A function to determine the preconsolidation stress (σ_P). It is a parameter obtained from
the soil compression curve and has been used as an indicator of soil loadbearing capacity as well as to
characterize the impacts suffered by the use of machines.
The function sigmaP()
contains implementations of the main methods for determining the preconsolidation
stress, such as the Casagrande method, the method of Pacheco Silva, the regression methods and the method of the
virgin compression line intercept.
1 2 3 
voidratio 
a numeric vector containing void ratio (or bulk density) values. 
stress 
a numeric vector containing the applied stress sequence. 
n4VCL 
the number of points for calculating the slope of the soil Virgin Compression Line (VCL), which is obtained by linear regression. 
method 
a character indicating which method is to be computed; one of the following:

mcp 
the maximum curvature point in log10 scale of 
graph 
logical; if TRUE (default) the compression curve is plotted. 
... 
further graphical arguments. 
casagrande
is the method proposed by Casagrande (1936). The preconsolidation stress obtained via VCLzero
corresponds
to the intersection of the soil Virgin Compression Line (VCL) with the xaxis at zero applied stress, as described by
Arvidsson & Keller (2004). reg1
, reg2
, reg3
and reg4
are regression methods that obtain the preconsolidation
stress value as the intercept of the VCL and a regression line fitted with the first two, three, four and five points of the curve, respectively,
as described by Dias Junior & Pierce (1995). pacheco
is the method of Pacheco Silva (ABNT, 1990).
You may follow the flowchart below to understand the determination of the preconsolidation stress through sigmaP()
.
A list of
sigmaP 
the preconsolidation stress. 
method 
the method used as argument. 
mcp 
the maximum curvature point in log10 scale of 
CI 
the compression index. 
SI 
the swelling index. 
Anderson Rodrigo da Silva <anderson.agro@hotmail.com>
ABNT  Associacao Brasileira de Normas Tecnicas. (1990). Ensaio de adensamento unidimensional: NBR 12007. Rio de Janeiro. 13p.
Arvidsson, J.; Keller, T. (2004). Soil precompression stress I. A survey of Swedish arable soils. Soil & Tillage Research, 77:8595.
Bowles, J. A. (1986). Engineering Properties of Soils and their Measurements, 3rd edition. McGrawHill Book Company, Inc. NY, 218pp.
Casagrande, A. (1936). The determination of the preconsolidation load and its practical significance. In: Proceedings of the International Conference on Soil Mech. and Found. Eng. (ICSMFE), Cambridge, MA, 2226 June 1936, vol. 3. Harvard University, Cambridge, MA, USA, pp. 6064.
Dias Junior, M. S.; Pierce, F. J. (1995). A simple procedure for estimating preconsolidation pressure from soil compression curves. Soil Technology, 8:139151.
voidratio
, maxcurv
, fitlbc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19  pres < c(1, 12.5, 25, 50, 100, 200, 400, 800, 1600)
VR < c(0.846, 0.829, 0.820, 0.802, 0.767, 0.717, 0.660, 0.595, 0.532)
plot(VR ~ log10(pres), type = "b") # find the 'mcp'
sigmaP(VR, pres, method = "casagrande", mcp = 1.6, n4VCL = 2)
# fitting the VCL
sigmaP(VR, pres, method = "casagrande", mcp = 1.6, n4VCL = 3)
# selfcalculation of "mcp" argument for Casagrande method
sigmaP(VR, pres, method = "casagrande", n4VCL = 3)
# Pacheco method
sigmaP(VR, pres, method = "pacheco")
# Regression method
sigmaP(VR, pres, method = "reg3")
# End (not run)

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