According to Rankine’s earth pressure theory, the coefficient of active earth pressure (Ka) for a horizontal backfill with internal friction angle phi is equal to:
A(1 + sin phi) / (1 - sin phi)
B(1 - sin phi) / (1 + sin phi)
C1 / (1 - sin phi)
D(1 - cos phi) / (1 + cos phi)
Explanation
Rankine’s active earth pressure coefficient Ka = (1 – sin phi) / (1 + sin phi) = tan^2(45° – phi/2). Passive earth pressure coefficient Kp = (1 + sin phi) / (1 – sin phi) = 1 / Ka.

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