Sediment Strength: User Guide

User Guide Information

User Guide Contents

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Introduction

Shear strength represents the resistance of a soil to failure in shear, a concept required in analysis for the stability of soil masses. If at a point on any plane within a soil mass the shear stress becomes equal to the shear strength of the soil, then failure will occur at that point, where material behavior changes from linearly elastic to perfectly plastic. Since shear stress in a soil can be resisted only by the skeleton of solid particles, shear strength (where the material yields: ¿t_f) should be expressed as a function of effective normal stress at failure (σ_f'’), where c' ’ is the effective cohesion intercept (stress-dependent component) and ¿' q’ is the effective angle of shearing resistance or the internal angle of friction (stress-independent component), respectively: ¿_f

tf =

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c¢ + s¢f tanf¢.

Failure will therefore occur at any point in the soil where a critical combination of shear stress and effective normal stress develops.

Marine sediments exhibit similar behavior in that the stress decreases as the sediment is strained beyond a peak stress. The effective cohesion intercept and the effective angle of shearing resistance are mathematical constants defining a linear relationship between shear strength and effective normal stress. Shearing resistance is created by interparticle forces; therefore, if effective normal stress = 0 then shearing resistance must be 0 (unless there is cementation between the particles) and c' ’ = 0.

States of stress in two dimensions can be represented on a plot of shear stress (τ) against effective normal stress (σ'σ’). A stress state can be represented either by a point with coordinates τ and σ' σ’ or by a Mohr circle defined by the effective principal stresses σ₁' ’ and σ₃' ’ (see Figure 1). The line through the stress points or the line tangent to the Mohr circle may be straight or slightly curved and is referred to as the failure envelope. The Mohr failure hypothesis states that the point of tangency of the Mohr failure envelope with the Mohr circle at failure determines the inclination of the failure plane. A state of stress represented by a stress point that plots above the failure envelope or by a part of the Mohr circle above the envelope is impossible.

There are two methods of specifying shear strength parameters:

1. The envelope is represented by the straight line defined by equation 1, from which the parameters c' ’and ¿' Φ’ are found. These are referred to as the tangent parameters and are only valid over a limited stress range. If the failure envelope is slightly curved the parameters are obtained from a straight line approximation to the curve over the stress range of interest. The use of tangent parameters does not infer that the shear strength is c' ’at zero effective normal stress.
2. A straight line is drawn between a particular stress point and the origin or a line is drawn through the origin and tangential to a particular Mohr circle. The parameter c' ’  = 0 and the slope of the line gives ¿¿Φ', the shear strength equation being

τ_f =  tanf¢.

