to use a Gravity stress field, in which the vertical stress is assumed to vary linearly with depth. The depth and the unit weight of overlying material determine the vertical stress distribution throughout the model. The horizontal stress is calculated by multiplying the vertical stress by the Horizontal/Vertical Stress Ratios.
The vertical and horizontal initial stresses are calculated using the following equations:
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Sv = gamma*H
Sh = L + K*Sv
Where
Sv = Vertical stress
Sh = Horizontal stress
L = Locked-in stress (horizontal stress at ground surface)
K = Horizontal stress ratio
gamma = average unit weight of the soil/rock
H = depth of element center below ground surface
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The equation for horizontal stress applies to both the in-plane and out-of-plane horizontal stresses.
Field Stress dialog (Gravity)
The depth can be measured from either:
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A single datum elevation
The actual ground surface of your model.
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The Use Actual Ground Surface option is very useful for surface excavations such as open pit mines, slopes, rock cuts etc. This method provides a better initial estimate of the vertical in-situ stress, for surface excavations where the ground surface is irregular (i.e. not defined by a single horizontal surface).
Open pit mine model (gravity field stress)
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Field Stress: Custom
There are situations when you may need to specify the field stress for individual materials in a model, rather than a single field stress which applies to all materials.
For example, individual layers of material in bedded geological structures will sometimes have an in-situ stress that is different from the stress in surrounding layers. When this situation occurs, the user can define a customized field stress for any material(s) in their model, as shown in the Field Stress dialog below.
Custom Field Stress per material
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Field Stress: Load Split
The Load Split option in Phase2 allows the user to "split" the field stress induced load, between any stages of the model, rather than applying the entire field stress load in the first stage. This allows the user to gradually apply the field stress load, as excavation progresses. A typical use of Load Splitting is to simulate the 3D effect of an advancing tunnel face, using a 2D model. This can be used, for example, to simulate the delayed installation of support at an advancing tunnel face.
Load Split dialog
For maximum flexibility, the Load Split can be customized on a per material basis, using the Advanced option in the Load Split dialog. When specified per material, the load can be initialized at any stage. It is also possible to apply the load split to body forces or field stress, independently.
Custom load split per material
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Field Stress: Initial Element Loading
Initial Element Loading is specified in the Define Material Properties dialog, for each material. The Initial Element Loading option allows the user to specify whether Field Stress and/or Body Force is applied as the initial element loading, for each material.
Initial Element Loading option
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If you are using Field Stress = Constant, then by default the
Initial Element Loading for all materials will be Field Stress Only.
If you are using Field Stress = Gravity, then by default the
Initial Element Loading for all materials will be Field Stress & Body Force.
You may also specify Initial Element Loading = Body Force Only
(this would be applicable for a backfill material, for example). Or you may
specify Initial Element Loading = None (no initial stress or force on the
elements).
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For more information about this option, see the Initial Element Loading help topic
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