Synthesis and Evaluation of The Service Limit State of Engineered Fills for Bridge Support

CHAPTER 2.
CURRENT SLS DESIGN PROCEDURES AND CRITERIA

2.1 AASHTO Load and resistance factor design (LRFD) Bridge Design Specifications

The AASHTO LRFD Bridge Design Specifications incorporates limit state design in LRFD.⁽⁸⁾ A “limit state” is a condition beyond which a bridge component ceases to satisfy the criteria for which it was designed. All possible structural and geotechnical failure modes for foundations that can lead to bridge failure are grouped into three distinct structural and geotechnical limit states: SLS, strength limit state, and extreme event limit state.⁽⁸⁾ The SLSs are defined as the limit states related to stress, deformation, and cracking under regular operating conditions. In SLSs, failure may be defined as exceeding the tolerable displacement. For example, the foundations must have adequate structural and geotechnical stiffness to keep the bridge displacements less than the bridge tolerable displacements. The failure modes in strength limit states are related to the strength and stability of the foundations under loads and conditions applied continuously or frequently during the bridge design life. The failure modes in extreme event limit states are related to the strength and stability of the foundations under loads and conditions applied during certain events that have a return period greater than the bridge design life. LRFD of spread footings at all limit states is prescribed in the AASHTO LRFD Bridge Design Specifications.⁽⁸⁾ SLS design (Article 10.6.2) covers the settlement and overall stability of spread footings, and Article 10.6.2.4 describes the methods that should be used to estimate settlement of spread footings of bridges on cohesionless and cohesive soils.⁽⁸⁾

Although not explicitly prescribed in the current AASHTO LRFD Bridge Design Specifications, the SLS design of engineered fills follows the same acceptance criterion used for bridge foundations.⁽⁸⁾ Namely, the estimated movement must not exceed the tolerable movement. Thus, current practice uses a variety of empirical, semi-empirical, and numerical modeling approaches to estimate fill deformation, and tolerable movement criteria are based on the permissible deformation of a bridge abutment, approach slab, pavement, or other structural feature in or on the fill.

2.2 Bridge Supports Using MSE

Guidelines for the static design of MSE walls have been published by AASHTO and FHWA. (See references 8 and 35–37.) A study by Koerner and Soong compared three static MSE design methods: the modified Rankine method, the FHWA method, and the National Concrete Masonry Association method.^(38,39) They found that the FHWA method provided a factor of safety value between those by the other two methods. Guidelines for the design of MSE as true bridge abutments are also available that limit MSE settlement to 0.5 inch (13 mm) if service loads are kept below 27.84 psi (192 kPa), but details on the parameters impacting the deformation response are still lacking.^(21,32,37) In addition, the parameters used (e.g., Φ, cohesion (c), etc.) may vary depending on the measurement technique and the design method selected, which may have an impact on the SLS analysis.

The FHWA National Highway Institute (NHI) reference manual, Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, provides a design method for MSE bridge abutments.⁽³⁶⁾ In addition, the FHWA NHI manual, Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes—Volume I, suggests that the following conditions be implemented in the design of MSE abutments:⁽³⁷⁾

• The tolerable angular distortions (i.e., limiting differential settlement) between abutments or between piers and abutments should be limited to 0.005L_as for simple-span bridges and 0.004L_as for continuous-span bridges, where L_as is the distance between adjacent supports.
  
  
• A minimum offset of 3.05 ft (1 m) from the front of the facing to the centerline of the bridge bearing is required.
  
  
• A clear distance of 6 inches (150 mm) between the back face of the facing units and front edge of footing is required.
  
  
• The abutment should be placed on a 3.05-ft (1-m)-thick bed of compacted coarse aggregate where significant frost penetration is anticipated.
  
  
• The bearing capacity on the reinforced volume should be limited to 4,000 lb/ft² (192 kPa).
  
  
• The maximum horizontal force at each reinforcement level should be used for the design of connections to the facing units.
  
  
• The density, length, and cross-section of reinforcements of the abutment should be extended to wing walls for a horizontal distance of 0.5H, where H is the height of the abutment wall.
  
  
• The seismic design forces should also include seismic forces transferred from the bridge through bearing supports that do not slide freely (e.g., elastomeric bearings).

2.3 Bridge Supports Using GRS

FHWA’s Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide provides the current design procedure for GRS abutments.⁽³²⁾ The guide notes that the design methods are appropriate for GRS structures (an abutment and wing walls) with a vertical or near vertical face and at a height that does not exceed 30 ft (9.15 m). The bearing stress on the GRS abutment is limited to a service load of 4,000 lb/ft² (192 kPa). The guide recommends that engineers should limit bridge spans to approximately 140 ft (42.7 m) since the SLS performances (such as thermal-induced movements) of longer spans on GRS-IBS were not as well understood.⁽³²⁾ GRS abutment capacities are dependent on a combination of the strength of the fill material and the strength of the reinforcement when built in accordance with the two rules of GRS construction: (1) good compaction (95 percent of maximum dry unit weight, according to AASHTO T99 for well-graded aggregate backfill) of high-quality granular fill and (2) closely spaced layers of reinforcement (less than or equal to 12 inches (304.8 mm)).⁽¹⁶⁾