The angle f¢ found in this way is referred to as a secant parameter and is valid only for one particular stress state. The value of secant (f¢) is generally the highest expected value of effective normal stress Wiki Markup¿{~}f~ = ¿¿{~}f~ tan¿¿. The angle ¿¿ found in this way is referred to as a secant parameter and is valid only for one particular stress state. The value of secant (¿¿) is generally the highest expected value of effective normal stress (i.e., the lowest value of the parameter for the stress range of interest). <ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="657c49b5-386e-421c-b297-d22db18f215b"><ac:parameter ac:name="">RTF38303032383a204669675469</ac:parameter></ac:structured-macro> !worddavc86152ccd71e8ce7e638f8d633a75ad5.png|height=257,width=384! <ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="9d4ea534-9201-4530-bc5e-8f252c920b20"><ac:parameter ac:name="">_Ref302564515</ac:parameter></ac:structured-macro>{*}Figure 1. Mohr-Coulomb Failure Criterion.* \\ The relationship between the shear strength parameters and the effective principal stress at failure at a particular point can be deduced. The general case w/{_}c{_}' > 0 is shown in <span style="color: #000d94"><strong><em>Figure 1</em></strong></span>, compressive strength taken as positive. The coordinates of the tangent point are (¿{~}f{~},¿¿{~}f{~}) where ¿{~}f~ = \[(¿¿{~}1~ - ¿¿{~}3{~})sin(2¿¿\]/2, and ¿¿{~}f~ = \[(¿¿{~}1~ + ¿¿{~}3{~})/2\] + \[(¿¿{~}1~ - ¿¿{~}3{~})cos(2¿¿¿/2 and is the theoretical angle between the major principal plane and the plane of failure. The Mohr-Coulomb strength criterion is the combination Mohr failure envelope, approximated by linear intervals over certain stress ranges, and the Coulomb strength parameters. Since ¿ = 45° + (¿¿/2), and sin(¿¿) = \[(¿¿{~}1~ - ¿¿{~}3{~})/2\]/\[c¿cot(¿¿) + (¿¿{~}1~ + ¿¿{~}3{~})/2\]; therefore, (¿¿{~}1~ - ¿¿{~}3{~}) = (¿¿{~}1~ + ¿¿{~}3{~})sin(¿¿) + 2c¿cos(¿¿), or ¿¿{~}1~ = ¿¿{~}3~ tan{^}2{^}\[45° + ¿¿¿/2)\] + 2c¿tan\[45° + ¿¿¿/2)\]. This equation is known as the Mohr-Coulomb failure criterion. When a material is sheared under a load or applied stress, excess pore water pressure is produced that may or may not escape depending on the permeability of the material and the time available. In the undrained shear scenario, volume changes translate into pore pressure changes and the assumption is made that the pore pressure and therefore the effective stress (¿¿ = ¿ - _u{_}) are identical to those in the field. The total, or the undrained, shear strength is used for stress analysis. Tests must be conducted rapidly enough so that undrained conditions prevail if draining is possible in the experimental setup. In the drained scenario, shear stress is used in terms of effective stresses. The excess hydrostatic pressure must be measured or estimated. Knowing the initial and the applied (total) stresses, the effective stress acting in the sediment can be calculated. This approach is philosophically more satisfying because pore water cannot carry any shear stress (i.e., shear strength is thought to be controlled by the effective stresses). Drained shear can ordinarily be determined only in the laboratory and the procedure is not popular because there are serious practical problems. Particularly in low-permeability material, the rate of loading must be sufficiently slow to avoid the development of excessive pore pressure, which can cause a test to take many days or weeks. For a given state of stress it is apparent that, because ¿¿{~}1~ = ¿{~}1~ - _u_ and ¿¿{~}3~ = ¿{~}3~ - _u{_}, the Mohr circles for total and effective stresses have the same diameter but their centers are separated by the corresponding pore water pressure _u{_}. Similarly, total and effective stress points are separated by the value of <span style="color: #5a5a5a"><strong><em>u</em></strong></span>. The shear strength parameters for a particular soil can be determined by means of laboratory tests on specimens taken from representative samples of the in situ soil. These tests should only be used as guides because there are many reasons why the results are only approximate. Particularly, the influence of pore pressure changes during the undrained experiment cannot be estimated.interest). 

Sediment Strength Measurement

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Shear measurements can be made using either the Giesa AVS or the Torvane. The AVS is controlled via Giesa's proprietary software (GeoLAB). The Megauploadatron (MUT) program is used to upload AVS results to the LIMS database. Torvane results are entered via the Java program PenStrength, which transfers the data automatically toLIMS. IODP x, y, and z-axis conventions are shown in Figure 2.

Compressive Strength

Compressive measurements are made with the pocket penetrometer, after which results are entered via the Java program PenStrength, which transfers the data automatically to LIMS.

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The AVS test is used for in situ determination of the undrained strength of intact, fully saturated clays (undrained strengths < 100 kN/m²); the test is not suitable for other types of soil or if the clay contains sand or silt laminations.
A four-bladed vane is inserted into the split core and rotated at a constant rate to determine the torque required to cause a cylindrical surface (with a diameter equal to the overall width of the vane) to be sheared by the vane. This destructive measurement is done in the working half, with the rotation axis parallel to the bedding plane. The torque required to shear the sediment along the vertical and horizontal edges of the vane is a relatively direct measure of the shear strength. Typical sampling rates are one per core section until the sediment becomes too firm for insertion of the vane.
The rate of rotation of the vane should be within the range of 6°–12°/min.
Clays may be classified on the basis of undrained shear strength as shown below. The GeoLab software that controls the AVS calculates shear stress as follows:
Stress (kN/m²) = torque (Nm) × Vane constant (1/m³) × 1/1000.


