Plasticity Chart – Local “family” of soils in a restricted area For soils with common geological origin from one locality, their LL–PI values usually plot as:

Introduction / Context:
On the plasticity chart (PI vs LL), soils from a single geological formation often share mineralogy and particle characteristics. Consequently, their Atterberg data tend to align along a “local line.”

Given Data / Assumptions:

• Soils sampled within a restricted geographic area and common origin.
• Atterberg limits tested consistently.

Concept / Approach:

Casagrande's plasticity chart distinguishes clays and silts using the A-line. Local families of soils often fall along a straight trend roughly parallel to the A-line, reflecting consistent clay mineral proportions and activity.

Step-by-Step Solution:

Verification / Alternative check:

Many site datasets show tight linear bands; departures often indicate contamination, different depositional histories, or testing inconsistencies.

Why Other Options Are Wrong:

(a) Describes a region, not a line; (c) perpendicular trend lacks basis; (d) ignores the observed geological consistency.

Common Pitfalls:

Mixing samples from unrelated formations or misclassifying by not checking the A-line offset (clay vs silt).

Final Answer:

A straight line roughly parallel to the A-line