Types of Structural Fill.

There are four general types of structural fill, as follows:

1. Select import. Select import refers to a processed material. The material may be derived from several different sources, then screened and mixed to provide a material of specified gradation.

Table 15.3 presents different methods that can be used to produce a select import material. A common type of select import is granular base material, which may have to meet specifications for gradation, wear resistance, and shear strength (Standard Specifications for Public Works Construction, 2003). Other uses for select import include backfill for retaining walls and utilities, and even for mass-graded fill.

The main characteristics of select import are a well-graded granular soil, which has a high laboratory maximum dry density, typically in the range of 125 to 135 pcf (2.0 to 2.2 Mg/m3). As a processed material, the particle size gradation for each batch of fill should be similar. Usually an import material will have all laboratory maximum dry density values within 3 pcf (0.05 Mg/m3) and a standard deviation of 1 pcf (0.02 Mg/m3) or less.

2. Uniform borrow. Uniform borrow typically refers to a natural material that will consistently have the same soil classification and similar grain size distribution. An example of a possible uniform borrow could be a natural deposit of beach sand. Other uniform borrow could be formational rock, such as deposits of sandstone or siltstone. As the name implies, the main characteristic of the material is its uniformity. Usually a uniform borrow material has consistently the same soil classification, with all laboratory maximum dry density values within 8 pcf (0.13 Mg/m3) and a standard deviation of 3 pcf (0.05 Mg/m3) or less.

Figure 15.15 presents laboratory test results on a uniform borrow material. The fill was derived from a formational rock, classified as a weakly cemented shale. When used as fill, the material is classified as silty clay, having a liquid limit between 41 and 50. The laboratory maximum dry density varies from 115 to 123 pcf (1.84 to 1.97 Mg/m3), with an average value of 120 pcf (1.92 Mg/m3) and a standard deviation of 2.2 pcf (0.035 Mg/m3).

FIGURE 15.15 Classification test data for a uniform borrow soil.

3. Mixed borrow. Mixed borrow contains material of different classifications. For example, mixed borrow could be a deposit of alluvium, that contains alternating layers of sand, silt, and clay.

Mixed borrow could also be formational rock that contains thin alternating layers of sandstone and claystone.

The main characteristics of mixed borrow are that each load of fill could have soils with significantly different grain size distributions and soil classifications. The fill commonly contains many different soil types, all jumbled up and mixed together.

One method to deal with mixed borrow material is to thoroughly mix each load of import and then perform a laboratory maximum dry density test on that batch of import soil. Another option is to thoroughly mix each batch of mixed borrow material and then perform a one-point Proctor test (see Sec. 3.6.3) in order to estimate the laboratory maximum dry density.

4. Borrow with oversize particles. The last basic type of fill is borrow having oversize particles, which are typically defined as those particles retained on the 3/4 in. (19 mm) U.S. standard sieve, i.e., coarse gravel and cobble size particles. The soil matrix is defined as those soil particles that pass the 3/4 in. (19 mm) U.S. standard sieve. When a field density test (such as a sand cone test) is performed, the soil excavated for the test can be sieved on the 3/4 in. (19 mm) sieve in order to determine the mass of oversize particles. The elimination method (Day, 1989) can then be used to mathematically eliminate the volume of oversize particles in order to calculate the dry density of the matrix material. The relative compaction is calculated by dividing the dry density of the matrix material by the laboratory maximum dry density, where the laboratory compaction test is performed on the matrix material. By using the elimination method to calculate the relative compaction of the matrix material, the compaction state of the matrix soil is controlled. This is desirable because it is the matrix soil (not the oversize particles) that usually govern the compressibility, shear strength, and permeability of the soil mass.

If the matrix soil can be considered to be a uniform borrow material, then the procedure for selecting the laboratory maximum dry density in the field is the same as previously discussed for a uniform borrow material. If the matrix material is a mixed borrow material, then the procedure for selecting the laboratory maximum dry density in the field is the same as previously discussed for a mixed borrow material. Other methods have been developed to deal with fill containing oversize particles (Saxena et al., 1984; Houston and Walsh, 1993).

TABLE 15.3 Methods Used to Produce a Select Import Material

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