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Publication Number: FHWA-HRT-11-026
Date: January 2011

Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY

2.1 NOTATION

α Angle between wall face and projection of the midline of the surcharge to the wall face [rad]
αb Angle between wall face and projection of the midline of the bridge surcharge to the wall face [rad]
β Angle between the projections of the inner and outer edge lines of the surcharge to the wall face [rad]
βb Angle between the projections of the inner and outer edge lines of the bridge surcharge to the wall face [rad]
γ Unit weight of soil [F/L3]
γb Unit weight of retained backfill [F/L3]
γDC MAX Maximum load factor for dead load (DL)
γDC MIN Minimum load factor for DL
γEH MAX Maximum load factor for horizontal earth pressure
γEH MIN Minimum load factor for horizontal earth pressure
γES MAX Maximum load factor for earth surcharge
γES MIN Minimum load factor for earth surcharge
γEV MAX Maximum load factor for vertical earth pressure
γEV MIN Minimum load factor for vertical earth pressure
γf Unit weight of foundation soil [F/L3]
γLS Load factor for live load (LL) surcharge
γr Unit weight of reinforced backfill [F/L3]
γrb Unit weight of road base material [F/L3]
εL Lateral strain
εV Vertical strain
μ Friction factor between the wall base and the foundation
σh Lateral pressure [F/L2]
σh,f Equivalent lateral stress distribution due to the retained soil behind the GRS abutment [F/L2]
σh,f Factored lateral pressure [F/L2]
σh,bridge Lateral pressure due to bridge DL surcharge within GRS [F/L2]
σh,bridge,eq Lateral pressure due to the equivalent bridge load [F/L2]
σh,bridge,f Factored lateral pressure due to the equivalent bridge load [F/L2]
σh,LL Lateral stress distribution due to the equivalent superstructure LL pressure [F/L2]
σh,q Lateral pressure due to surcharge loading [F/L2]
σh,rb pressure due to road base surcharge within GRS [F/L2]
σh,rb,f Factored lateral pressure due to road base surcharge within GRS [F/L2]
σh,t Lateral pressure due to traffic surcharge within GRS [F/L2]
σh,t,f Factored lateral pressure due to traffic surcharge within GRS [F/L2]
σh,total Total lateral pressure due to loads on GRS mass [F/L2]
σh,W Lateral stress due to weight of GRS [F/L2]
σv,base,n Nominal vertical pressure at the base of the GRS mass [F/L2]
σv,base,R Factored vertical pressure at the base of the GRS mass [F/L2]
ΣMD Total driving moment [L-F/L]
ΣMD,R Total factored driving moment [L-F/L]
ΣMR Total resisting moment [L-F/L]
ΣMR,R Total factored resisting moment [L-F/L]
ΣV Total vertical load [F/L]
ΣVR Total factored vertical load [F/L]
φ Soil friction angle [deg]
φb Friction angle of retained backfill [deg]
φcrit Critical friction angle [deg]
φdesign Friction angle of reinforced fill used in design [deg]
φf Friction angle of foundation soil [deg]
φr Friction angle of reinforced backfill [deg]
φrb Friction angle of road base material [deg]
φreb Repose angle [deg]
φtest Friction angle of reinforced fill found from standard direct shear test [deg]
Φτ Resistance factor for shear resistance
Φbc Resistance factor for bearing capacity
Φcap Resistance factor for ultimate capacity
Φreinf Resistance factor of the required reinforcement strength
ω Batter angle [deg]
a Distance between the back of the wall face and a surcharge (setback) [L]
ab Setback distance between the back of the face and the beam seat [L]
arb Setback distance for the road base surcharge over the GRS mass [L]
at Setback distance for the traffic surcharge over the GRS mass [L]
b Bearing width for bridge; beam seat [L]
bblock Width of the facing element [L]
bq Width of surcharge loading [L]
bq, vol Width of the load along the top of the wall (including the setback) [L]
brb, t Distance over which the road base DL and roadway LL surcharges act over the GRS mass [L]
B Base length of reinforcement not including the wall face [L]
B' Effective foundation width [L]
Bb Width of the bridge [L]
BRSF Width of the RSF [L]
Btotal Total base width of the GRS abutment including the block face
c Cohesion [F/L2]
Cb Cohesion of retained backfill [F/L2]
Cf Cohesion of foundation soil [F/L2]
Cr Cohesion of reinforced backfill [F/L2]
Cu Undrained shear strength of foundation soil [F/L2]
de Clear space distance [L]
