Soils 205- General Soils

Lecture 6- Engineering uses of soils

I.  Properties relevant to engineering uses

    a.  strength- measure of the capacity of a soil mass to withstand stresses without rupturing or    becoming deformed.  Failure- occurs when weight exceeds the bearing strength of a soil.  Causes buildings to topple and failure of earthen dams.

        i.  Components of soil strength

            - in cohesive (>15% clay) soils

                1)

                2)

                3)

                What happens when cohesive soils wet up?

            -in non cohesive (<15% clay) soils

                1)

                2)

small amounts of water increase strength, but as sandy soils continue to wet up, thick water films develop and the strength decreases.

        ii.  collapsible soils- soils that exhibit strength at low water contents, but fail suddenly when they become wet.

       These soils undergo thixotrophy which is the liquification of a wet soil mass subject to vibrations (earthquakes, blasting).  Usually these soils consist of sand grains cemented by small amounts of gypsum or clay.  As they wet up the water dissolves gypsum and disperses clay, decreasing soil strength.

        iii.  Measurement of soil strength

                        Compression test-

                           Angle of repose-

                            Steepest angle to which sand can be piled without slumping

                            Smooth rounded grains < rough, interlocking grains

    b.  compressibility- extent to which a soil's volume is reduced by a given applied force

        Sandy soils-

        Uneven settlement can crack foundations.  A famous example of uneven settlement is the Tower of Pisa.

    c.  Atterberg limits- describes how soil behaves with increasing water content.

            Plasticity index (PI)-

            PI = liquid limit (LL) - plastic limit (PL)

II.  Expansive soils

        - soils that are rich in clays (smectites) that swell when wet; 20% of US

        - $4 billion in damages/year

        -Mostly in regions with long dry periods alternating with rain (CA, TX, WY)

       a.  Measurement of shrink-swell potential

            COLE-

            =

            LM = length of soil bar, moist

            LD = length of soil bar, dry