PRECONSOLIDATION PRESSURE.  This pressure value, P+c,, forms theboundary between recompression and virgin compression ranges and isapproximately the maximum normal effective stress to which the material insitu has been consolidated by a previous loading.  Desicccation produces asimilar effect.  The preconsolidation pressure cannot be determinedprecisely, but can be estimated from consolidation tests on high qualityundisturbed samples.

a.  Graphical Determination.  Estimate preconsolidation pressure fromsemilogarithmic pressure-void ratio curve using the procedure given in thecentral panel of Figure 2.  Alternative methods are given in Reference 17,Foundation Engineering, by Leonards, and Reference 18, The UndisturbedConsolidation of Clay, by Schmer tmann .  Maximum test pressures should exceedpreconsolidation by an amount sufficient to define the slope of virgincompression.  Generally, this requires application of three or more loadincrements exceeding the preconsolidation value.

b.  Approximate Values.  See Figure 3 for a relationship betweenpreconsolidation pressure and liquidity index.  For samples with naturalmoisture at the liquid limit (liquidity index of 1), preconsolidation rangesbetween about 0.1 and 0.8 tsf depending on soil sensitivity.  For naturalmoisture at the plastic limit (liquidity index equal to zero),preconsolidation ranges from about 12 to 25 tsf.q+u,/2

))))))))))))))))Alternately estimate:  P+c, = 0.11 + 0.0037 PI  in which q+u, isthe unconfined compressive strength, and PI is the soil plasticity index.

4.  VIRGIN COMPRESSION.  Virgin compression is deformation caused by loadingin the range of pressures exceeding that to which the sample has beensubjected in the past.a.  Compression Index.  The semilogarithmic, pressure-void ratio curveis roughly linear in the virgin range.  The semilogarithmic, straight lineslope for virgin compression is expressed by the compression index C+c,.(See Figure 2.)b.  Approximate Values.  The compression index of silts, clays, andorganic soils has been correlated with the natural water content, initialvoid ratio and the liquid limit.  Approximate correlations are given inChapter 5.  The approximate values of C+c, for uniform sands in the loadrange of 1 to 4 tsf may vary from 0.05 to 0.06 (loose condition), and from0.02 to 0.03 (dense condition).5.  RECOMPRESSION AND SWELL.  Depending on the magnitude ofpreconsolidation, pressures applied by new construction may lie partly orwholly in the recompression range.  If the load is decreased by excavation,fine-grained soil will undergo a volumetric expansion in the stress range

below preconsolidation.a.  Swelling Index.  The slope of straight-line rebound of thesemilogarithmic pressure-void ratio curve is defined by C+s, (see Figure 2).The swelling index is generally one-fifth to one-tenth of the compressionindex except for soils with very high swell potential.  For typical valuesof C+s,, see Chapter 5.7.1-141