Although its use has become more widespread, controlled low-strength material, or CLSM, has fallen through the crack between geotechnical engineering and materials engineering research. The National Ready Mix Association states that CLSM is not a low strength concrete, and geotechnical engineers do not consider it as a conventional aggregate backfill. The use of CLSM as a bridge abutment backfill material brings up the need to understand the passive force versus backwall displacement relationship for this application. To safely account for forces generated due to seismic activity and thermal expansion in bridge design, it is important to understand the passive force versus backwall displacement relationship. Previous researchers have pointed out the fallacy of designing skewed bridges the same as non-skewed bridges. They observed that as the bridge skew angle increases, the peak passive force is significantly diminished which could lead to poor or even unsafe performance. The literature agrees that a displacement of 3-5% of the wall height is required to mobilize the peak passive resistance. The shape of the passive force displacement curve is best represented as hyperbolic in shape, and the Log Spiral method has been confirmed to be the most accurate at predicting the peak passive force and the shape of the failure plane. All of the previous research on this topic, whether full-scale field tests or large-scale laboratory tests, has been done with dense compacted sand, dense granular backfill, or computer modeling of these types of conventional backfill materials. However, the use of CLSM is increasing because of the product's satisfactory performance as a conventional backfill replacement and the time saving, or economic, benefits. To determine the relationship of passive force versus backwall displacement for a CLSM backfilled bridge abutment, two laboratory large-scale lateral load tests were conducted at skew angles of 0 and 30°. The model backwall was a 4.13 ft (1.26 m) wide and 2 ft (0.61 m) tall reinforced concrete block skewed to either 0 or 30°. The passive force-displacement curves for the two tests were hyperbolic in shape, and the displacement required to reach the peak passive resistance was approximately 0.75-2% of the wall height. The effect of skew angle on the magnitude of passive resistance in the CLSM backfill was much less significant than for conventional backfill materials. However, within displacements of 4-5% of the backwall height, the passive force-displacement curve reached a relatively constant residual or ultimate strength. The residual strength ranged from 20-40% of the measured peak passive resistance. The failure plane did not follow the logarithmic spiral pattern as the conventional backfill materials did. Instead, the failure plane was nearly linear and the failed wedge was displaced more like a block with very low compressive strains.

Bro 2004, BV Bro and the Eurocodes give guidelines for how to consider earth pressure induced by change in temperature and braking forces when designing backwalls. In this thesis those demands are investigated using PLAXIS for evaluation of the earth pressure. The results show that the model in PLAXIS corresponds quite well with the conventions in Bro 2004 and that modelling in PLAXIS gives reliable results. The demand in Bro 2004 that backwalls always shall be designed for passive earth pressure has been found to be pessimistic. In case of long bridges and short backwalls passive earth pressure is most often reached but for shorter bridge lengths in combination with longer backwalls this is almost never the case. It was also found that PLAXIS is sensitive and that the structure of the model and the choice of input are essential. A model in PLAXIS doesn’t make the design more effective but it may be a good tool for analysing the effect of the earth pressure combined with other effects such as the patterns for displacement as well as moment- and force distributions. / Bro 2004, BV Bro och Eurocode ger råd för hur jordtryck som uppkommer på grund av temperaturändring och bromskraft skall tas hänsyn till vid dimensionering av ändskärmar. I detta examensarbete undersöks dessa dimensioneringskrav med hjälp av PLAXIS för att göra en bedömning av jordtrycket. Resultaten visar att modellen i PLAXIS överensstämmer ganska väl med de konventioner som ges i Bro 2004 och att PLAXIS ger tillförlitliga resultat. Kravet att ändskärmar alltid ska dimensioneras för passivt jordtryck visade sig vara pessimistiskt. I fall med långa broar och korta ändskärmar nås ofta passivt jordtryck men för kortare broar med djupare ändskärmar är detta nästan aldrig fallet. PLAXIS visade sig vara känsligt för hur modellen byggs upp och vilka indata som ändvänds, varför dessa bör väljas försiktigt. En modell i PLAXIS medför inte en mer effektiv dimensionering men kan vara ett bra verktyg för analys av jordtryck i kombination med andra effekter så som förskjutningsmönster samt moment- och kraftdiagram.

