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Part 2 Expansive Soils Promote Slope Creep
Butt-bowed trees in northern California
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• Cut slope along Pleasant Hill Rd in Martinez, CA
  in 1954 (left) and 30 years later, in 1984 (at
  right).

• Many natural slopes are blanketed by residual
  soils which are expansive
• These materials swell and shrink with seasonal
  regularity, during alternating wet and dry
  seasons
• This cyclic movement allows for plastic creep, or
  strain under sustained loading.

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• Slope creep sketched by Sharpe (1938)

• Apparent creep of a cut slope in Lafayette, CA

• Seasonal slope creep occurs on most inclined
  ground, whether engineered fill or natural
  hillsides.
• The longer the slope, the greater the amount of
  creep, because strain increments are accumulated
  upslope, as an integral function.
• Foundation elements situated on slopes can be
  expected to move with the slope, if not deeply
  embedded

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• So called butt-bowed or pistol-butted tree
  trunks are common descriptors for tree trunks
  that exhibit a curvalinear form, due to down
  slope creep, as shown in these aspen trees near
  Dunckley Pass, CO, around el. 9500 ft.

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Slope creep accumulates upslope
Maximum creep

• Slope creep accumulates upslope as an integral
  function
• This shows flexural slope creep in the Vishnu
  Schist along Clear Creek, a tributary in the
  Grand a Canyons Granite Gorge

Mimimal creep
Creep increases upslope
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• Desiccation cracks forming more or less normal to
  the slope surface create zero tension boundaries,
  which allow for seasonal translation and
  bending, as sketched here

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• The annual or seasonal creep increment is caused
  by expansion and contraction of near surface
  soils, especially on sloping ground
• When soil expands, it heaves upward, more or less
  normal to the slope surface (expansion arrow).
• When the soil subsequently shrinks (contracts),
  it moves down, vertically (contraction arrow).
• The downslope offset between the beginning and
  ending points of a single cycle of heave and
  shrinkage is called the creep increment

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• As surface soils creep downslope, it may
  encounter a stiff or fixed structural element,
  such as a foundation. As the soil movement
  continues, passive soil pressures can develop
  against the embedded element, as shown in the
  inverted loading diagram at lower right.