LITHOMORPHIC STRESSES AND CLEAVAGE OF LOESS

Lateral stresses existing in soil deposits are important clues to rece nt stress history because they tend to remain ''locked in'' following adjustments in the overburden load: Thus a high lateral stress is part ly retained following a reduction in vertical stress, as by glacial me lting or erosion. Conversely, during loess deposition a low lateral st ress should be retained if the soil structure is preserved under incre asing vertical stress. This has not previously been verified in the fi eld because of the difficulty of measuring lateral stresses in situ in an hydrocollapsible material. Tests were conducted at two sites in Mi ssouri river valley loess using the K(o) Stepped Blade, which uses dat a extrapolation to give stress on a zero thickness blade. In basal loe ss where the moisture content was at or above the liquid limit, K(o) = 1.0 +/- 0.02. In the upper to middle loess zone K(o) is as low as 0.2 to 0.4, lower than the values of 0.4 to 0.5 that would be expected if the soil were normally consolidated. Near the ground surface, lateral stresses are higher so K(o) > 1, attributed to post-depositional weat hering and the presence of smectite clay minerals. The vertical sequen ce of lateral stress from high to low to high again should contribute a tension-induced cleavage if lateral confinement is removed by excava tion or erosion. This stress condition is most pronounced in loess clo se to its source, where the potential tension zone was found to extend to a depth of about 7 m compared to 4 m at the other site. Loess clea vage appears to result from stress relief and is not a unique directio nal property of this material.