The pore water pressure in the soil sample of a consolidometer test is ______.

Asked in SSC JE 29/01/2018 (Morning)

Detailed Explanation

In a standard consolidometer (oedometer) test, the soil specimen sits in a ring between two porous stones, one on top and one below. Water squeezed out of the sample can escape through both faces. This is double drainage, and the drainage path d is half the specimen thickness.

Why “maximum at center” Is Correct

Water near the top and bottom faces escapes almost immediately, so excess pore pressure there drops to zero. Water at mid-height has the longest path to travel, so it is the last to drain. As a result, at any time during the test the excess pore water pressure is maximum at the centre of the sample, as the isochrones show.

Isochrones from Terzaghi's theory for a clay layer drained at top and bottom (thickness 2H_d). Each curve shows how excess pore water pressure varies with depth at one instant (one value of T_v). Pressure is zero at the drainage faces and largest at mid-depth, and the curves move left as time passes.

Why the Other Options Are Wrong

A. maximum at bottom: The bottom face rests on a porous stone and drains freely, so pore pressure there is zero, not maximum.
C. maximum at top: The top face also drains through a porous stone.
D. minimum at center: The centre is where pore pressure is highest, not lowest.

Key Point

Double drainage: u max at mid-depth, d = H/2. Single drainage: u max at the impervious boundary, d = H.

Key Concepts for Students

In the field, a clay layer between two sand layers is doubly drained; a clay layer on rock is singly drained.
Pore pressure is measured at the base in some tests by sealing the bottom stone (single drainage), which makes d = H.
The hydraulic gradient at any depth equals the slope of the isochrone divided by γw.

Practice more: this is question 16 in Soil Consolidation MCQs – Part 2. Try the whole set and check every answer instantly.