Geotechnical Engineering

DHARM

248 GEOTECHNICAL ENGINEERING

Summary of Main Points

1. Specifically, the compressibility of a soil depends on the structural arrangement of the soil par-
   ticles. Grain-shape also influences this aspect.


2. Consolidation means expulsion of pore water from a saturated soil; it is relevant to clays and is
   a function of the effective stress rather than the total stress.


3. Oedometer or consolidometer is the device used for investigating the compressibility character-
   istics of a soil in the laboratory—both the total compression and its time-rate under specific
   pressure.


4. In a sand, the total or ultimate compression under the influence of a stress increment occurs
   very fast, almost instantaneously; however, in a clay it takes at least 24 hours or even much
   more.


5. The slope of the straight line portion of pressure (logarithmic scale) and void ratio (natural
   scale) is called the ‘compression index’.


6. Preconsolidation pressure or past maximum pressure is important since compression is very
   little until this pressure is reached. The concept is applicable primarily to field deposits but can
   also be applied to laboratory samples.


7. ‘Normally consolidated’ refers to a condition wherein the existing effective stress is the maxi-
   mum which the soil has ever been subjected to in its stress history; ‘overconsolidated’, on the
   other hand, refers to a condition wherein the present effective stress is smaller than the past
   maximum pressure.


8. The compression index, Cc, is related to the liquid limit, LL, as established by Skempton and his
   associates.


9. The total consolidation settlement is given by:

Sc = HC^0 ec

0

10 0

(^10)
()
log

• F +
  HG
  I
  KJ
  σσ
  σ
  ∆.

10. Terzaghi’s one-dimensional consolidation theory, expressed mathematically, states:

∂

∂

= ∂

∂

u

t

C u

v z

2

2 , where cv =

k

mvwγ , coefficient of consolidation.

11. Fitting methods are those used to compare laboratory time-compression curves with the theo-
    retical curve, with a view to evaluating the coefficient of consolidation.


12. Plastic readjustment of clay platelets and certain colloidal chemical processes lead to continued
    consolidation even after cent per cent dissipation of excess pore water pressure, which is termed
    ‘secondary consolidation’; however, it is very slow and negligible compared to primary consolida-
    tion.

References

1. A.S.K. Buisman: “Results of long duration settlement tests,” Proceedings, First International
   Conference on Soil Mechanics and Foundation Engineering, Combridge, Mass., USA, June, 1936.


2. A. Casagrande: “The Determination of the Pre-consolidation load and its practical significance,”
   Proceedings, First International Conference on Soil Mechanics and Foundation Engineering,
   Cambridge, Mass., USA, June, 1936.