dmax Maximum grain size [L]
D50riprap Mean grain size for riprap
Df Depth of embedment [L]
DL Maximum lateral displacement [L]
DV Vertical settlement in the GRS mass [L]
DRSF RSF depth [L]
eB,n Nominal eccentricity for bearing capacity calculations [L]
eB,R Factored eccentricity for bearing capacity calculations [L]
Fb Lateral force due to the retained backfill [F/L]
Fn Nominal driving force for direct sliding calculations [F/L]
Frb Lateral force due to the road base surcharge [F/L]
FR Factored driving force for direct sliding calculations [F/L]
Ft Lateral force due to LL on the roadway [F/L]
FS Factor of safety
FSbearing Factor of safety against bearing failure
FScapacity Factor of safety for vertical capacity using the empirical method
FSreinf Factor of safety for required reinforcement strength
FSslide Factor of safety against direct sliding
G Grade [L/L]
heq Equivalent height of overburden for traffic surcharge [L]
hrb Height of road base (equals height of bridge beam) [L]
H Height of the GRS abutment including the clear space distance [L]
Habut Height of the GRS abutment [L]
Ka Coefficient of active earth pressure
Kab Coefficient of active earth pressure for the retained backfill
Kar Coefficient of active earth pressure for the reinforced backfill
Kpr Coefficient of passive earth pressure for the reinforced backfill
Labut Abutment length [L]
Lblock Length of a facing block [L]
Lspan Span length of the bridge [L]
(LL + IM)total Governing abutment reaction for the HL-93 LL model for one lane
Nγ Dimensionless bearing capacity coefficient
Nblock Number of facing blocks in a column
Nc Dimensionless bearing capacity coefficient
Nlanes Number of lanes
Nq Dimensionless bearing capacity coefficient
q Surcharge load [F/L2]
qb Equivalent superstructure DL pressure [F/L2]
qLL Equivalent superstructure LL pressure [F/L2]
qn Bearing capacity of the foundation soil [F/L2]
qn,an Nominal ultimate load-carrying capacity of the foundation using the analytical method [F/L2]
qn,emp Nominal ultimate load-carrying capacity of the foundation using the empirical method [F/L2]
qR Factored bearing resistance [F/L2]
qrb Surcharge due to the structural backfill (road base) DL [F/L2]
qt Equivalent roadway LL surcharge [F/L2]
qult,an Ultimate load-carrying capacity of GRS using the analytical method [F/L2]
qult,emp Ultimate load-carrying capacity of GRS using the empirical method [F/L2]
QLL LL reaction load [F]
Rn Nominal resisting force for direct sliding calculations [F/L]
RR Factored resisting force for direct sliding calculations [F/L]
RFglobal Global reduction factor for the geosynthetic to account for long-term strength losses due to installation damage, creep, and durability [dimensionless]
Se Superelevation angle [deg]
Sk Skew angle [deg]
Sv Reinforcement spacing [L]
T@ε = 2% Reinforcement strength at 2 percent reinforcement strain [F/L]
Tallow Allowable reinforcement strength [F/L]
Tf Ultimate reinforcement strength [F/L]
Tf,f Factored reinforcement strength [F/L]
Treq Required reinforcement strength [F/L]
Treq,f Factored required reinforcement strength [F/L]
Vallow,an Factored applied stress on top of GRS mass using the analytical method [F/L2]
Vallow,emp Factored applied stress on top of GRS mass using the empirical method [F/L2]
Vapplied Applied stress on top of GRS mass [F/L2]
Vapplied,f Factored applied stress on top of GRS mass [F/L2]
W Weight of the GRS abutment backfill [F/L]
WB Total width of riprap [L]
Wblock Weight of an individual facing block [F]
Wface Weight of the facing elements [F/L]
WL Distance between abutment faces [L]
WRSF Weight of the RSF [F/L]
Wt Total weight (weight of GRS plus weight of bridge beam plus weight of the road base over the GRS mass only) [F/L]
Wt,R Factored total resisting weight (weight of GRS plus weight of bridge beam plus weight of the road base over the GRS mass only) [F/L]
WT Width of level riprap along the top [L]
x Distance from the edge of the load to the point of interest for lateral pressure [L]
Ysc Contraction scour plus long-term degradation scour referenced to the thalweg [L]
YTot Distance from the top of riprip to the bottom of riprap [L]
xRSF Length of the RSF in front of the abutment wall face [L]
Z Location along height of wall (measured from the top of the wall) [L]

