A house foundation is being constructed on expansive soil with a plasticity index of 35. The soil report recommends a specific moisture content be maintained. What is the primary reason for this requirement?
Correct Answer
A) To minimize future soil volume changes
Maintaining specific moisture content in expansive soils helps minimize future volume changes (swelling and shrinking) that could cause foundation movement and structural damage.
Why This Is the Correct Answer
Option A (to minimize future soil volume changes) is correct. Expansive soils contain clay minerals that absorb water and swell, then shrink when they dry out. Maintaining a consistent moisture content limits the amplitude of these volume changes, preventing the cyclic heaving and settlement that damages foundations.
Why the Other Options Are Wrong
Option B: To increase soil bearing capacity
Bearing capacity is influenced by moisture, but this is not the primary reason for moisture control in expansive soils. In fact, wetting expansive clay can reduce bearing capacity. The dominant concern is volumetric stability, not load-carrying ability.
Option C: To prevent soil liquefaction
Liquefaction is a phenomenon of saturated, loose granular soils (sands and silts) losing strength during seismic shaking. Clay-rich expansive soils with a plasticity index of 35 are not susceptible to liquefaction — this answer reflects a fundamental misunderstanding of soil behavior.
Option D: To improve soil compaction characteristics
Compaction characteristics relate to achieving maximum dry density during earthwork — a construction-phase concern. Once the foundation is placed, maintaining moisture content is about long-term volumetric stability, not improving compaction.
Memory Technique
Plasticity Index (PI) > 20 = expansive soil risk. High PI clays swell when wet, shrink when dry. Moisture control = keeping the soil at a stable moisture level so it neither swells nor shrinks. Think of it as keeping clay 'calm' — no dramatic wet/dry cycles, no dramatic volume swings.
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