3. PRECONSOLIDATION PRESSURE. This pressure value, P+c,, forms the
boundary between recompression and virgin compression ranges and is
approximately the maximum normal effective stress to which the material in
situ has been consolidated by a previous loading. Desicccation produces a
similar effect. The preconsolidation pressure cannot be determined
precisely, but can be estimated from consolidation tests on high quality
a. Graphical Determination. Estimate preconsolidation pressure from
semilogarithmic pressure-void ratio curve using the procedure given in the
central panel of Figure 2. Alternative methods are given in Reference 17,
Foundation Engineering, by Leonards, and Reference 18, The Undisturbed
Consolidation of Clay, by Schmertmann. Maximum test pressures should exceed
preconsolidation by an amount sufficient to define the slope of virgin
compression. Generally, this requires application of three or more load
increments exceeding the preconsolidation value.
b. Approximate Values. See Figure 3 for a relationship between
preconsolidation pressure and liquidity index. For samples with natural
moisture at the liquid limit (liquidity index of 1), preconsolidation ranges
between about 0.1 and 0.8 tsf depending on soil sensitivity. For natural
moisture at the plastic limit (liquidity index equal to zero),
preconsolidation ranges from about 12 to 25 tsf.
Alternately estimate: P+c, = 0.11 + 0.0037 PI in which q+u, is
the unconfined compressive strength, and PI is the soil plasticity index.
4. VIRGIN COMPRESSION. Virgin compression is deformation caused by loading
in the range of pressures exceeding that to which the sample has been
subjected in the past.
a. Compression Index. The semilogarithmic, pressure-void ratio curve
is roughly linear in the virgin range. The semilogarithmic, straight line
slope for virgin compression is expressed by the compression index C+c,.
(See Figure 2.)
b. Approximate Values. The compression index of silts, clays, and
organic soils has been correlated with the natural water content, initial
void ratio and the liquid limit. Approximate correlations are given in
Chapter 5. The approximate values of C+c, for uniform sands in the load
range of 1 to 4 tsf may vary from 0.05 to 0.06 (loose condition), and from
0.02 to 0.03 (dense condition).
5. RECOMPRESSION AND SWELL. Depending on the magnitude of
preconsolidation, pressures applied by new construction may lie partly or
wholly in the recompression range. If the load is decreased by excavation,
fine-grained soil will undergo a volumetric expansion in the stress range
a. Swelling Index. The slope of straight-line rebound of the
semilogarithmic pressure-void ratio curve is defined by C+s, (see Figure 2).
The swelling index is generally one-fifth to one-tenth of the compression
index except for soils with very high swell potential. For typical values
of C+s,, see Chapter 5.