4.1 Introduction
Five tests were performed for this study using the instrumented retaining wall facility.
The test procedures, materials, and results for the instrumented retaining wall tests are presented
in the following sections.
4.2 Test Procedures and Materials
This section describes the test procedures and materials used in the instrumented
retaining wall tests. The backfill material, compaction equipment, wall preparation activities,
backfill placement and compaction procedures, cyclic testing procedures, and the instrumented
retaining wall test schedule are described.
4.2.1 Backfill
The backfill used for the instrumented retaining wall tests is Light Castle sand obtained
from a quarry in Craig County, Virginia. Light Castle sand is a clean, fine sand consisting
predominantly of subangular quartz grains. Filz and Duncan (1992) performed various
laboratory tests on Light Castle sand. For this sand, it was found that 68 percent of the material
passes the No. 40 sieve and less than 1 percent passes the No. 200 sieve. The coefficient of
uniformity and coefficient of curvature were determined to be 1.8 and 0.9, respectively.
Therefore, the sand classifies as a poorly graded sand (SP) according to the Unified Soil
Classification System. The specific gravity of solids is 2.65. The maximum and minimum
densities determined by ASTM D4253-83 and ASTM D4254-83 are 106 and 88.5 pounds per
cubic foot, respectively.
Filz and Duncan (1992) performed two instrumented retaining wall tests using Light
Castle sand, but without a compressible inclusion. The average unit weight of the compacted
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sand was approximately 105.5 pcf, corresponding to a relative density of nearly 100 percent.
The estimated friction angle of the compacted sand was 42 degrees.
4.2.2 Compaction Equipment
For this study, two hand-operated compactors were used: a Wacker model BS60Y
(rammer compactor) and a Wacker model BPU 2240A (vibrating plate compactor). Schematic
diagrams of both compactors are shown in Figure 4.1. The rammer compactor is powered by a 4
horsepower, 2-cycle engine that drives a steel ramming shoe into contact with the soil at a
percussion rate of 10 blows per second. The operating weight of the rammer compactor is 137
pounds. The vibrating plate compactor is powered by a 5 horsepower, 4-cycle engine that drives
counter-rotating eccentric weights. These weights rotate at a frequency of about 100 Hz and are
connected by axles to a steel plate that contacts the soil. The operating weight of the vibrating
plate compactor is 275 pounds.
The rammer and vibrating plate compactor used for this study are commonly employed
for compaction in confined areas and adjacent to retaining wall structures.
These compactors are different in their mode of operation. In a study by Filz and Duncan (1992)
on the two compactors used for this research, it was found that the rammer compactor delivered
higher peak contact forces to the soil than the vibrating plate compactor. Thus, higher
compaction-induced earth pressures can be expected in backfill compacted with the rammer
compactor than in backfill compacted with the vibrating plate compactor.
4.2.3 Wall Preparation Prior to Compaction
Wall preparation consisted of lubricating the end and far walls of the backfill area and
placing TerraFlex on the instrumented wall. Lubrication of the end and far walls was
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a) Rammer compactor
Figure 4.1: Schematic diagrams of a) Rammer Compactor and b) Vibratory Plate
Compactor (After Filz and Duncan 1992)
Eccentric
Weights
Base
Plate
Shock
Absorber
b) Vibrating plate compactor
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performed in order to minimize the buildup of shear stresses along these walls, which could
influence the test results. Lubrication allows the facility to more closely model a 2-D case of an
infinitely long wall and infinitely wide backfill area (Filz and Duncan 1992). To lubricate the
end and far walls, a sheet of 6-mil polyethylene was taped in place on these walls. A thin layer
of wheel bearing grease was applied to the polyethylene sheet, which was then covered with a
second polyethylene sheet. The walls were lubricated for all five tests performed.
The TerraFlex was delivered in pre-cut blocks of the desired thickness. The TerraFlex
was then placed on the face of the instrumented retaining wall using GeoTech DB-784 adhesive
supplied by GeoTech Systems Corporation. The TerraFlex was applied over the full height and
length of the instrumented wall and extended 2.5 feet from the instrumented panels onto the wall
in the access ramp area.
4.2.4 Backfill Placement and Compaction
Before it was used as backfill in the instrumented retaining wall test facility, the Light
Castle sand was dried to less than 0.1 percent hydroscopic moisture and placed in a dry stockpile
area. The sand was moved from the stockpile area to the backfill area by a hopper lifted by an
overhead crane. After depositing the sand in the backfill area, it was spread by hand in loose lifts
of sufficient thickness to produce a compacted lift thickness of 6 inches. Backfill was placed
approximately 6.5 feet high against the instrumented wall for each test.
The rammer compactor delivers higher peak forces to the soil than the vibrating plate
compactor. For tests using the rammer compactor, each backfill lift was compacted with 2
passes. For tests using the vibrating plate compactor, 5 passes were used to compact each lift.
Both procedures produced relative densities near 100 percent.