Geosynthetic Reinforced Soil Performance Testing - Axial Load Deformation Relationships

2. TESTING CONDITIONS

A series of 19 performance tests have been conducted (table 1); 5 at the Defiance County, OH, highway maintenance facility and 14 at the TFHRC.

Φ = the peak friction angle, c = the cohesion at peak strength, d_max = the maximum aggregate size, T_ƒ = the ultimate reinforcement strength, expressed as the minimum average roll value (MARV) from ASTM D4595 testing,⁽¹⁾ and Sν = the reinforcement spacing.
^ MARV value.
*Rounded pea-gravel angularity.
**Two courses of bearing bed reinforcement placed at the top of the PT.
***Uncompacted sample, +technical difficulties required termination during testing.
++Technical difficulties resulted in unloading/reloading of the composite.

2.1 BACKFILL CONDITIONS

In total, six unique backfill types were used in the PTs: (1) an AASHTO No. 8 crushed, manufactured limestone aggregate obtained from Defiance County OH, (2) an AASHTO No. 8 rounded quartz pea gravel (PG) obtained from Defiance County, OH, (3) an AASHTO No. 57 crushed, manufactured limestone aggregate obtained from Defiance County, OH, (4) an AASHTO No. 9 crushed, manufactured limestone aggregate obtained from Defiance County, OH, (5) an AASHTO No. 8 crushed, manufactured diabase aggregate obtained from Loudon County, VA, and (6) an AASHTO A-1-a aggregate obtained from Loudon County, VA (also referred to locally as a Virginia DOT, or VDOT, 21A material).

2.1.1 Sieve Analysis

All backfills used for testing, except for the AASHTO No. 9 aggregates, meet the FHWA specifications for use in bridge abutments. ⁽¹⁾ The gradations of each aggregate are shown in table 2 and figure 14.

Blank cell = no value was measured for that particular sieve number.

Most of the backfills tested are open or poorly graded materials (e.g., AASHTO Nos. 57, 8,and 9); however, the A-1-a material is a well-graded material. Table 3 shows the classification (based on the USCS) along with the maximum aggregate size (d_max), other relevant grain sizes for various percent passing values, and the coefficient of curvature (Cc) and coefficient of uniformity (Cu) for each material tested.

d_max = the maximum aggregate size.
D₈₅ = the aggregate size in which 85 percent of the sample is finer.
D₆₀ = the aggregate size in which 60 percent of the sample is finer.
D₃₀ = the aggregate size in which 30 percent of the sample is finer.
D₁₀ = the aggregate size in which 10 percent of the sample is finer.
Cc = the coefficient of curvature.
Cu = the coefficient of uniformity.

2.1.2 Density

To determine the in-place compaction requirements of the backfill material, a standard Proctor test was conducted according to Method D of AASHTO T99 for the AASHTO A-1-a (VDOT 21A) material.⁽¹⁵⁾ In addition, vibratory tests were conducted according to ASTM D4252 for the open-graded materials to determine the maximum dry density.⁽¹⁶⁾ The results are shown in table 4.

N/A = Not applicable, there is no optimum moisture content for open-graded aggregates since they are free draining.

2.1.3  Friction Angle

The strength properties of each aggregate were determined using a large scale direct shear (LSDS) device according to ASTM D3080. ⁽¹³⁾ The LSDS device at TFHRC is 12 x 12 x 8 inches in dimension and is capable of testing aggregates up to 1.2 inches. For this series of experiments, the unscalped aggregates were tested at four applied normal stresses, 5, 10, 20, and 30 psi, at a shear rate of 0.015 inches/min and a gap size equal to the D₈₅ of the material (i.e., the aggregate size where 85 percent of the sample is smaller; see table 3).

The open-graded materials were tested in a dry, uncompacted state prior to the consolidation phase in the LSDS device. The well-graded material (VDOT 21A) was tested at 100 percent of the maximum dry density (i.e., the level of compaction achieved during each PT with the backfill), and at the optimum moisture content (see table 4). Since the shear strength failure envelope for these aggregates is non-linear, the reported cohesion for the well-graded material (VDOT21A) was determined through a series of LSDS tests that were performed with the compacted state fully saturated; note that the resulting peak friction angle in this case was similar to the non-saturated condition.

The results of the LSDS testing are shown in table 5 and in figure 15. Note that the reported friction angle is based on the measured peak strength during testing and assumes a linear Mohr-Coulomb envelope for the range of confining stresses tested. Note that peak strength for these backfill materials is mobilized at typically 0.5- to 1-inch lateral displacement in the LSDS device, which corresponds to about 4- to 8-percent lateral strain for the 12-inch shear box. In the context of the PT though, the GRS composite is tested to failure, sometimes well beyond 8-percent lateral strain. From a theoretical perspective, it may be more appropriate to model the failure of a GRS composite by using the friction angle at the fully softened state of the backfill material during the LSDS test; however, to conform to the current standard-of-practice, the peak strength of the reinforced backfill material was selected to calibrate the design as it is the commonly reported value and easiest to ascertain from typical testing. Figure 15 provides the raw data for LSDS testing.

2.2 REINFORCEMENT CONDITIONS

In all of the tests, a biaxial, woven polypropylene geotextile was used as the reinforcement element; however, different strengths and stiffness of material were used among the PTs. The manufacturer supplied MARV data is shown in table 6. A more recent property of the geosynthetic used in design is the wide width tensile strength at 2-percent strain which provides an indication of GRS performance at the service limit state.⁽¹⁾ Currently, many of the manufacturers do not make this value publicly available, although they can supply it on request. Note that in the field, the actual strength of the reinforcement will be higher than the reported MARV.

¹Values for the machine (warp) by cross machine (fill) directions, respectively.

2.3 FACING CONDITIONS

A concrete masonry unit (CMU) facing was used on 11 of the 17 tests; the remaining 6 tests were conducted without any facing element. The CMU is a dry-cast, split-faced product with dimensions of 7⅝ x 7⅝ x 15⅝ inches and an approximate weight of 42 lb. The CMUs are frictionally connected to the geotextile reinforcement. The reinforcement overlaps at least 85 percent of the block depth, termed the coverage ratio, as specified by Adams et al. 2011a.⁽¹⁾