Undrained Shear Strength and Consistency Term

Undrained Shear Strength 1

Undrained Shear Strength 1 (s_u) in Undrained_Shear_Strength_1 is defined as:

When qt has data,

s_u = \frac {(q_t - \sigma_{v0})}{N_{kt}}

Otherwise,

s_u = \frac {(q_c - \sigma_{v0})}{N_{k}}

NCHRP Synthesis 368 p 39

N_kNk and N_{kt} are defined on the CPT_POINT_MATERIAL_PROPERTIESCPT_POINT_CORRELATION_PARAMETERS and CPT_PROJECT_CORRELATION_PARAMETERS tables. Different values for N_k and N_{kt} can be defined for when q_c or q_t is either greater than or equal to, or less than a defined break point. Each of these tables have six fields from which the N_k and N_{kt} values, and the q_c and qt break point values are stored and retrieved for the calculation of s_u .

Field NameDescription
Undrained_Shear_Strength_1_Nk_qc_BP

Break point used to determine which N_k to use in the s_u calculation

Undrained_Shear_Strength_1_Nk_qc_LT_BP

Used as the N_k in the s_u calculation, where q_t does not have data, and q_c is less than the break point.

Undrained_Shear_Strength_1_Nk_qc_GTE_BP

Used as the N_k in the s_u calculation, where qt does not have data, and q_c is greater than or equal to the break point.

Undrained_Shear_Strength_1_Nkt_qt_BP

Break point used to determine which N_{kt} to use in the s_u calculation

Undrained_Shear_Strength_1_Nkt_qt_LT_BP

Used as the N_{kt} in the s_u calculation, where q_t has data, and q_t is less than the break point.

Undrained_Shear_Strength_1_Nkt_qt_GTE_BP

Used as the N_{kt} in the s_u calculation, where q_t has data, and q_t is greater than or equal to the break point.

The fields in the CPT_POINT_MATERIAL_PROPERTIES table allow the N_k and N_{kt} to be defined for depth ranges for a particular PointID. A top and bottom depth must be specified when using the CPT_POINT_MATERIAL_PROPERTIES table, and the depth ranges must not overlap.

If the CPT_POINT_MATERIAL_PROPERTIES table is populated for the calculated PointID, the N_k or N_{kt} that lies within the depth range of the current depth of the row in the CPT_DATA table is used to calculate s_u. If a matching depth range for the current depth is not found, or N_k or N_{kt}  is missing, then the value is taken from the CPT_POINT_CORRELATION_PARAMETERS or the CPT_PROJECT_CORRELATION_PARAMETERS tables.

Undrained Shear Strength 1 Term

The Undrained_Shear_Strength_Term_1 is defined based on the value of the Undrained_Shear_Strength_1 field and the values in the DG_LOG_CONSISTENCY_DENSITY library table.

Undrained Shear Strength 2

Undrained Shear Strength 2 (s_u) in Undrained_Shear_Strength_2 is defined as:

When q_t has data,

s_u = \frac {(q_t)}{N_{kt}}

Otherwise,

s_u = \frac {(q_c - \sigma_{v0})}{N_{kt}}

Where:

     N_{kt} is stored in the Undrained_Shear_Strength_2_Nkt field.

     N_k is stored in the Undrained_Shear_Strength_2_Nk field.

The variables are stored on CPT_POINT_MATERIAL_PROPERTIESCPT_POINT_CORRELATION_PARAMETERS and CPT_PROJECT_CORRELATION_PARAMETERS tables.

The fields in the CPT_POINT_MATERIAL_PROPERTIES table allow the N_k and N_{kt} to be defined for depth ranges for a particular PointID. A top and bottom depth must be specified when using the CPT_POINT_MATERIAL_PROPERTIES table, and the depth ranges must not overlap.

If the CPT_POINT_MATERIAL_PROPERTIES table is populated for the calculated PointID, the N_k or N_{kt} that lies within the depth range of the current depth of the row in the CPT_DATA table is used to calculate s_u. If a matching depth range for the current depth is not found, or N_k or N_{kt} is missing, then the value is taken from the CPT_POINT_CORRELATION_PARAMETERS or the CPT_PROJECT_CORRELATION_PARAMETERS tables.

Undrained Shear Strength 2 Term

The Undrained_Shear_Strength_Term_2 is defined based on the value of the Undrained_Shear_Strength_2 field and the values in the DG_LOG_CONSISTENCY_DENSITY library table.

Undrained Shear Strength 3

Undrained Shear Strength 3 (s_u) based on CSSM in Undrained_Shear_Strength_3 is defined as:

When q_t has data,

s_u = 0.5 \cdot \sin \Phi' \cdot OCR ^\Lambda \cdot \sigma'_{v0}\\

Wroth (1984), NCHRP Synthesis 368 p 39

Where:

     \Phi'  is taken from the first of these fields with data: Friction_Angle_3 and Friction_Angle_1 
    OCR is taken from Overconsolidation_Ratio_1
     \Lambda = 1 - \frac{C_s}{C_c}\\ , typically low to medium sensitivity clays are  0.7 \leq \Lambda \leq0.8 , and sensitive and structured clays are  0.9 \leq \Lambda \leq1.0and is stored in the Undrained_Shear_Strength_3_Lambda field.

\Lambda  is stored on CPT_POINT_CORRELATION_PARAMETERS and CPT_PROJECT_CORRELATION_PARAMETERS tables.

Undrained Shear Strength 3 Term

The Undrained_Shear_Strength_Term_3 is defined based on the value of the Undrained_Shear_Strength_3 field and the values similar to that in DG_LOG_CONSISTENCY_DENSITY library table.

Undrained Shear Strength 4

Undrained Shear Strength 4 (s_u) in Undrained_Shear_Strength_4 is defined as:

s_u = {C_1} \cdot {\sigma'_p}


Trak et al. (1980), Terzaghi et al. (1996), NCHRP Synthesis 368 p 40

Where:

      \sigma'_p  is taken from Preconsolidation_Stress_1
      C_1  is 0.22 in the published formula, and is stored in the Undrained_Shear_Strength_4_C1 field.
      C_1  is stored on CPT_POINT_CORRELATION_PARAMETERS and CPT_PROJECT_CORRELATION_PARAMETERS tables.

Undrained Shear Strength 4 Term

The Undrained_Shear_Strength_Term_4 is defined based on the value of the Undrained_Shear_Strength_4 field and the values similar to that in DG_LOG_CONSISTENCY_DENSITY library table.

Undrained Shear Strength 5

Undrained Shear Strength 5 (s_u) in Undrained_Shear_Strength_5 is defined as:

s_u = \frac {\Delta u}{N_{\Delta u}}

Robertson (2009), pp 28-29

Where:

      \Delta u  is taken from Excess_Pore_Pressure field
      {N_{\Delta u}}  is taken from Undrained_Shear_Strength_5_N field
      {N_{\Delta u}}  is stored on CPT_POINT_CORRELATION_PARAMETERS and CPT_PROJECT_CORRELATION_PARAMETERS tables.

Undrained Shear Strength 5 Term

The Undrained_Shear_Strength_Term_5 is defined based on the value of the Undrained_Shear_Strength_5 field and the values similar to that in DG_LOG_CONSISTENCY_DENSITY library table.

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