Part 2 Expansive Soils Promote Slope Creep Butt-bowed trees in northern California

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. Cut slope along Pleasant Hill Rd in Martinez, CA in 1954 (left) and 30 years later, in 1984 (at right).

 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 Slope creep sketched by Sharpe (1938) Apparent creep of a cut slope in Lafayette, CA

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 ft.

Slope creep accumulates upslope 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 Canyon’s Granite Gorge Mimimal creep Maximum creep Creep increases upslope

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

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

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.