[web] Added the documentation of the new test of BGRa creep model

web/content/docs/benchmarks/creep-after-excavation-bgra/CreepAfterExcavation.md

++++
+date = "2018-08-10T11:46:45+01:00"
+title = "Creep in a rectangle domain with hole under pressure on its top surface"
+weight = 171
+project = "ThermoMechanics/CreepBGRa/CreepAfterExcavation/CreepAfterExcavation.prj"
+author = "Wenqing Wang"
+
+[menu]
+  [menu.benchmarks]
+    parent = "thermo-mechanics"
+
++++
+
+{{< data-link >}}
+
+-------------------------------------------------------------------------
+Following up the benchmark about the BGRa creep model described on the 
+[this page](https://www.opengeosys.org/docs/benchmarks/creepbgra/creepbrga/), 
+this example represents the creep in the near field of
+drift in the deep rock salt after excavation. The domain and the
+geometry are shown in the following figure:
+<figure>
+    <img src="../mesh.png" alt="Mesh and Geometry" id="fig_6"
+ style="height:400px;width:490px;">
+    <figcaption>Mesh and Geometry.</figcaption>
+</figure>
+
+The domain has two material groups, which are highlighted by different
+colors. The material group that is in the
+top part of the domain represents a cap rock type, while the other
+material group is for rock salt. The material properties of the rocks are given in the following table:
+
+---------------------- ---------- ----------- ------------
+|         |Cap rock |  Rock salt|   Unit|
+| --- |:---------:| -----:|----:|
+|Density            |    2000   |    2170 |       kg/m$^{3}$|
+|Young’s Modulus    |    7.0    |    7.65  |      GPa       |
+|Poisson ratio      |    0.3    |    2.7   |      -          |
+|Thermal conductivity |   5     |     5    |       W/(mK)    |
+---------------------- ---------- ----------- ------------
+
+The parameters of the BGRa creep model are $A=0.18\, \mbox{d}^{-1}$,
+$m=5$, $Q=54 \mbox{ kJ/mol}$.
+
+
+The width
+and the height of of the domain are 300 m and 340 m, respectively. The
+height of the cap rock portion is 40 m. The drift to be excavated has a
+radius of 50 m.
+
+Here we consider the creep of the rock after excavation. Therefore, we
+assume a steady state of stress and temperature of excavation at the
+beginning of the current simulation. For this assumption, the boundary
+conditions are given as:
+
+-   top boundary: $\tau_x = \sigma_x=0$, $\tau_y=\sigma_y=70$ MPa,
+    $T=310$ K.
+
+-   two lateral boundaries: normal displacement is fixed, and no heat
+    flux.
+
+-   bottom boundary: normal displacement is fixed, and $T=320$ K.
+
+-   circle of drift surface: traction free
+    (${ \mathbf\sigma}\cdot \mathbf n = 0$), and $T=300$ K as excavation
+    conditions.
+
+The boundary conditions of the mechanical process lead to a distinct
+stress field after excavation. Besides, heat capacity is neglected in
+order to pose a steady state temperature field after excavation. The
+initial stresses are zero and the initial temperature is linearly
+distributed from top to bottom with the boundary values of temperature.
+
+The time step sizes of the simulation are: One step of 0.001 day, 10 steps of 0.1 day, and the
+remaining steps of 1 day.
+
+The following three figures are plotted by using the results of the simulation of 1000 days creep. 
+The three figures display the distribution of horizontal and vertical stresses at times of
+108 days, 409 days and 1000 days, respectively. 
+
+In theses three figures,
+the left sub-figure show the time variations of horizontal and vertical stresses
+ at a position just close to the top of the drift, and the
+green vertical line in it marks the time of the displayed stress field.
+
+<figure>
+    <img src="../stress_xx_yy_20.png" alt="Stress distribution at the time of 109 days." id="fig_2">
+    <figcaption>Stress distribution at the time of 109 days.</figcaption>
+</figure>
+
+<figure>
+    <img src="../stress_xx_yy_50.png" alt="Stress distribution at the time of 409 days." id="fig_3">
+    <figcaption>Stress distribution at the time of 409 days.</figcaption>
+</figure>
+
+<figure>
+    <img src="../stress_xx_yy_110.png" alt="Stress distribution at the time of 1000 days." id="fig_4">
+    <figcaption>Stress distribution at the time of 1000 days.</figcaption>
+</figure>
+
+
+The above three figures show that the absolute value of the horizontal stress increase
+signification in the areas above and beneath the drift due to the creep,
+and the change of the vertical stress is slow compared to that of the
+horizontal stress.
+
+The following figure shows the strain distribution at the
+end of the simulation time at 1000 days.
+<figure>
+    <img src="../strain.png" alt="Strain distribution at the time of 1000 days." id="fig_5">
+    <figcaption>Strain distribution at the time of 1000 days.</figcaption>
+</figure>
+
+
+The steady-state temperature distribution is displayed in the following figure
+<figure>
+    <img src="../T.png" alt="Temperature distribution at the time of 1000 days." id="fig_6">
+    <figcaption>Temperature distribution at the time of 1000 days.</figcaption>
+</figure>
+
+## Note:
+For the automatic benchmarking, the time duration of creep is reduced to 50 days in order to reduce the run time.
+ If one wants to test this benchmark for 1000 days' creep, please change the end time in the tag of `<t_end> `
+in the project file as
+ ```
+<t_end>1000</t_end>
+```
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