The effects of seismic forces and thermal expansion on bridge performance necessitate an accurate understanding of the relationship between passive force and backwall deflection. In past case studies, skewed bridges exhibited significantly more damage than non-skewed bridges. These findings prompted studies involving numerical modeling, lab-scale tests, and large-scale tests that each showed a dramatic reduction in passive force with increased skew. Using these results, a correlation was developed between peak passive force and backwall skew angle. The majority of these tests had length to height ratios of 2.0; however, for several abutments in the field, the length to height ratio might be considerably higher than 2.0. This change in geometry could potentially affect the validity of the previously found passive force reduction correlation. To explore this issue, laterally loaded, large-scale pile cap tests were performed with densely compacted sand at a length of 11 ft (3.35 m) and a height of 3 ft (0.91 m), resulting in a length to height ratio of 3.7. The backwall interface was adjusted to fit three various skew angles including: 0°, 15° and 30°. The behavior of both the pile cap and adjacent soil backfill were monitored under these conditions. The peak passive force for the 15° and 30° tests were found to be 71% and 45%, respectively, of the peak passive force for the 0° skew test. These findings are relatively consistent with previously performed tests. Passive forces peaked at deflections between 2% and 5% of the backwall height, decreasing with skew angle. All skews exhibited a log spiral failure plane that transitioned into a linear plane. These results also agreed with previously reported values for large-scale passive force-deflection tests. Rotation of the pile cap was detected in the direction opposite to the skew. Higher pressures were found to be on both corners of the pile cap than in the middle portion, as is suggested by the elastic theory.

Passive force-deflection behavior for densely compacted backfills must be considered in bridge design to ensure adequate resistance to both seismic and thermally induced forces. Current codes and practices do not distinguish between skewed and non-skewed bridge abutment geometries; however, in recent years, numerical models and small-scale, plane-strain laboratory tests have suggested a significant reduction in passive force for skewed bridge abutments. Also, various case studies have suggested higher soil stresses might be experienced on the acute side of the skew angle. For these reasons, three large-scale tests were performed with abutment skew angles of 0, 15 and 30 degrees using an existing pile cap [11-ft (3.35-m) wide by 15-ft (4.57-m) long by 5.5-ft (1.68-m) high] and densely compacted sand backfill confined by MSE wingwalls. These tests showed a significant reduction in passive force (approximately 38% as a result of the 15 degree skew angle and 51% as a result of the 30° skew angle. The maximum passive force was achieved at a deflection of approximately 5% of the backwall height; however, a substantial loss in the rate of strength gain was observed at a deflection of approximately 3% of the backwall height for the 15° and 30° skew tests. Additionally, the soil stiffness appears to be largely unaffected by skew angle for small displacements. These results correlate very well with data available from numerical modeling and small-scale lab tests. Maximum vertical backfill displacement and maximum soil pressure measured normal to the skewed backwall face were located on the acute side of the skew for the 15° and 30° skew test. This observation appears to be consistent with observations made in various case studies for skewed bridge abutments. Also, the maximum outward displacement of the MSE wingwalls was located on the obtuse side of the skew. These findings suggest that changes should be made to current codes and practices to properly account for skew angle in bridge design.

Bridge abutments are designed to withstand lateral pressures from thermal expansion and seismic forces. Current design curves have been seen to dangerously over- and under-estimate the peak passive resistance and corresponding deflection of abutment backfills. Similar studies on passive pressure have shown that passive resistance changes with different types of constructed backfills. The effects of changing the length to width ratio, or including MSE wingwalls determine passive force-deflection relationships. The purpose of this study is to determine the effects of the wall heights and of the MSE support on passive pressure and backfill failure, and to compare the field results with various predictive methods. To compare the effects of backfill geometries, three large-scale tests with dense compact sand were performed with abutment backfill heights of 3 ft (0.91 m), 5.5 ft (1.68 m), and 5.5 ft (1.68 m) confined with MSE wingwalls. Using an existing pile cap 11 ft (3.35 m) wide and 5.5 ft (1.68 m) high, width to height ratios for the abutment backfills were 3.7 for the 3ft test, and 2.0 for the 5.5ft and MSE tests. The failure surface for the unconfined backfills exhibited a 3D geometry with failure surfaces extending beyond the edge of the cap, increasing the "effective width", and producing a failure "bulb". In contrast, the constraint provided by the MSE wingwalls produced a more 2D failure geometry. The "effective width" of the failure surface increased as the width to height ratio decreased. In terms of total passive force, the unconfined 5.5ft wall provided about 6% more resistance than the 5.5ft MSE wall. However, in terms of passive force/width the MSE wall provided about 70% more resistance than the unconfined wall, which is more consistent with a plane strain, or 2D, failure geometry. In comparison with predicted forces, the MSE curve never seemed to fit, while the 3ft and 5.5ft curves were better represented with different methods. Even with optimizing between both the unconfined curves, the predicted Log Spiral peak passive forces were most accurate, within 12% of the measured peak resistances. The components of passive force between the unconfined tests suggest the passive force is influenced more by frictional resistance and less by the cohesion as the height of the backwall increases.