AMENDED May 24, 2012

 

2.2 ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials
ASD Allowable Stress Design
ASTM American Society for Testing and Materials (known as ASTM International)
CIP Cast-in-place
CMU Concrete masonry unit
DL Dead load
EDM Electronic distance measurement
FHWA Federal Highway Administration
GRS Geosynthetic Reinforced Soil
HEC Hydraulic Engineering Circular
IBS Integrated Bridge System
IDOT Illinois Department of Transportation
LL Live load
LRFD Load and Resistance Factor Design
MSE Mechanically stabilized earth
NCHRP National Cooperative Highway Research Program
NYSDOT New York State Department of Transportation
PET Polyethylene terephtalate (polyester)
PP Polypropylene
HDPE High density polyethylene
QA Quality assurance
QC Quality control
RSF Reinforced soil foundation
SPT Standard penetration test
SRW Segmental retaining wall
VDOT Virginia Department of Transportation
WSDOT Washington State Department of Transportation

 

2.3 TERMINOLOGY

Biaxial: Reinforcement strength is approximately equal in both the machine and the cross machine directions.

Clear space: The vertical distance between the top of the wall face (block) and base superstructure. Typically, this distance is about 3 inches or at least 2 percent of the wall height.

GRS: Alternating layers of compacted granular fill reinforced with geosynthetic reinforcement (e.g., geotextiles, geogrids). The primary reinforcement spacing in GRS is less than or equal to 12 inches. Facing elements can be frictionally connected to the reinforcement layers to form the outer wall. The facing elements do not need mechanical connections to each other or the layers of reinforcement. The outer wall facing can be built with natural rock, concrete modular block, gabions, timber, or geosynthetic wrapped face. GRS is generic and can be built with any combination of geosynthetic reinforcement, compacted granular fill, and facing system, although some combinations of the three components are more compatible than others.

GRS abutment: A GRS system designed and built to support a bridge. Usually, GRS abutments have three sides: the abutment face wall and two wing walls. All GRS abutments must have the abutment face wall. In some circumstances, depending on the layout, a GRS abutment can be built with one or none of the wing walls.

GRS abutment face wall: The vertical or near vertical wall parallel to the center of bearing and designed to support the bridge. The length of a GRS abutment face wall is typically the total width of the bridge structure plus any additional width necessary to accommodate the structure (e.g., guardrail deflection distance).

GRS–IBS: A unique application of GRS technology in the specific context of bridge abutments. GRS–IBS is different from other, more general GRS abutments that use many common elements associated with traditional bridge abutments. GRS–IBS bridge abutments are built to economically support a bridge on the granular fill directly behind the block face. GRS-IBS can be used to integrate the bridge structure with the bridge approach to create a jointless bridge system. One version of GRS–IBS uses adjacent concrete box beams or void slabs supported directly on the GRS abutments without a concrete footing or elastomeric pads. The bridge has no CIP concrete or approach slab. A typical cross section of IBS shows a GRS mass compacted directly behind the bridge beams to form the approach way and to create a smooth transition from the roadway to the bridge. Another version of GRS–IBS uses steel girders with either a CIP footing or a precast sill. The footing or sill is placed directly on the GRS abutment. The reinforcement layers behind the beam ends are wrapped to confine the compacted approach fill against the beam ends and the adjacent side slopes to prevent lateral spreading. Since the wrapped–face GRS mass behind the beam ends is free standing, the active lateral pressure against the beam ends is considered negligible. The wrapped–face fill also prevents migration of fill during thermal bridge cycles and vehicle LL.

GRS mass or GRS structure: A composite mass built with GRS that creates a freestanding, internally supported structure with reduced lateral earth pressures with considerable strength. This design permits the use of lightweight modular blocks and the elimination of mechanical connections between blocks and the reinforcement. A GRS mass is not rigid and is therefore tolerant to differential foundation settlement.

GRS wall: Any wall built with GRS.

GRS wing wall: A wall attached and adjacent to the abutment face wall. The wing walls are built at the same time as the abutment face wall and at a right or other angle to the abutment face wall. The wing walls are built to support the roadway and the approach embankment. The wing walls must be designed to retain the soil fill in the core of the approach embankment and to protect the abutment from erosion.

Setback: The lateral distance from the back of the wall face to the front of the bearing area. This distance must be a minimum of 8 inches.

Uniaxial: Reinforcement strength is larger is one direction than the other.

 

 

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