Part 2Part 2

Expansive Soils Expansive Soils

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Slope CreepSlope Creep

Butt-bowed trees in northern CaliforniaButt-bowed trees in northern California

Many natural slopes are blanketed by Many natural slopes are blanketed by residual residual soilssoils which are which are expansiveexpansive

These materials These materials swellswell and and shrinkshrink with with seasonal regularity, during alternating wet seasonal regularity, during alternating wet and dry seasonsand dry seasons

This cyclic movement allows for This cyclic movement allows for plastic creepplastic creep, , or strain under sustained loading.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 “So called “butt-bowedbutt-bowed” or “” or “pistol-buttedpistol-butted” tree trunks ” tree trunks are common descriptors for tree trunks that exhibit a are common descriptors for tree trunks that exhibit a curvalinear form, due to curvalinear form, due to down slope creepdown slope creep, as shown , as shown in these aspen trees near Dunckley Pass, CO, around in these aspen trees near Dunckley Pass, CO, around el. 9500 ft.el. 9500 ft.

Slope creep Slope creep accumulates accumulates

upslopeupslope

Slope creep Slope creep accumulates upslope accumulates upslope as an integral as an integral functionfunction

This shows flexural This shows flexural slope creep in the slope creep in the Vishnu Schist along Vishnu Schist along Clear Creek, a Clear Creek, a tributary in the Grand tributary in the Grand a Canyon’s Granite a Canyon’s Granite Gorge Gorge

Mimimal creepMimimal creep

Maximum creepMaximum creep

Creep increases upslopeCreep 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 seasonalThe annual or seasonal creep increment creep increment is caused by is caused by expansion and contraction of near surface soils, expansion and contraction of near surface soils, especially on sloping groundespecially on sloping ground

When soil When soil expandsexpands, it heaves upward, more or less , it heaves upward, more or less normal to the slope surface (expansion arrow).normal to the slope surface (expansion arrow).

When the soil subsequently When the soil subsequently shrinksshrinks (contracts), it (contracts), it moves down, vertically (contraction arrow). moves down, vertically (contraction arrow).

The downslope offset between the beginning and The downslope offset between the beginning and ending points of a single cycle of heave and shrinkage ending points of a single cycle of heave and shrinkage is called the creep incrementis called the creep increment

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