Open wells and “Mota layer” (foundation stratum) Pick the <em>incorrect</em> statement regarding the Mota layer and well terminology used in field practice.
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AAn impervious layer a few metres below the water table is colloquially called the Mota layer; it provides good structural support to open deep wells during sinking.
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BFor deep wells, a borehole is generally drilled into the Mota layer to increase yield.
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CBy definition, shallow wells are of lesser depth than deep wells.
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DNone of these
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EThe Mota layer is preferred for seating well curbs because of its bearing strength.
Answer
Correct Answer: For deep wells, a borehole is generally drilled into the Mota layer to increase yield.
Explanation
Introduction / Context:In many practical hydrogeology and well-construction contexts, “Mota layer” refers to a hard, relatively impervious stratum encountered below the water table. Understanding its role helps in correct siting and sinking of open wells.
Given Data / Assumptions:
- Mota layer is hard/impervious, offering bearing support.
- Open (dug) wells are constructed to intercept water-bearing strata, not to seat screens in impervious layers.
Concept / Approach:Open wells are typically sunk through overburden until they rest on or are seated within a competent stratum for stability. Yield is obtained from surrounding permeable horizons (aquifers), not from the impervious “Mota” layer. Therefore, drilling into an impervious layer to “increase yield” is illogical.
Step-by-Step Solution:
Evaluate (a) and (e): they emphasize structural support—true for seating curbs.Evaluate (c): definitional statement is true.Evaluate (b): suggests boring into an impervious support layer to increase yield—this contradicts hydrogeologic function.Conclude (b) is incorrect.Verification / Alternative check:Site logs show yields are associated with permeable strata (sands, gravels, fractured rock); impervious layers are used for support or act as confining beds.
Why Other Options Are Wrong:
- (a) and (e) are consistent with well-sinking practice (support/bearing).
- (c) is tautologically correct.
- (d) “None of these” is false because (b) is indeed incorrect.
Common Pitfalls:Assuming that harder strata always give more water; in fact, permeability governs yield.
Final Answer:For deep wells, a borehole is generally drilled into the Mota layer to increase yield.