Foundation, Concrete and Earthquake Engineering

Rainfall and Settlement Behavior of Foundation Soil, Loess

Whatever the internal structures loess soil show more or less collapse nature and threaten foundation safety. We know consolidation is associated with settlement. In this post we will learn about typical bearing capacity of loess in relation to dry density and rainfall induced failure of foundation on loess soil.

An annual average rainfall of a region has strong correlation with moisture content. Though water table rises with seasonal change, average moisture content depends on average rainfall for a year. The internal structure of loess such that rain water can do the harm quickly.

Accumulated rain water on loess soil of Tibetan Plateau
Recall the structure of loess soil; it is homogeneous, as usual for wind transported soil, have high void ratio and capillaries traverse in vertical direction which results fractures in sediments and vertical bluffs are formed.

Dear reader we have discussed void ratio and permeability of loess soil in previous posts. We are providing necessary links for convenience.

These loosely arranged structure of loess with numerous voids, allows rainwater to infiltrate quickly and we have known in previous post that moisture content increment alone can collapse in the presence of small consolidation pressure.

We know that horizontal permeability of loess soil is less than vertical permeability which is an exception in soil mechanics. The moisture content of loess soil (in-situ) lies between (4-49)%. Now we are going to summarize some findings about loess that correlates moisture content, density and bearing capacity of this soil.

Bandyopadhyay (1983), Holland and Gibbs (1960):

Saturated loess soil under rainfall
They provided settlement relation with density:

-Settlement is suspected to be large when dry unit weight is less than 80 pcf

-Settlement is suspected to be small when dry unit weight is greater than 90 pcf

From this finding Bandyopadhyay concluded that such soils that have low in-situ densities and also low clay cementation are considered high consolidation and collapse susceptibility.

A dry loess soil may have bearing capacity more than 10 Ksf which may fall down to 0.5 Ksf under saturation.

Now some points about moisture and density correlation:

Holland and Gibbs:

At moisture content less than 10 percent, dry density reached its maximum value and as discussed above highest resistance against settlement is expected.

Moisture content (10-15) % ---soil shows moderate high strength

Moisture content around 20% ---strength decreased gradually

Moisture content > 20%----this moisture content is considered high for loess soil that leads to full consolidation under load.

Moisture content 35%---soil becomes saturated.

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