PARTICLE BREAKDOWN IN COLLAPSIBLE LOESS DEPOSITS IN SE IDAHO

Loess deposits from the Snake River plain blanket large areas of southeastern Idaho. The loess is collapsible and typically undergoes a 0.9 to 2.2% vertical strain when saturated in a consolidometer at 2 ksf. The process of collapse is generally tied to a reduction in void ratio from an initial open work structure to a more stable configuration.

One of the secondary features of SE Idaho loess that can contribute to the collapse potential and cause an increased fines content in compacted loessal soils is breakdown of the sand size particles. This behavior was discovered in a series of mechanical gradation tests in which the mass on an individual sieves did not stabilize after three-five minute cycles of agitation. The greatest breakdown (10.9%) occurred in the #100 size fraction. On the #200 sieve, the mass of the sand increased and decreased during the second and third cycles as fragments accumulated and fell into the pan. The percent fines (-#200) increased 9.2%, whereas there was no change in mass on the #10 sieve. A reduction in grain size during multiple cycles of mechanical gradation tests is a good index of the breakdown characteristics in the coarse fraction of soils.

Petrographic study of the sand fractions reveals sharp angular fragments (shards) and agglomerated particles which readily break down under slight finger pressure. The shards and agglomerated particles are most frequent in the #40 to #200 size fractions. The geometry of the sand particles in the #10 and #20 size fractions are typically prismatic and thus are much less susceptible to breakdown.

Attrition tests performed on a washed sand fraction also show a large percentage breakdown. The attrition test is carried out by performing mechanical gradation tests on the sand fraction before and then after immersing the sample in water and using a soil blender to agitate the slurry. The blender is the standard equipment used in preparation of a soil for a hydrometer test (ASTM D-422). The tests show that the southeastern Idaho loess is susceptible to breakdown. This feature is significant in soil collapse beneath foundations and in reducing the permeability of loess used in compacted fill.