us8 Parameter Definitions

Under Construction through March 2002: refer to Chris Jenkins with any queries

dbSEABED
Geoacoustic and Geotechnical Input Parameters

Contents

Vane Shear Strength Sensitivity Over Consolidation Ratio (OCR)
Atterberg Limits & Indexes
Unified Soil Classification System
Porosity, Void Ratio
Densities
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Vane Shear Strength

A vane shear apparatus measures the strain caused per rotational stress applied by a vane inserted into the sediment. The vane is usually inserted normal to bedding. Vane shear devices range in size from desktop ('miniature-vane-shear apparatus'; the most common; 3-20mm blade sizes) to barged (1m blade). Note procedure ASTM (1997a, 1997b) Standard D 4648; the vane height and diameter and rotation rate (about 90°/min) should be noted in metadata. The parameter "Vane Shear Strength" usually refers to the peak strength (see figure); the remolded vane shear strength refers to the final strength (see figure).

Unconfined shear strength can be calculated from Vane Shear Strength measurements. The results are valid only for cohesive sediments of grainsize << vane size.

Sensitivity (St): "the effect of remolding on the shear strength of an undrained cohesive soil." (CanSIS).
It is calculated as the intact vane shear strength/remolded vane shear strength. High values indicate that the sediment classifies as “sensitive” (Bowles, 1979)". [Quote from: Winters, 2000]

Not to be confused with "soil sensitivity": a soil's general tendency to allow a chemical to be transported through the soil to ground-water (OSU Extension Soil Sensitivity Database).

Over Consolidation Ratio (OCR)

Overconsolidation means that a soil deposit "has been subjected to an effective pressure greater than the present overburden pressure". (CanSIS). The OCR can be used to qualitatively interpret stress history.

One form is calculated as:

where S_u the (vane-shear or pocket-penetrometer) strength, is divided by the cumulative product of submerged unit weight, W_s, and sub-bottom depth, d.

The more strictly correct form is calculated using the preconsolidation stress, P'c:

Pc can be "determined from consolidation test results that have a straight virgin line by the Casagrande (1936) graphical technique and is assumed to equal the in situ maximum past stress." [Quote from: Winters, 2000]

Atterberg Limits (LL, PL) and Indexes (LI, PI)

The Atterberg Limits and associated indexes describe a material's geotechnical behaviour in terms of the water content.

Parameter	Meaning	Method of determination
Liquid Limit (LL, wL)	"1. The water content corresponding to an arbitrary limit between the liquid and plastic states of consistence of a soil." (CanSIS).	2. The water content at which a pat of soil, cut by a standard-sized groove, will flow together for a distance of 12 mm under the impact of 25 blows in a standard liquid-limit apparatus. " (CanSIS).
Plastic Limit (PL, wP)	"1.The water content corresponding to an arbitrary limit between the plastic and the semisolid states of consistence of a soil. (CanSIS)	2.The water content at which a soil will just begin to crumble when rolled into a thread approximately 3 mm in diameter. " (CanSIS)
Plasticity index (PI, IP)		PI= wL - wP
Liquidity Index (LI, IL)		LI = (w-wP)/PI = (wc -wP )/(wL -wP ).

Observed, linked forms of behaviour include the following:

Viscosity or brittleness	IL<0 and w<PL 0<IL<1 and PL<w<LL IL>1 and w>LL	brittle, stiff, high strength plastic, lower stiffness, lower strength viscous liquid or quick clay
Sensitivity	w > wL or LI > 1	ultra sensitive (quick)
Plasticity chart	For a given water content PI increases with % clay (though relationship depends on clay minerals present). The A -line: above = Clays; below = Silts/Organic Soil (From Winters 2000)
Activity	Is the slope of the plot of PI vs % Clay
Plasticity behaviour	LL > 50 LL < 50	High plasticity Low plasticity

Unified Soil Classification System

Specific Criteria for Unified Classification System

   1. Size criterea:
        < 50% passes # 200 - Coarse grained
        > 50% passes # 200 - Fine grained
        % Fines > 12%         - Clayey or Silty
        Well graded, Poorly graded, % of fines <5%

    For Sand, Gravel
                    Use:
                        Cu = D60 / D10
                        Cc = (D30)*2 / (D10.D60)

Archie Exponent

The 'm' exponent in the empirical relationship between (i) formation resistivity factor (F) and (ii) porosity (phi):

The exponent reflects the properties of the rigid rock which influence the behaviour of electrolytic pore fluids, for instance the cementation, pore shape.

Porosity, Void Ratio, Moisture Content

Porosity (n) is the percent of voidspace in total volume. Porosity is dimensionless, and in dbSEABED is always expressed in percent.
Void Ratio (e) is ratio of voidspace to total solids volume, and is dimensionless.

In these definitions, Vv, Vw and Vs are the void, water and solid volumes; Ww and Ws are the water and solid weights; V and W are the sample bulk volumes and weights. These relations hold for non-gassy materials.

Porosity	Void Ratio	Water Content
Ratio of void volume to total volume	Ratio of void volume to solids volume	Ratio of pore water weight to solids weight
n = Vv / V	e = Vv / Vs	w = Ww / Ws

(Hein, F.J. The need for grain size analyses in geotechnical studies; In Syvitski, J.P. [Ed.], 1991).

For reference, conversions between these parameters.

Densities > Void Ratio	Porosity > Void Ratio
n = (Dg-Ds) / (Dg-Dw) * 100.	e = n / (1 - n)

Where Ds, Dg and Dw are the sample (bulk), grain, pore water densities; n is the fractional (not percentage) porosity:

Densities

Here, RHOw and RHOs water and solid component denstries. The following relations hold for non-gassy materials.

Bulk Density (Saturated)
Ratio of void volume to total volume
RHOsat = nRHOw +(1-n)RHOs

(Hein, F.J. The need for grain size analyses in geotechnical studies; In Syvitski, J.P. [Ed.], 1991).
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Chris Jenkins (Email)
INSTAAR, University of Colorado
5-Feb-2002