GATE Civil Engineering (CE) 2018 Shift 1 Solved Paper

Question : 56 of 65

Marks: +1, -0

A conventional drained triaxial compression test was conducted on a normally consolidated clay sample under an effective confining pressure of 200 KPa. The deviator stress at failure was found to be

400 {kPa}

. An identical specimen of the same clay sample is isotropically consolidated to a confining pressure of

200 {kPa}

and subjected to standard undrained triaxial compression test. If the deviator stress at failure is

150 {kPa}

, the pore pressure developed (in kPa, up to one decimal place) is

Your Answer:

Validate

Solution:

Ist Specimen : Drained condition

\ov{σ_{3}}=200 {kPa}: \ov{σ_{d}}=400 {kPa}: \ov{σ_{1}}=600 {kPa}

^{\;\text "nd "\;}

Specimen : Undrained condition

\ov{σ_{3}}=200 {kPa}: \ov{σ_{d}}=150 {kPa}: \ov{σ_{1}}=\ov{σ_{3}}+σ_{d}

=350 {kPa}

Let pore pressure developed is u

∴ \;\; \ov{σ_{3}}=(200-u)

\ov{σ_{1}}=(350-u)

From stress relationship

\ov{σ_{1}}=\ov{σ_{3}} \tan ^{2}(45^{∘}+\;{φ}/{2})+2 c \tan (45^{∘}+\;{φ}/{2})

For clay under drained condition

∴ \;\; \ov{σ_{1}}=\ov{σ_{3}} \tan ^{2}(45^{∘}+\;{φ}/{2})

600=200 \tan ^{2}(45^{∘}+\;{φ}/{2})

φ=30^{∘}

For second specimen,

\ov{σ_{1}}=\ov{σ_{3}} \tan ^{2}(45^{∘}+\;{φ}/{2})

(350-u)=(200-u) \tan ^{2}(45^{∘}+\;{30^{∘}}/{2})

u=125 {kPa}

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