Undrained shear strength of clays are as follows:

AVS Hardware

The Giesa AVS consists of the following hardware components:

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FL2SS Controller with Step-Down Converter

Vane Blades

Figure 6. Vane Blades.

AVS Specifications

Specification	Description
Torque limit (Nm)	–1.5 to ~2.5 (only forward turning to ~3.4)
Torque measurement:	2 analog force sensors
Measurement range	~1000 stage partitions/sensor
Event timing (ms)	100
Total torque	Sum of individual sensor standardized values
Dissolution (Nm)	0.001 ± 0.3% of final value
Angle measurement:	Digital stepper (IGR) in motor
Accuracy	Dependent on flexural rigidity of test rack and force sensors
Dissolution	~0.04°
Rotation measurement:	2 encoders: rotation of applied stress and rotation of vane
Rotation limit (°/s)	0–20

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Figure 7. Giesa GeoLAB Main Control Screen.

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Figure 8. Selecting Giesa Main Control Template.

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Figure 9. Giesa Excel Main Control Template.

Measuring Strength Using the AVS

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Strength limits of the Torvane adapters are as follows:

Vane Diameter(mm)	Vane Identifier	Vane Height (mm)	Maximum shear strength(kPa)
19	2.5	3	250
25	1.0	5	100
48	0.2	5	20

Torvane Hardware

–Part number EI26-2261: includes driver, 3 interchangeable vanes, and carrying case
–Scale: 1 kg/cm² × 0.1 kg/cm² divisions
–Vane driver: 1.6 inch (41 mm) diam. × 3.2 inch (81 mm) with vane attached
–Vanes:

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–Part number: EI29-3729
–Scale divisions: 0.25 kg/cm²
–Load piston: ¼ inch (6 mm) diameter stainless steel
–Dimensions: ¾ inch diameter × 6-3/8 inch long (19 mm × 162 mm)
–Weight: 7 oz (198 g)
–1 inch adapter foot: P/N EI29-3729/10

Measurement Specifications

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Sample and Analysis Attributes and Components

Analysis	Component	Unit	Explanation
VANE_SHEAR	offset	cm	Offset from top of section; location of measurement in section
	penetration_direction into X into Y into Z into –Z		x-, y-, or z-axis measurement
	rotation_rate	deg/s	Geisa vane rotation rate
	shear_strength	Pascal
	residual_strength	Pascal
	max_torque_angle	°circ
	residual_torque_angle	°circ
	comment		From AVS data entry form
PENETRATE	offset	cm	Offset from top of section; location of measurement in section
	penetration_direction into X into Y into Z into –Z		x-, y-, or z-axis measurement
	adapter_foot yes (1/16) no (1)		Was adapter foot used for very soft sediment?
	compression_strength	kg/cm2	Converted result
	comment		From PenStrength data entry form
TOR_SHEAR	offset	cm	Offset from top of section; location of measurement in section
	penetration_direction into X into Y into Z into –Z		x-, y-, or z-axis measurement
	adapter_id regular (1.0) large (0.2) small (2.5)		Which blade was used on the device?
	shear_strength	kg/cm2	Converted result
	comment		From PenStrength data entry form

Viewing Logfile

At any time you may peruse the logfile. Click on the view logfile button from the AVS main control (Excel) program.
All actions and results are entered in this file. For example, if there is a problem connecting to the LIMS database, you can just go to this log to look at your results.

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Troubleshooting

AVS Status Displays

Status message	Meaning
ready for use	FL2 is not busy; press START to begin a test
attempt runs	Manually initiated test; terminate using the STOP button
equipment disturbed	Error detected; see below
ST attempt runs	PC-initated test; START and STOP buttons are deactivated

AVS Error Displays

When the AVS controller detects an error, the NET light on the front of the controller flashes and an error message displays.

Error message	Explanation	Action
Storage error	Internal memory error	Requires electronics repair
Maximum strength	At least 1 internal force sensor has reached maximum value; further testing may damage force sensor	Terminate test and restart with a smaller vane
Overflow condition	Vane rotated > 8000°	Add 8000° to final result for this test
Force sensor is missing	–A force sensor is not connected –Force sensors have been switched	Check cable connections at back of FLSS

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Giesa Installation Information

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