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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: τ_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 Φ' is the effective angle of shearing resistance or the internal angle of friction (stress-independent component), respectively:

τ_f =  c' + σ'_ftanΦ'.

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

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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’ 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.

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1. The envelope is represented by the straight line defined by equation 1, from which the parameters c’' and Φ’ 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

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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 Figure 1, compressive strength taken as positive. The coordinates of the tangent point are (τ_f,σ'_f) where

τ_f = [(σ'₁ – σ'₃)sin(2θ)]/2, and

s¢f σ'_f = [(s¢1 + s¢3σ'₁ + σ'₃)/2] + [(s¢1 – s¢3σ'₁ – σ'₃)cos(2q2θ)]/2

andis 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

q θ = 45° + (q¢θ'/2), and

sin(q¢θ') = [(s¢1 – s¢3σ'₁ – σ'₃)/2]/[c¢cot(q¢c'cot(θ') + (s¢1 + s¢3σ'₁ + σ'₃)/2]; therefore,

(s¢1 – s¢3σ'₁ – σ'₃) = (s¢1 + s¢3σ'₁ + σ'₃)sin(q¢θ') + 2c¢cos(q¢2c'cos(θ'), or

s¢1 = s¢3 tan2σ'₁ = σ'₃ tan²[45° + (q¢θ'/2)] + 2c¢tan2c'tan[45° + (q¢θ'/2)].

This equation is known as the Mohr-Coulomb failure criterion.

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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 (s¢ = s – σ' =σ – 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.

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For a given state of stress it is apparent that, because s¢1 = s1 σ'₁ = σ₁ – u and s¢3 = s3 σ'₃ = σ₃ – 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 u.

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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 toLIMSto LIMS. IODP x, y, and z-axis conventions are shown in Figure 2.

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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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Sample preparation

There is no sample preparation required for these tests; however, the test destroys the sample so approval must be granted by the curator.

• Choose a test location with care to avoid particles that would influence the reading.
• Avoid obviously disturbed areas.
• For saturated cohesive sediments, take the reading in a fresh cut surface.
• Rapid drying will greatly influence the reading.

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Figure 2. Core x-, y-, and z-directions.

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Giesa AVS

Automated Vane Shear (AVS) Test

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: AnchorRTF35303433353a205461626c65RTF35303433353a205461626c65

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The vane blade is manually raised and lowered using the crank handle at the top of the test rack. The vane blade attaches to a keyless chuck.

Anchor_Ref302565771_Ref302565771Figure 3. Giesa FL2 Frame with Blade.

FL2SS Controller with Step-Down Converter

Vane Blades

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_Ref302565793_Ref302565793Figure 6. Vane Blades.

AVS Specifications

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Software

The AVS system is controlled by a PC with Windows environment that has the following software installed to run and upload AVS measurements:

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• Vane blade ID (vane type)
• Rotation rate (green "speed level" cell)
• Expedition, Site, Hole, etc.
• Start with a rotation time of 15°/s.

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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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Anchor_Ref302565886_Ref302565886Figure 9. Giesa Excel Main Control Template.

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1. Choose an appropriate test location on the sample (see Sample preparation).
2. Measure the offset from the top of the sample section in centimeters and record in the logsheet, along with section text ID and whether the adapter foot is being used.
3. Insert the vane manually (by turning handle on top of the Giesa) into the material until it is completely immersed (critical for accuracy and precision) in the sediment.

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Important! If the brass spacer begins to extend on the top of the frame, the sediment is too hard for the Giesa AVS; discontinue the measurement.

1. Select init on the Giesa GeoLAB main control screen to open a monitoring screen that shows near real-time values of torque and angle of rotation (see Figure 10).
2. Set the sampling frequency in the sample-time field; a fair estimate is 1 measurement every 2 s.
3. Click on the traffic light button to begin the test.
4. Go to the Excel template screen to monitor the test in progress. When the maximum shear strength of the sediment has been reached, stop the test by clicking on the red stop-light button.
5. Record the maximum torque and angle results on the logsheet.
6. To save the results, go to the Excel template:

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1. • Select File > Save As >
   • Save to location: c:\data\strength\in
   • Name: text ID{_}offset_AVS.csv
   • Save as type: CSV (comma delimited)
2. Close Excel after saving the file.

