Gradation Design of Sand and Gravel Filters, and Drainage: SCS_Filter_Design PDF

from the PDF above:

Drains are often used downstream of or in addition to a filter to provide outlet capacity. Combined filters and drains are commonly used. The filter is designed to function as a filter and as a drain.

### Determining filter gradation limits

**Step 1: Plot the gradation curve (grain-size distribution) of the base soil material. **

Use enough samples to define the range of grain sizes for the base soil or soils. Design the filter using the base soil that requires the smallest D15 size for filtering purposes. Base the design for drainage purposes on the base soil that has the largest D15 size.

**Step 2: Proceed to step 4 if the base soil contains no gravel (material larger than No. 4 sieve).**

**Step 3: Prepare adjusted gradation curves for base soils that have particles larger than the No. 4 (4.75 mm) sieve.**

- Obtain a correction factor by dividing 100 by the percent passing the No. 4 (4.75 mm) sieve.
- Multiply the percentage passing each sieve size of the base soil smaller than No. 4 (4.75 mm) sieve by the correction factor determined above.
- Plot these adjusted percentages to obtain a new gradation curve.
- Use the adjusted curve to determine the percentage passing the No. 200 (0.075 mm) sieve in step 4.

**Step 4: Place the base soil in a category determined by the percent passing the No. 200 (0.075 mm) sieve from the regraded gradation curve data according to table 26–1.**

**Step 5: To satisfy filtration requirements, determine the maximum allowable D15 size for the filter in accordance with the table 26–2.**

If desired, the maximum D15 may be adjusted for certain noncritical uses of filters where significant hydraulic gradients are not predicted, such as bedding beneath riprap and concrete slabs. For fine clay base soil that has d85 sizes between 0.03 and 0.1 mm, a maximum D15 of ≤ 0.5 mm is still conservative.

For fine-grained silt that has low sand content, plotting below the “A” line, a maximum D15 of 0.3 mm may be used.

**Step 6: If permeability is a requirement **(see section 633.2602)**, determine the minimum allowable D15 in accordance with table 26–3. **

Note: The permeability requirement is determined from the d15 size of the base soil gradation before regrading.

**Step 7: The width of the allowable filter design band must be kept relatively narrow to prevent the use of possibly gap-graded filters. Adjust the maximum and minimum D15 sizes for the filter band determined in steps 5 and 6 so that the ratio is 5 or less at any given percentage passing of 60 or less.** Criteria are summarized in table 26–4.

This step is required to avoid the use of gap-graded filters. The use of a broad range of particle sizes to specify a filter gradation could result in allowing the use of gap-graded (skip-graded) materials. These materials have a grain size distribution curve with sharp breaks or other undesirable characteristics. Materials that have a broad range of particle sizes may also be susceptible to segregation during placement.

The requirements of step 9 should prevent segregation, but other steps are needed to eliminate the use of any gap-graded filters. Gap-graded materials generally can be recognized by simply looking at their grain size distribution curve.

However, for specification purposes, more precise controls are needed. In designing an acceptable filter band using the preliminary control points obtained in steps 1 through 6, the following additional requirements should be followed to decrease the probability of using a gap-graded filter

First, calculate the ratio of the maximum D15 to the minimum D15 sizes determined in steps 5 and 6. If this ratio is greater than 5, adjust the values of these control points so that the ratio of the maximum D15 to the minimum D15 is no greater than 5. If the ratio is 5 or less, no adjustments are necessary. Label the maximum D15 size as Control point 1 and the minimum D15 size as Control point 2. Proceed to step 8.

