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CHAPTER 3
INTERPRETATION, APPRAISAL, AND APPLICATIONS
ASSESSMENT OF THE NCHRP TEST still exhibiting a near-linear load-settlement relationship
ABUTMENTS at 814 kPa, about four times the typical design pressure
of 200 kPa, although the deformation had become fairly
The NCHRP test abutments were assessed in two ways: large (average sill settlement = 163 mm, maximum
(1) the measured performance and observed behavior were lateral movement = 82 mm)--an indication of high duc-
evaluated against existing performance criteria for bridge tility of the abutment system. With a weak reinforce-
abutments; and (2) the safety factors and failure loads of the ment (Tult = 21 kN/m, per ASTM D 4595), however, the
abutments were evaluated using the design method in the ductility was significantly compromised. As the applied
current NHI manual (Elias et al., 2001) through the computer pressure increased beyond 200 kPa, the rate of settle-
program, MSEW. ment continued to increase with increasing applied pres-
sure, and the failure state was only about twice the typ-
ical design pressure of 200 kPa.
Assessment of Measured Performance and
Observed Behavior
The assessment of the measured performance and observed Sill Settlement and Angular Distortion
behavior of the two full-scale test abutments was made
against performance criteria previously established based on · The settlement of the sill was much smaller in the
experiences with real bridges. Load-carrying capacity and Amoco test section than in the Mirafi test section. Under
ductility, sill settlement and angular distortion, and maxi- an applied pressure of 200 kPa, the average sill settle-
mum lateral movement of the abutment wall were all ment was 40 mm and 72 mm in the Amoco test section
assessed. The performance criteria, referred to as "the exist- and the Mirafi test section, respectively. As the loading
ing criteria" in this chapter, are from studies conducted by was being terminated (814 kPa for the Amoco test sec-
Bozozuk (1978), Walkinshaw (1978), Grover (1978), Moul- tion and 414 kPa for the Mirafi test section), the average
ton et al. (1985), and Wahls (1990). sill settlement of the two test sections was comparable,
The differences in performance between the two test abut- 163 mm and 170 mm, respectively.
ments were caused by the difference in the geosynthetic rein- · The existing bridge settlement criteria for ride quality
forcement used for the two test sections. The tensile strength and structural distress range from 50 to 100 mm. The
of the reinforcement used was 70 kN/m for the Amoco test average sill settlement of 40 mm for the Amoco test
section and 21 kN/m for the Mirafi test section. section under a typical design pressure of 200 kPa met
the settlement criterion. For the Mirafi test section,
the average sill settlement of 72 mm under 200 kPa
Load-Carrying Capacity and Ductility might have compromised ride quality, but was still
considered "tolerable." The performance criteria are
· As the loading was being terminated (814 kPa for the for bridges resting on "conventional" abutments.
Amoco test section and 414 kPa for the Mirafi test Given that GRS abutments can provide a "smoother"
section), the Mirafi test section approached a bearing transition between the approach fill and the bridge
failure condition while the Amoco test section appeared structure, the settlement criteria may be relaxed to
to still be sufficiently stable. For a typical design pres- some extent.
sure of 200 kPa, the safety margin in terms of the load- · The typical tolerable maximum angular distortion for
carrying capacity was "acceptable" for the Amoco test single-span bridges is 1:200. For an 18-m (60-ft)-long
section and "marginally acceptable" for the Mirafi test single-span bridge, the "maximum possible" angular
section. distortion would be 1:450 for the Amoco test section
· With a sufficiently strong reinforcement (Tult = 70 and 1:250 for the Mirafi test section--both below 1:200.
kN/m, per ASTM D 4595), the Amoco test section was The test abutments were constructed over a rigid
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97
foundation, thus the settlement that would occur in the (24 mm and 36 mm) were somewhat below the lateral
foundation was not accounted for. movement criterion.
