The design of a proposed earth dam is to include a gravel drain to control the groundwater inside the dam. This drain is to be protected with a graded soil filter zone. The grain-size distribution curves of the soils used to build the core of the dam and the gravel used to build the drain are shown

Question

The design of a proposed earth dam is to include a gravel drain to control the groundwater inside the dam. This drain is to be protected with a graded soil filter zone. The grain-size distribution curves of the soils used to build the core of the dam and the gravel used to build the drain are shown in Figure 8.36. Determine the range of acceptable filter material.

Accepted Answer

Migration of core soils into filter:
91% passing #200 sieve, so this is soil type 1 per Table 8.2

Soil Group N0.	Soil Description	[latex]D_{15}[/latex] Design Criteria
1	Fine silts and clays with >85% passing the #200 sieve	[latex]D_{15} ≤9d_{85} [/latex], but not smaller than 0.2 mm
2	Silty and clayey sands and sandy silts and clays with 40-85% passing the #200 sieve	[latex]D_{15}≤ 0.7[/latex] mm
3	Silty and clayey sands and gravels with 15-39% passing the #200 sieve	[latex]D_{15}\leq \left\lgroup\frac{40 - A}{25} \right\rgroup \left(4d_{85} - 0.7 mm\right) + 0.7 mm[/latex]
4	Silty and clayey sands and gravelly sands with ≤ 15% passing the #200 sieve	[latex]D_{15}≤4d_{85}[/latex]

[latex]d_{85}= 0.056 mm[/latex]
[latex]D_{15} ≤ 9d_{85} → D_{15} ≤ (9)(0.056 mm) → D_{15} ≤ 0.50 mm[/latex]

Therefore, the filter soils must have [latex]D_{15} ≤ 0.50 mm[/latex], as shown by the mark on Figure8.36.

Migration of filter soils into drain:
We use the same analysis, but now [latex]D[/latex] represents the drain and [latex]d[/latex] represents the filter.
Based on the earlier results, the filter will be a type 4 soil
[latex]D_ {15} =4.0 mm[/latex]
[latex]D_{15} ≤ 4d_{85} → 4.0 mm ≤ (4)(d_{85} ) → 1.0 ≤ d_{85}[/latex]

Therefore, the filter soils must have [latex]D_{85} ≥ 1.0 mm[/latex], as shown by the mark on Figure 8.36.

Based on these two marks, the range of acceptable filter material is as shown in Figure 8.36.

Question 8.7: The design of a proposed earth dam is to include a gravel dr...

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