The decision on where to locate the final D15 sizes within the range established with previous criteria should be based on one of the following considerations:

- Locate the design filter band at the maximum D15 side of the range if the filter will be required to transmit large quantities of water (serve as a drain as well as a filter). With the maximum D15 size as the control point, establish a new minimum D15 size by dividing the maximum D15 size by 5, and locate a new minimum D15 size. Label the maximum D15 size Control point 1 and the minimum D15 size Control point
- Locate the band at the minimum D15 side of the range if it is probable there are finer base materials than those sampled and filtering is the most important function of the zone. With the minimum D15 size as the control point, establish a new maximum D15 size by multiplying the minimum D15 size by 5, and locate a new maximum D15 size. Label the maximum D15 size Control point 1 and the minimum D15 size Control point 2.
- The most important consideration may be to locate the maximum and minimum D15 sizes, within the acceptable range of sizes determined in steps 5 and 6, so that a standard gradation available from a commercial source or other gradations from a natural source near the site would fall within the limits. Locate a new maximum D15 and minimum D15 within the permissible range to coincide with the readily available material. Ensure that the ratio of these sizes is 5 or less. Label the maximum D15 size Control point 1 and the minimum D15 size Control point 2.

**Step 8: The designed filter band must not have an extremely broad range of particle sizes to prevent the use of possibly gap-graded filters. Adjust the limits of the design filter band so that the coarse and fine sides have a coefficient of uniformity of 6 or less. The width of the filter band should be such that the ratio of maximum to minimum diameters is less than or equal to 5 for all percent passing values of 60 or less. Other filter design criteria in step 8.**

*To prevent gap-graded filters* —Both sides of the design filter band will have a coefficient of uniformity, defined as:

CU = D60/D10 ≤ 6

Initial design filter bands by this step will have CU values of 6. For final design, filter bands may be adjusted to a steeper configuration, with CU values less than 6, if needed. This is acceptable so long as other filter and permeability criteria are satisfied.

Calculate a maximum D10 value equal to the maximum D15 size divided by 1.2. (This factor of 1.2 is based on the assumption that the slope of the line connecting D15 and D10 should be on a coefficient of uniformity of about 6.) Calculate the maximum permissible D60 size by multiplying the maximum D10 value by 6. Label this Control point 3.

Determine the minimum allowable D60 size for the fine side of the band by dividing the determined maximum D60 size by 5. Label this Control point 4.

**Step 9: Determine the minimum D5 and maximum D100 sizes of the filter according to table 26–5.**

Label as Control points 5 and 6, respectively.

**Step 10: To minimize segregation during construction, the relationship between the maximum D90 and the minimum D10 of the filter is important. Calculate a preliminary minimum D10 size by dividing the minimum D15 size by 1.2. (This factor of 1.2 is based on the assumption that the slope of the line connecting D15 and D10 should be on a coefficient of uniformity of about 6.) Determine the maximum D90 using table 26–6. Label this as Control point 7.**

Sand filters that have a D90 less than about 20 mm generally do not require special adjustments for the broadness of the filter band. For coarser filters and gravel zones that serve both as filters and drains, the ratio of D90/D10 should decrease rapidly with increasing D10 sizes.

**Step 11: Connect Control points 4, 2, and 5 to form a partial design for the fine side of the filter band. Connect Control points 6, 7, 3, and 1 to form a design for the coarse side of the filter band. This results in a preliminary design for a filter band. Complete the design by extrapolating the coarse and fine curves to the 100 percent finer value. For purposes of writing specifications, select appropriate sieves and corresponding percent finer values that best reconstrstruct the design band and tabulate the values.**

**Step 12: Design filters adjacent to perforated pipe to have a D85 size no smaller than shown in table 26–7.**

For critical structure drains where rapid gradient reversal (surging) is probable, it is recommended that the D15 size of the material surrounding the pipe be no smaller than the perforation size.

**Additional design considerations:** Note that these steps provide a filter band design that is as well graded as possible and still meets criteria. This generally provides the most desirable filter characteristics. However, in some cases a more poorly graded filter band may be preferable; for example, if more readily available standard gradations are needed or where onsite filters are used for economy.

The design filter band obtained in steps 1 through 12 may be adjusted to a steeper configuration in such cases. The width of the filter band should be maintained so that the ratio of the maximum diameters to the minimum diameters at a given percent finer is no greater than 5 below the 60 percent finer value.

Only the portion of the design filter band above the previously established minimum and maximum D15 sizes should be adjusted. The design band may be adjusted so that the coefficients of uniformity of both the coarse and fine sides of the design band are less than 6, but not less than 2, to prevent use of very poorly graded filters.