Shallow Foundations MCQ with answer and solution

Concept

Determination of ultimate load from a plate load test using a double logarithmic (log-log) plot for soils that exhibit local shear or punching shear failure.

Step-by-step solution

1. In dense sands or stiff clays, a distinct failure point is visible on an arithmetic load-settlement plot. 2. In loose to medium soils, no clear failure peak is observed, so the data is plotted on a log-log scale (log load vs. log settlement). 3. On this scale, two distinct linear trends appear representing elastic and plastic phases. 4. The ultimate load corresponds to the point of intersection of these two straight lines.

Explanation

According to IS 1888 (Method of Load Test on Soils), when the load-settlement curve on normal arithmetic scales does not show a well-defined break or failure point, the data is plotted on a log-log scale. On a log-log plot, the curve typically breaks into two straight lines, and the intersection point of these two straight lines gives the ultimate bearing capacity.

Why the other options are wrong

B: Drawing tangents to find the yield point is typically used on an arithmetic load-settlement curve when a transition zone exists, not on a log-log graph. C: The secant method is used for defining deformation moduli, not for locating ultimate bearing capacity on a log-log plot. D: 0.2 % offset is standard for finding proof stress in metals, not for plate load test interpretation.

Shortcut

Log-log plot in soil mechanics usually linearizes bilinear curves; failure is given by the intersection of the two straight lines.

Common mistake

Candidates often confuse the arithmetic plot method (tangent method or yield point method) with the log-log plot method, or confuse it with the 0.2% proof stress concept from structural engineering.