Performance of Different Systems Employed for Construction of Bridge Approaches

Performance and Cost-Effectiveness Analyses of Flowfill and MSE Approaches. In these

analyses, a total of 98 bridges were constructed with flowfill backfill material (202 approaches), 14 with MSE Class 1 backfill (28 approaches), and 10 with MSE Class B Backfill (20 approaches). A list of these bridges and detailed performance results for each structure are given in Tables B.4 through Table B.7. Performance results for these approaches are briefly summarized in Table 4.5. Results of the cost-effectiveness analysis are summarized in Table 4.6.

The performance and cost-effectiveness data presented in Tables 4.5 and 6 are based on limited number of data and service years for MSE approaches and therefore should be considered with precaution. In addition, the performance and repair of approaches are not only controlled by abutment backfill but more related to drainage, construction workmanship, embankment, and foundation soil, as discussed before. Findings are

‰ Most of the flowfill and MSE bridge approaches constructed by CDOT since 1993 are

performing well, with no settlement or cracking problems.

‰ Most of the settlement problems for the flowfill approaches occurred with the older

bridge approaches constructed in 1994 when CDOT initiated flowfill operations. The 2005 unit construction cost for flowfill is $76/CY. The estimated total 2005 unit cost of flowfill approaches over their service life of 40 years ranges from $95/CY if no additional repair costs will not be needed in the future to $176/CY if repair will be needed in the future (assuming that past and future repair rates are identical). If flowfill approaches constructed before 1994 were not considered in the cost-effectivness analysis, the unit cost of the flowfill over the entire design life would drop to around $80/CY. This suggests that that the costly flowfill backfill should remain a viable alternative in special applications because it has outstanding performance.

‰ Out of 28 bridge approaches constructed with MSE Class-1 backfill, 4 approaches at two bridge structures failed. Pure construction problems caused the failure of the MSE approaches that could be avoided in the future with better and tighter construction specifications for the backfill and embankment materials. These failures would not be eliminated if flowfill backfill was employed because the primary sources of the settlement problem were the embankment and foundation soils, not the 4-ft thick backfill placed beneath the sleeper slab. However, it is possible that the extent of these failures would be reduced with flowfill because construction problems in MSE backfill, like lack of compaction or construction timing, will not be of concern with flowfill. Generally, MSE backfill is more sensitive to construction problems than flowfill.

‰ The use of MSE Class 1 backfill is cost effective ONLY if the rate of repair of MSE

approaches will decline significantly in the future. In this case, the unit cost of MSE approaches over the entire service life would be much less than that for flowfill and comparable to approaches constructed with MSE Class B. However, if the past rate of repair continues in the future, the unit cost will be much higher because: 1) the repair costs of the four failed MSE Class-1 Backfill approaches were significant; 2) the limited number of constructed MSE approaches; 3) and the relatively short service period of MSE approaches.

‰ The MSE Class B filter material as abutment backfill has the lowest cost. This is because

no repair was reported for the MSE Class B approaches and their current performance is adequate. A more scientific evaluation on the performance and cost-effectiveness of the MSE Class B Backfill material should be made after five or 10 years when both number and average service life of MSE Class B approaches have increased.

Performance of the Founders/Meadows MSE Approaches (Integral Abutment)

•

The overall short- and long-term performance of the approaches of the

Founders/Meadows structure is excellent because there is no evidence of the “bump at the bridge” problem after five years in service, and there are no signs of structural distress to the approach slab and the bridge expansion device.

Drainage measures included placement of impervious membranes with collector pipes at the top of the MSE backfill beneath the approach slab (was not extended beneath the sleeper slab) and a wick drain filtration and collection system at the interface of the backfill and the existing ground.

•

The placed drainage protection system was not fully effective in preventing surface water

from reaching deep inside the reinforced soil mass. The backfill beneath the sleeper slab becomes fully saturated during late June of each year. There are signs of gradual softening of the soil beneath the sleeper slab with time due to increase of retained soil moisture.

•

The design procedure significantly overestimates the loads carried by reinforcement behind

the abutment wall by 3 to 4 times. This implies that the actual factor of safety against breakage and pullout failure of the reinforcement is 3 to 4 times higher than what is estimated in the design.

•

The largest measured horizontal geogrid tensile strains were experienced around the sleeper

slab location (where traffic and approach slab loads are transferred to the soil mass).

•

An overall continuous stretching or expansion of the MSE backfill over the five years of

service was noticed, of approximately 1.5 to 2 mm every year. This behavior could lead to the settlement of the sleeper slab in the long-term and, if confirmed, should be minimized or prevented. This response could be attributed to continuous shortening of the bridge or growth of the roadway (both reported in the literature). Also, it is possible that this behavior is due to the continuous softening of the polystyrene sheets.

A compressible 75 mm thick low-density expanded polystyrene sheet was placed between the reinforced backfill and the abutment walls to accommodate the horizontal expansion movements of the bridge superstructure without affecting the backfill (reduce passive earth pressures) and allow for the free expansion of the MSE backfill (reduce active lateral earth pressures).

•

The presence of polystyrene sheets behind the abutment wall allowed for about 12 mm of

free lateral expansion movement of the MSE backfill. This led to the mobilization of the friction resistance of the backfill and tensile resistance of the reinforcements in the MSE backfill, thus taking most of the lateral earth pressure load off the abutment facing.

Temperatures rise from March or May (warming season), remain high around the summer time from June to August, decrease from September to November (cooling season), and remain low around the winter time from December to February.

•

Each year, the bridge abutment wall pulls away from the backfill due to thermal

contraction movements of the bridge superstructure during the cooling and winter times. This leads to the expansion of the MSE backfill (increase in the reinforcement tensile strains) and reduction of the lateral earth pressures on the abutment wall, even to zero when a void develops between the backfill and the abutment wall during the coldest time of the winter season.

•

Tensile horizontal strains should be expected in the approach slab during the colder

nights and seasons of the year.

•

Each year, the bridge abutment wall is pushed into the backfill due to the expansion

movements of the bridge superstructure during the warming and summer times. This lead to exerting high lateral passive earth pressure (or compression lateral strains) by the abutment wall on the reinforced soil mass (the MSE backfill felt the thermal expansion movement of the bridge superstructure).

•

The presence of compressible polystyrene sheets accommodate to a large extent (but not

entirely) the expansion thermal movement of the bridge superstructure.

o The expansion lateral movements of the MSE fill due to the seasonal changes in

temperatures were estimated around 4 mm. This is much less than the expected expansion lateral movement of the bridge superstructure of 100 mm.

o The measured passive lateral earth pressures were below those reported for integral

abutments where polystyrene sheet was not used and the backfill was not reinforced.

o In the upper zone of the abutment wall, although the presence of polystyrene reduced

the lateral passive pressures by 10 kPa to 15 kPa, high passive lateral earth pressures of 35 kPa were exerted by the abutment on the MSE backfill. This indicates that the thermal expansion movements of the bridge superstructure would have significant influence on the upper zone of the abutment backfill during the summer time. This could lead to the fill settlement problem and should be avoided in the future.

7.3 Recommendations

These recommendations are valid for CDOT current practice for geotechnical investigation, construction, and repair of bridge approaches as presented in Chapter 2.

In document FLOWFILL AND MSE BRIDGE APPROACHES: PERFORMANCE, COST, AND RECOMMENDATIONS FOR IMPROVEMENTS (Page 172-176)