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Figure 10. Giesa Test Parameters and GeoLab "init" Window.

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All data is moved from the C:/Data/In to C:Data/Archive folder when uploaded correctly. If there is an error with the file it will be moved to the C:/Data/Error folder.
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Figure 11. Logging into MUT Uploader.
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Anchor_Ref302565935_Ref302565935Figure 12. MUT upload screen. Green check means file is in correct format for uploading to LIMS.
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Giesa Vendor Contact

Giesa mbH, Heinrich-Heine-Straße 16
01723 Wilsdruff
T : +49 (0) 35204 40602, F : +49 (0) 35204 40604
mail@giesa.de, www.giesa.de

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Torvane

The Torvane shear device (ELE International model 26-2261; Figure 13) enables the user to quickly determine the shear strength of cohesive soils. All that is required is a relatively flat sample area at least 2 inches (50 mm) in diameter. The device has a stress range of 0–2.5 kg/cm² (tons/ft²), the approximate range of torque that can be easily applied by the fingers. The dial head is equipped with a mechanism to hold the maximum reading after release. The smallest division on the dial is 0.1 kg/cm², permitting visual interpolation to the nearest 0.05 kg/cm².
The Torvane is most accurate in fully saturated cohesive soils whose undrained strength is independent of normal pressure. The stress range permits it to be used for very soft to stiff clays. Extensive laboratory testing indicates excellent agreement in homogeneous clays between the unconfined compression test and the Torvane. The shear strength of a cohesive soil is dependent upon many factors, including rate of loading, progressive failure, orientation of the failure plane, and pore water migration during testing. The Torvane does not eliminate the effects of any of the variables. AnchorRTF37323937323a204669675469
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Figure 13. Torvane Shear Device.
Three sizes of vanes are used with the Torvane:

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Strength limits of the Torvane adapters are as follows:

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1. Choose an appropriate test location on the sample.
2. Measure the offset from the top of the section (cm) and record in the logsheet, along with the section text ID and which vane adapter foot is used.
3. Select the desired vane size and attach to the driver and attach to the stem by pressing the end of the stem all the way into the square recess on the vane.
   
   • 1 inch regular vane (25.4 mm diameter, 1.0 multiplier): 0–1.0 kg/cm²
   • 1-7/8 inch large vane (47.6 mm diameter, 0.2 multiplier): 0–0.2 kg/cm²
   • ¾ inch small vane (19 mm diameter, 2.5 multiplier): 0–1.5 kg/cm²
4. Make sure that the dial is aligned with the zero mark. Turn the dial face counterclockwise (while holding onto the handle) until the zero mark aligns with the index mark on the handle.
5. Hold the Torvane at a right angle to the surface and press into the soil to a depth of the blades.
6. Maintaining a constant vertical pressure, turn the handle at a constant rate such that failure occurs in 5–10 s.
7. After failure occurs, slowly release the remaining spring tension.
8. Take a reading on the dial where the handle's index mark rested—this is the maximum shear value—and record the result on the logsheet.
9. Before taking the next measurement, re-zero the scale by rotating counterclockwise until it aligns with the index.

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1. Log into the application with your LIMS username/password (Figure 14) to open the main application window (Figure 15).
2. Select the Torvane radio button.
3. Select Label ID info. (Figure 16). Select the sample from the Sample Table underneath the hierarchy search.
4. Enter pertinent info (Offset, Adapter ID, Alignment, and Raw Instrument Reading).
5. Press Save. Clicking Save uploads the measurement into LIMS and shows the correction applied for the selected adapter foot.

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Figure 14. Logging into the Application/LIMS Database.
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Figure 15. Control Screen for Entering Results and Uploading to LIMS.
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Figure 16. Entering Torvane Results.

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Figure 18. Logging in to the Application/LIMS Database.

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Figure 19. Control Screen for Entering Results and Uploading to LIMS.

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Figure 20. Entering Penetrometer Results.

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LIMS Integration

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