If both soils are dry, adding water will cause it to drain first from which soil?

Question

BlackTom AI · Accepted Answer

Sand drains faster because it has larger pore spaces and a much higher hydraulic conductivity than clay.

1) Relevant knowledge points and concepts
- Pore size and connectivity: Sand consists of larger, well-connected pores; clay has many tiny pores and plate-like particles.
- Hydraulic conductivity (K): Measures how easily water moves through a soil; K is orders of magnitude larger in sand than in clay.
- Capillary forces and water retention: Small pores (clay) create strong capillary suction and high surface area, so clay holds water more tightly; sand has low capillary retention.
- Darcy’s law (conceptual): q = K · i, so for a given hydraulic gradient i, the volumetric flux q is proportional to K.

2) Step-by-step reasoning
- Both soils start dry and you add water at the surface. Water moves downward under gravity and the soil’s hydraulic properties.
- In sand, large pores give high K, so water moves quickly downward and gravitational drainage removes water rapidly.
- In clay, tiny pores and strong capillary forces slow both infiltration and drainage; water is retained longer and moves slowly.

3) Intermediate/quantitative comparison
- Typical saturated hydraulic conductivity: sand ~10^-3 to 10^-5 m/s; clay ~10^-9 to 10^-7 m/s. Even at the low end for sand and high end for clay, sand’s K is thousands to millions of times greater.
- Using Darcy’s law qualitatively: q_sand >> q_clay → much faster drainage in sand.

4) Why the correct answer is correct
- Because sand’s high hydraulic conductivity and low water-holding capacity allow added water to percolate and drain first. The physics of flow and retention favor rapid drainage in coarse-textured soils.

5) Why the other option is incorrect
- Clay retains water strongly due to small pores and high surface area; its low hydraulic conductivity prevents rapid drainage. Therefore clay will not drain first.

6) Conclusion
- The correct choice is a. Sand, which matches the provided answer.

If both soils are dry, adding water will cause it to drain first from which soil?单项选择题

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