GDSLab will apply the membrane and side drain corrections using British Standard (BS) when B1377 is selected in the Sample details (see Fig. 1). For ASTM calculations, select ASTM_Triaxial from the drop-down list.

1. Membrane Correction

1.1.  British Standard

A membrane correction when selected in the Specimen Details in GDSLab (Fig. 1) applies restraining effect of the membrane.

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The curve in Fig. 2 gives the membrane correction σ_mb (kN/m² or kPa) applied to a specimen initially 38mm diameter enclosed in a membrane 0.2 mm thick. For other conditions, GDSLab multiplies the correction obtained from Fig. 2 by:

to give the value of  σ_mb to be used,

where:

D_o is the initial specimen diameter, mm and

t_m is the total thickness of membrane (which may consist of more than one layer), mm

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1.2. ASTM Standard

where:

D_c is the diameter of specimen after consolidation, mm

E_m is the Young’s modulus for the membrane material, kPa (refer to ASTM D4767 for calculating E_m), and

ε is the axial strain (decimal form)

2. Side Drain Correction

2.1. British Standard

The side drain correction (σ_dr, kPa) is applied when selected in the specimen details in GDSLab (Fig. 1). Please refer to Table 1 for the side drain correction in GDSLab.

 Table 1: Corrections for Side Drains

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2.2. ASTM Standard

• For values of axial strain above 2%, the following equation is used to compute the correction:

where:

K_fp is the load carried by filter-paper strips per unit length of perimeter covered by filter-paper, kN/mm (For filter-paper generally used in triaxial testing, K_fp is approximately 0.00019 kN/mm)

P_fp is the perimeter covered by filter-paper, mm

• For values of axial strain below 2%, the following equation is used to compute the correction:

where:

50 is the limiting axial strain decimal format, and

ε is the axial strain (decimal form) for the given axial load