Basic design guidelines for GRS foundations are available that outline recommended spacing along with length and depth of reinforcement layers for pier foundations and abutment supports.^(32,40) The work is largely based on load tests on reinforced soil shallow foundations and performance tests (PTs) on abutment supports.^(41,22,42) However, the design guidelines are limited to the conditions and parameters under which testing has occurred. While recent FHWA research has added to the database and provided recommendations on methods to limit deformations to a target value, quantifying the vertical and lateral deformations is still empirically based.⁽⁴²⁾

2.4 Current SLS Criteria for Bridge Supports

In general, foundation movement criteria should be consistent with the function and type of structure, anticipated service life, and consequences of unacceptable movements on structure performance. Specifications and reports published by FHWA, AASHTO, and some State transportation departments provide criteria for the settlement of shallow foundations of bridges and vertical and horizontal deformations of bridge abutments and piers. The following section summarizes the current SLS criteria for bridge supports using shallow foundations.

Criteria for Settlement of Shallow Foundations of Bridges

According to section 11 in AASHTO LRFD Bridge Design Specifications, “Abutments, piers, and walls shall be investigated for excessive vertical and lateral displacement, and overall stability, at the service limit state.”⁽⁸⁾ The vertical settlements of bridge foundations can be expressed in terms of angular distortion, which is defined as the differential settlement divided by span length. Uneven displacements of bridge abutments and pier foundations can affect the ride quality, functioning of deck drainage, and the safety of the traveling public as well as the structural integrity and aesthetics of the bridge. Such movements often lead to costly maintenance and repair measures.⁽¹⁰⁾ Table 3 shows the criteria from various reports. Three different reports show the same criteria for the angular distortion for continuous span bridges, while the criteria for the angular distortion for simple span bridges vary.

WSDOT provided the differential settlement criteria and the associated action based on the total settlement. Table 4 lists the settlement criteria for pier and abutment.⁽⁴⁶⁾

Chapter 10 of Arizona Department of Transportation’s (ADOT) Bridge Design Guidelines states the following:

• “The bridge designer should limit the total settlement of a foundation per 30 m (100 ft) span to 13 mm (0.5 in). Linear interpolation should be used for other span lengths. Higher total settlement limits may be used when the superstructure is adequately designed for such settlements. The designer shall also check other factors such as rideability and aesthetics. Any total settlement that is higher than 2.5 in, per 30 m (100 ft) span, must be approved by the ADOT Bridge Group.”⁽⁴⁷⁾

Through tolerable movement analysis of 148 highway bridges supported by spread footings on compacted fill throughout Washington, an FHWA report concluded that these bridges have easily tolerated differential settlement of 1 to 3 inches (25.4 to 76.2 mm) without serious distress.⁽²⁾ Based on field studies of 314 bridges and theoretical analyses, Moulton et al. found that the bridges that performed acceptably had average settlement of 2 inches (50.8 mm).⁽⁴³⁾

Tolerable Horizontal Deformation Criteria for Bridge Supports

Horizontal deformations cause more severe and widespread problems for highway bridge structures than do equal magnitudes of vertical movement.⁽¹⁰⁾ The data presented by Moulton et al. also show that horizontal movements resulted in more damage when accompanied by settlement than when occurring alone.⁽⁴⁴⁾ Tolerance of the superstructure to horizontal (lateral) movement depends on bridge seat or joint widths, bearing type(s), structure type, and load distribution effects. Moulton et al. found that horizontal movements less than 1 inch (25.4 mm) were almost always tolerable, while horizontal movements greater than 2 inches (50.8 mm) were typically considered to be intolerable.⁽⁴⁴⁾ Wahls states, “Horizontal movements in excess of 2 inches (50 mm) appear likely to cause structural distress.”⁽⁴⁸⁾ Moulton et al. recommends that horizontal movements be limited to 1.5 inches (38.1 mm).⁽⁴⁰⁾ Similarly, surveys of the performance of bridges by Bozozuk, Walkinshaw, and Wahls also indicate that horizontal abutment movements less than 1.5 inches (38.1 mm) can usually be tolerated by bridge superstructures without significant damage.^(49–51)

On the other hand, abutments are often designed for active lateral earth pressure conditions, which require a certain amount of movement. Depending on the configuration of the bridge end spans and expansion joints, horizontal movements of an abutment can be restrained. However, such restraint can lead to an increase in the lateral earth pressures above the active earth pressures normally used in design. Samtani and Nowatzki recommends that, “Design of expansion joints should allow for sufficient movement to keep earth pressures at or close to their design values and still allow the joints to perform properly under all temperature conditions.”(Chapter 8, pp. 69)⁽¹⁸⁾

Tolerable Deformation Criteria for Reinforced Bridge Supports

MSE walls can tolerate larger total and differential vertical deflections than rigid walls. The amount of total and differential vertical deflections that can be tolerated depends on the wall facing material, configuration, and timing of facing construction.⁽⁸⁾ AASHTO states that abutments should not be constructed on MSE walls if the anticipated angular distortion is greater than 50 percent of the values recommended by Moulton et al. as shown in table 3.^(52,44) For GRS abutment, the vertical strain should be limited to 0.5 percent unless the engineer decides to permit additional deformation, and the lateral strain should be limited to 1 percent.⁽³²⁾