3 August 1998
(d) In the hydraulic rotary method of drilling, the screen may be attached directly to
the bottom of the casing before lowering the whole assembly into the well.
e. Filter packing. Filter packing (sometimes referred to as gravel packing)is primarily
sand placed around the well screen to stabilize the aquifer and provide a radius of high
permeability around the screen. This differs from the naturally developed well in that the zone
around the screen is made more permeable by the addition of coarse material. Filter-pack
material is more effective when it is composed of clean rounded silicious sand or gravel.
(1) Size. Grain size of the filter pack is selected on the basis of information obtained
from sieve analyses of the material in the aquifer. The well screen aperture size will be
selected so that between 85 and 100 percent of the filter pack is larger than the screen
openings. Criteria for sizing the filter pack are as follows:
(a) Perform sieve analyses on all strata within the aquifer. The sieve sizes to be
used in performing these analyses are:
80 mm (3 in)
50 mm (2 in)
40 mm (1-1/2 in)
25 mm (1 in)
20 mm (3/4 in)
The results of the analysis of any particular sample should be recorded as the percent (by
weight) of the sample retained on each sieve and the cumulative percent retained on each
sieve (i.e., the total of the percentages for that sieve and all larger sieve sizes). Based on
these sieve analyses, determine the aquifer stratum which is composed of the finest material.
(b) Using the results of the sieve analysis for the finest aquifer material, plot the
cumulative percent of the aquifer material retained versus the size of the mesh for each sieve.
Fit a smooth curve to these points. Find the size corresponding to a 70 percent cumulative
retention of aquifer material. This size should be multiplied by a factor between 4 and 6, 4 if
the formation is fine and uniform and 6 if the formation is coarse and nonuniform. Use 9 if the
formation includes silt. The product is the 70 percent retained size (i.e., the sieve size on
which a cumulative 70 percent of the sample would be retained) of the material to be used in
(c) A uniformity coefficient of 2.5 for the filter pack is desirable. The uniformity
coefficient is defined 40 percent of the retained grain size divided by 90 percent retained size.
Lower size represents a more uniform material and is more meaningful for values less than 5.
(d) The plot of cumulative percent retention versus grain size for the filter pack
should be approximately parallel to same plot for the aquifer material, should pass through the
70 percent retention value and should have 40 and 90 percent retention values such that the
uniformity coefficient is less than 2.5. Filter pack material will be specified by determining the
sieve sizes that cover the range of the curve and then defining an allowable range for the
percent retention on each sieve.