mujoco

<mujoco model="Sand castles">
  <!-- Adhesion is Mohr-Coulomb cohesion: the shear strength of a contact is
       mu*(N + adhesion), nonzero even without confining pressure, and contacts
       resist tension up to the summed adhesion of the two geoms. Three identical
       cubes of grains differ only in adhesion: dry sand avalanches to its angle
       of repose, damp sand slumps but holds a mound, wet sand stands as a castle.
       The small gap is the moisture film: grains within it form tensile bridges. -->

  <option solver="CG" timestep="4e-3"/>

  <statistic center="0 0 .15" extent="1.2"/>

  <visual>
    <headlight diffuse=".8 .8 .8" ambient=".3 .3 .3"/>
    <quality shadowsize="8192"/>
  </visual>

  <asset>
    <texture type="skybox" builtin="gradient" rgb1=".4 .6 .8" rgb2=".9 .9 .95" width="32" height="512"/>
    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
    <material name="grid" texture="grid" texrepeat="6 3" reflectance=".2"/>
  </asset>

  <default>
    <geom type="sphere" size=".01" friction="1 .005 .005" solref=".01 1"/>
    <default class="dry">
      <geom rgba=".87 .78 .55 1" adhesion=".002" gap=".001"/>
    </default>
    <default class="damp">
      <geom rgba=".72 .58 .38 1" adhesion=".02" gap=".001"/>
    </default>
    <default class="wet">
      <geom rgba=".5 .38 .25 1" adhesion=".05" gap=".001"/>
    </default>
  </default>

  <worldbody>
    <light pos=".5 -1 1.5" dir="-.3 .6 -.8" diffuse=".7 .7 .7"/>
    <geom name="floor" type="plane" size="1.2 .6 .01" material="grid"/>
    <geom name="rail-x" type="plane" size=".6 .05 .01" pos="-1.2 0 .05" xyaxes="0 1 0 0 0 1" rgba=".5 .5 .5 .15"/>
    <geom name="rail+x" type="plane" size=".6 .05 .01" pos="1.2 0 .05" xyaxes="0 -1 0 0 0 1" rgba=".5 .5 .5 .15"/>
    <geom name="rail-y" type="plane" size="1.2 .05 .01" pos="0 -.6 .05" xyaxes="-1 0 0 0 0 1" rgba=".5 .5 .5 .15"/>
    <geom name="rail+y" type="plane" size="1.2 .05 .01" pos="0 .6 .05" xyaxes="1 0 0 0 0 1" rgba=".5 .5 .5 .15"/>

    <body name="ramp1" mocap="true" pos="-.4 0 .02" euler="0 18 0">
      <geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
    </body>
    <body name="ramp2" mocap="true" pos="0 0 .02" euler="0 18 0">
      <geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
    </body>
    <body name="ramp3" mocap="true" pos=".4 0 .02" euler="0 18 0">
      <geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
    </body>

    <site name="adhesion = 0.002" size="1e-5" pos="-.45 0 .4"/>
    <replicate count="3" offset="0 0 .02828">
      <replicate count="6" offset=".02 0 0">
        <replicate count="6" offset="0 .02 0">
          <body name="a" pos="-0.45 -.05 .29" childclass="dry">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
      <replicate count="5" offset=".02 0 0">
        <replicate count="5" offset="0 .02 0">
          <body name="a2" pos="-0.44 -.04 .30414" childclass="dry">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
    </replicate>

    <site name="adhesion = 0.02" size="1e-5" pos="-.05 0 .4"/>
    <replicate count="3" offset="0 0 .02828">
      <replicate count="6" offset=".02 0 0">
        <replicate count="6" offset="0 .02 0">
          <body name="b" pos="-0.05 -.05 .29" childclass="damp">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
      <replicate count="5" offset=".02 0 0">
        <replicate count="5" offset="0 .02 0">
          <body name="b2" pos="-0.04 -.04 .30414" childclass="damp">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
    </replicate>

    <site name="adhesion = 0.05" size="1e-5" pos=".35 0 .4"/>
    <replicate count="3" offset="0 0 .02828">
      <replicate count="6" offset=".02 0 0">
        <replicate count="6" offset="0 .02 0">
          <body name="c" pos="0.35 -.05 .29" childclass="wet">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
      <replicate count="5" offset=".02 0 0">
        <replicate count="5" offset="0 .02 0">
          <body name="c2" pos="0.36 -.04 .30414" childclass="wet">
            <freejoint/>
            <geom/>
          </body>
        </replicate>
      </replicate>
    </replicate>
  </worldbody>
</mujoco>