MARCO REFERENCIAL

Lightweight materials may be used in place of a select soil backfill to control the settlement of walls constructed over soft compressible soils. According to FHWA NHI-06-019 (Elias et al., 2006), lightweight fills can be grouped into two categories: (1) materials that behave and have similar properties to granular soils; and (2) materials that have an inherent compressive strength and behave similar to cohesive soils. Examples of the first group include wood fiber, blast furnace slag, fly ash, boiler slag, expanded clay or shale, and shredded tires.

Examples of the second group include geofoam and foamed concrete.

The dry loose unit weight of expanded shale, for example, typically varies from 45 to 65 pcf, while its compacted (Standard Proctor) dry unit weight ranges from 70 to 85 pcf. The internal friction angle for these materials may be 40^o or higher, which corresponds to lower active earth pressure coefficients than for typical granular backfill soils. The lower earth pressure coefficient coupled with lower unit weight results in considerably lower earth pressures and corresponding reduction in bending moments in the wall structure and a reduction of the wall section. Table 2-7 provides a summary of typical dry densities for lightweight fills.

In addition to the above advantages, the gradation of lightweight fills can be controlled to produce a free-draining backfill, thereby avoiding development of water pressures. Before using these materials in construction, however, laboratory testing should be performed to define their shear strength, unit weight, corrosivity and durability (soundness and resistance to abrasion). Lightweight fills must have adequate hardness and durability to resist degradation during placement and compaction, and to resist long-term deterioration in the underground environment.

Table 2-7. Range of Dry Densities for Lightweight Fills (after Elias et al., 2006).

Fill Type Range in Dry Density (pcf)

Geofoam (EPS) 0.75 - 2

Foamed Concrete 20 - 60

Wood Fiber 35 - 60

Shredded Tires 38 - 56

Expanded Shale, Clay,

and Slate (ESCS) 38 - 65

Fly Ash 70 - 90

Boiler Slag 70 - 94

The applicability of lightweight material may not be appropriate for all cases. For example, due to their light weight such materials may not be suitable for mechanically stabilized earth walls which rely on the overburden weight to generate friction along the reinforcing elements within the backfill material.

Refer to the Lightweight Fill technical summaries in Ground Improvement Methods manual, FHWA NHI-06-019 (Elias et al., 2006) for discussions and details of lightweight fills.

Geofoam blocks have been used in construction of several highway wall and embankment projects.

2.5.9.2 Flowable Fill

Another backfill option is flowable fill. This backfill material is composed of cement in which air voids are distributed in the form of small, homogeneous, non-interconnected foam cells. High flowability and pumpability permits complete backfilling. In its hardened state, this type of backfill is stable. However, since it is in a fluid state initially, formwork is usually required.

2.5.9.3 Swelling Soils

Swelling (or expansive) soils are typically clayey soils that undergo large volume changes in direct response to moisture changes in the soil and generally are not used as wall backfill.

Swelling soils tend to increase in volume (i.e., swell) as the moisture content of the soil is increased and decrease in volume (i.e., shrink) as the moisture content of the soil is decreased. Although the expansion potential of a soil can be related to many factors (e.g., soil structure and fabric, environmental conditions, etc.), it is primarily controlled by the clay

mineralogy. Soils that contain low-plasticity kaolinite will tend to exhibit a lower swell potential than soils containing high-plasticity montmorillonite.

To identify expansive soils in the laboratory, several classification methods have been developed. Generally, soils with a plasticity index less than 15 percent will not exhibit expansive behavior. For soils with a plasticity index greater than 15 percent, the clay content of the soil should be evaluated in addition to the Atterberg Limits. Figure 2-7 shows the swelling potential of a remolded soil as related to the soil activity and clay fraction. For the purposes of evaluating expansion potential of a soil, activity can be defined as:

( )

( )

Activity = (2-18)

where CF is the clay fraction that corresponds to the percentage of particles exhibiting an equivalent diameter (ds) < 0.078 mil (0.002 mm) as calculated from a hydrometer test performed in accordance with ASTM D422, Standard Test Method for Particle Size Analysis of Soils.

Note: 0.002 mm = 0.078 mil

Figure 2-7. Classification Chart for Swelling Potential (after Seed et al., 1962).

2.5.9.4 Degradable Materials

On some transportation projects, construction activities involve the use of potentially degradable materials. Although the material may, at first, exhibit rock-like characteristics, it has the potential to degrade to soil-size particles. The gradual but ultimate degradation of the rock to the original parent soil material can occur within minutes or after several years of exposure to air and/or water. Shale, the most common member of this family of materials, can generically be considered to include claystone, siltstone, and mudstone.

In many parts of the U.S., high-quality granular material is not locally available for use as borrow material. As a result, degradable materials that, at first, appear to be competent granular materials are used. However, once in contact with water, these materials may degrade causing problems and/or failures during the service life of the structure.

Many rock types are prone to degradation when exposed to the cyclic wet/dry and freeze/thaw weathering processes. Rock types that are particularly susceptible to degradation due to these processes are poorly indurated shale and claystone exhibiting high clay content.

The degradation can take the form of swelling, weakening, and ultimately disintegration. For wall backfills, the shear strength of the material may decrease with time resulting in greater earth pressures and continued wall lateral displacements. Methods to evaluate degradation potential are provided in Chapter 7 of GEC No. 5 (Sabatini et al., 2002).

2.6 PARAMETERS FOR IN-SITU SOILS