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Publication Number: FHWA-HRT-11-027
Date: JANUARY 2011

 

Geosynthetic Reinforced Soil Integrated Bridge System Synthesis Report

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY

2.1 NOTATION

γUnit weight of soil [F/L3]
fbBulk unit weight of facing block [F/L3]
δFriction angle between the geosynthetic and the facing block [rad]
Δσ3Change in lateral pressure due to the reinforcement [F/L2]
ΔSDifferential bridge settlement [L]
ΔSabutDifferential abutment settlement [L]
ΔVfaceVolume gained at the face of the GRS mass [L3]
ΔVtopVolume lost at the top of the GRS mass [L3]
εLLateral strain
εVVertical strain
σcLateral confining pressure [F/L2]
σhLateral pressure [F/L2]
σh,binLateral pressure at the face due to bin pressure [F/L2]
σvVertical earth pressure [F/L2]
φSoil friction angle [deg]
abSetback distance between the back of the face and the beam seat [L]
bBearing width for bridge; beam seat [L]
bq,volWidth of the load along the top of the wall (including the setback) [L]
cCohesion [F/L2]
dmaxMaximum grain size [L]
DfDepth of facing block unit [L]
DLMaximum lateral displacement [L]
DVVertical settlement in the GRS mass [L]
EModulus of GRS composite [F/L2]
FbinThrust force found from bin theory [F/L]
HHeight of GRS abutment [L]
KaCoefficient of active earth pressure
KarCoefficient of active earth pressure for the reinforced backfill
KprCoefficient of passive earth pressure for the reinforced backfill
LLength of the wall [L]
qbEquivalent superstructure DL pressure [F/L2]
qcalcCalculated ultimate capacity [F/L2]
qmeasuredMeasured ultimate capacity [F/L2]
qruptureMeasured vertical capacity at reinforcement rupture [F/L2]
qultUltimate applied vertical load [F/L2]
qult,anUltimate load-carrying capacity of GRS using the analytical method [F/L2]
qult,an,cUltimate load-carrying capacity of GRS using the analytical method with cohesion [F/L2]
qult,emp.Ultimate load-carrying capacity of GRS using the empirical method [F/L2]
SvReinforcement spacing [L]
TReinforcement strength [F/L]
TactualActual reinforcement strength at rupture [F/L]
TcalcCalculated reinforcement strength [F/L]
TfUltimate reinforcement strength [F/L]
TreqRequired reinforcement strength [F/L]
Treq,cRequired reinforcement strength including effect of cohesion [F/L]
wFactor accounting for reinforcement spacing and aggregate size

2.2 ABBREVIATIONS

AASHTOAmerican Association of State and Highway Transportation Officials
ASDAllowable Stress Design
CDOTColorado Department of Transportation
CIPCast-in-place
CISCompaction-induced stresses
CMUoncrete masonry unit
COBCenter of bearing
EDMElectronic distance measurement
FHWAFederal Highway Administration
GRSGeosynthetic Reinforced Soil
GSGCGeneric Soil-Geosynthetic Composite
IBSIntegrated Bridge System
LRFDLoad and Resistance Factor Design
MSEMechanically stabilized earth
NCHRPNational Cooperative Highway Research Program
RSFReinforced soil foundation
SRWSegmental retaining wall
TFHRCTurner-Fairbank Highway Research Center

2.3 TERMINOLOGY

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 live loads.

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.

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