Soil mechanics terminology – pore pressures under loading: In geotechnical engineering, the pressure that develops in the pore water immediately due to an increment of load on the soil mass is termed as what?
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AExcess pore pressure
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BExcess hydrostatic pressure
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CHydrodynamic pressure
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DAll of the above
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ENeutral pore pressure at initial equilibrium
Answer
Correct Answer: Excess pore pressure
Explanation
Introduction / Context:When a load is applied to a saturated or partially saturated soil, water within the voids initially carries part of the stress. This generates a transient increase in pore-water pressure above the equilibrium value. Understanding this “excess pore pressure” is central to consolidation theory, slope stability during rapid loading, and earthquake-induced soil behavior.
Given Data / Assumptions:
- No numerical values are required; the question asks for the correct term.
- Hydrostatic pore pressure refers to the equilibrium pressure due to the water column alone.
- Excess pore pressure is the additional component created by load or disturbance and dissipates with time as water drains.
Concept / Approach:Immediately after loading a low-permeability soil (e.g., clay), the soil skeleton cannot compress quickly, so the pore water takes the extra stress, creating excess pore pressure u_excess. Over time, drainage reduces u_excess and transfers effective stress to the soil skeleton, causing settlement. In rapid shearing (undrained), u_excess may persist and can reduce effective stress, leading to instability.
Step-by-Step Solution:
Define hydrostatic pressure: u_h = gamma_w * h (equilibrium).Define excess pore pressure: u_excess = u_total − u_h immediately after loading or disturbance.Recognize dissipation: u_excess → 0 with time as water flows out and effective stress increases.Hence, the correct term for load-induced transient pore pressure is “excess pore pressure.”Verification / Alternative check:Piezometer readings during construction often spike above hydrostatic immediately after embankment placement, then decline as consolidation proceeds—direct evidence of u_excess generation and dissipation.
Why Other Options Are Wrong:
- Excess hydrostatic pressure: hydrostatic is an equilibrium state, not the transient rise from loading.
- Hydrodynamic pressure: used for flowing fluids around structures, not for consolidation response.
- “All of the above” is incorrect because only the first term is standard.
- Neutral pore pressure at initial equilibrium is the baseline, not the increment.
Common Pitfalls:Confusing total pore pressure with its components; assuming u_excess is always positive (it can be negative in dilative soils).
Final Answer:Excess pore pressure