· In the load tests, the sill was loaded in equal increments · There is little information in the literature about long-
of 50 kPa average vertical pressure and maintained for term lateral movement of GRS bridge abutments. For
about 30 minutes between increments. Maintaining the Founders/Meadows abutment, with a fill of 40° fric-
each load increment for 30 minutes was intended to tion angle, the maximum outward movement of the
allow the stresses to be transferred to the entire soil abutment wall caused by placement of bridge super-
mass, not to determine creep deformation. The AASHTO structure was about 9 mm. The additional outward
guideline suggests that creep tests be conducted for movement over 18 months after the bridge was opened
10,000 hours and extrapolated for a maximum of two to the traffic was about 13 mm. Assuming that the long-
log cycles in time. term lateral movement is about the same magnitude as
· Based on the soil-geosynthetic interactive performance the "immediate" lateral movement, the Amoco test
(SGIP) tests performed on Amoco 2044 with a road base section would be "marginally acceptable" and the Mirafi
material by Ketchart and Wu (2001 and 2002), the mag- test section would be "unacceptable."
nitude of creep deformation over the lifetime of a GRS
mass under 200 kPa (30 psi) and in an "unconfined
condition" (a conservative condition) would be about the Observed Behavior
same as the magnitude of the "immediate" settlement.
This means that the Amoco 2044 test under 200 kPa · A tension crack was observed on the wall crest in both
would settle about 80 mm and the Mirafi test section load test sections. The tension crack was first observed
would settle about 145 mm over the design life. at an applied pressure of 150 to 200 kPa. The distinct
This would render the Amoco test section only "margin- tension crack was parallel to the abutment wall face and
ally acceptable" and the Mirafi test section "unaccept- located at end of the reinforcement. The location of the
able" under a design pressure of 200 kPa. The measured tension crack suggests that the assumption of rigid rein-
data from the Founders/Meadows abutment (with a soil forced soil mass in the existing design methods for
friction angle of 40 deg from the standard direct shear evaluating external stability is a sound procedure. The
tests) showed that the sill settled 13 mm because of place- tension cracks might be suppressed by lengthening the
ment of bridge superstructure and an additional settle- top few layers (e.g., three layers) of the reinforcement.
ment of 11 mm occurred over 18 months after the bridge If an upper wall had been constructed over the test
was opened to the traffic. abutment, as in the case of typical bridge abutments, the
· From the standpoint of angular distortion, the maximum tension crack would not have been visible and perhaps
possible long-term angular distortion for the Amoco and would have been less likely to occur.
Mirafi test sections would be 1:225 and 1:125, respec- · Under higher applied loads, the facing blocks in the top
tively, for an 18-m (60-ft)-long single-span bridge under three courses were pushed outward as the sill tilted for-
200 kPa pressure. Again, the Amoco test section would ward. This suggests (1) that the sill clear distance of
only be "marginally acceptable" and the Mirafi test 0.15 m, the minimum value stipulated by the NHI man-
section would be "unacceptable" under a design pres- ual, may be too small, and (2) that it may be beneficial
sure of 200 kPa. to increase the connection strengthening in the top three
to four courses of the facing. The authors believe that the
strengthening effect will be most effective if the facing
blocks are "inter-connected" after all the facing units are
Lateral Movement of Abutment Wall
in place.
· In both test sections, the abutment wall moved outward
with the maximum movement occurring near the top of Assessment of Safety Factors and Failure
the wall. At an average applied pressure of 200 kPa, the Loads by the MSEW Program
maximum movements in the abutment wall were 24 mm
in the Amoco test section and 36 mm in the Mirafi test The NCHRP test abutments were evaluated by MSEW, a
section. The wing-walls also moved outward with the computer program developed by ADAMA Engineering, Inc.,
maximum movement occurring at about H/6 from the for design and analysis of mechanically stabilized earth
top of the wall. At an average applied pressure of 200 walls. The MSEW program follows the design guidelines
kPa, the maximum movement was 18 mm in the Amoco presented in the FHWA Demo-82 manual (Elias et al., 1997)
test section and 30 mm in the Mirafi test section. and the NHI manual (Elias et al., 2001) and is completely
· The existing lateral movement criteria for ride quality compatible with AASHTO Standard Specifications for High-
and structural distress range from 25 to 50 mm. The way Bridges, 16th edition, 1996; as amended by the 1998
maximum lateral displacements for both test sections interim revisions.
