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ROADEX approach on deterioration of low volume roads
Johan Ullberg, VägverketPauli Kolisoja, Tampere University of Technology
Introduction of the ROADEX project
Classification of rutting modes on low volume roads
Basic solutions to rutting problems
ROADEX design approach against rutting
Conclusions
OUTLINE OF THE PRESENTATION
We can define three modes of rutting, depending on how and where the plastic strain accumulates
- Mode 0 = Compaction strain in upper layers
- Mode 1 = Shear strains in the near-surface layers
- Mode 2 = Shear strains in deeper layers (especially the subgrade)
THREE MODES OF RUTTING
During spring thaw:
- deeper soil / sub-base is frozen
- upper aggregate is thawed
- upper aggregate can’t drain due to frozen underlayer, so tends to contain excess moisture
- thus upper aggregate prone to Mode 1 rutting
MODE 1 RUTTING DURING THEEARLY SRPING THAW
Frozen
Use a stronger base layer if Mode 1 (upper layer) rutting is the problem
- Inappropriate if Mode 0 or Mode 2 is the problem
Use a stiffer base layer if Mode 2 (lower layer) rutting is the problem OR Use same material in thicker layer
- Inappropriate if Mode 0 or Mode 1 is the problem
BASIC SOLUTIONS TO RUTTING
Structures to be analysed are unsurfaced/thinly sealed
Enables different wheel configurations/tyre inflation pressures to be analysed
Is based on a mechanistic analysis approach
Is still simple to use; at least shouldn’t assume the availability of sophisticated computational tools
Determination/estimation of the required design parameters should be a ’reasonable’ task
REQUIREMENTS SET FOR THE DESIGN APPROACH
Wheel configuration: dual wheel/super single
Tyre inflation pressure: 800 kPa/400 kPa
Thickness of the unbound layer (in relation to radius of the loaded area)
Aggregate stiffness/subgrade stiffness ratio
Mohr-Coulomb strength parameters (c and ϕ) for the unbound base course aggregate
Undrained shear strength of the subgrade
VARIABLES/PARAMETERS CONSIDERED IN THE ROADEX III DESIGN APPROACH
An analysis of stress corresponding to the actual loading case is made; in practise a simple parameter Sis determiner by using a set of graphs or tabulated values or a spreadsheet tool
Strength of the base course material is described by a parameter Sf (derived based on the values of c and ϕ)
S/Sf ratio should not exceed 0,90 in dry conditions and 0,75 in wet conditions
BASIC IDEA OF MODE 1 RUTTING ANALYSIS
GRAPHICAL DETERMINATION OF S
1.3
4
242.9kPaDifferent
graphs are available
for different wheel
configurationsand tyre inflation
pressures.
DETERMINATION OF S USING THE TABULATED VALUES
Table continues for 800kPa/DT, 400kPa/SS and 800kPa/SS
Vertical stress acting on top of the subgrade is estimated using a set of ready-made graphs.
Undrained shear strength of the subgrade is determined/estimated.
Vertical stress should not exceed 4 x undrained shear strength of the subgrade.
BASIC IDEA OF MODE 2 RUTTING ANALYSIS
GRAPHICAL DETERMINATION OF THE STRESS AT THE TOP OF SUBGRADE
0
50
100
150
200
250
300
350
400
450
500
1.0 1.5 2.0 2.5 3.0 3.5
Agg.Thickness/Load Radius Ratio
Str
ess
at th
e to
p of
sub
grad
e (k
Pa)
Sup. Singles - 800kPa - Ebas/Esub=2
Sup. Singles - 800kPa - Ebas/Esub=4
Sup. Singles - 800kPa - Ebas/Esub=8
Sup. Singles - 400kPa - Ebas/Esub=2
Sup. Singles - 400kPa - Ebas/Esub=4
Sup. Singles - 400kPa - Ebas/Esub=8
SS-800-R:2
SS-800-R:4
SS-800-R:8
SS-400-R:2
SS-400-R:4
SS-400-R:8
Different graphs are available
for dual wheel and super
single wheel configurations.
Rutting is clearly the dominating structural problem on the Low Volume Roads of the Northern Periphery Area
Efficient design requires a correct diagnosis of the failure mechanism – identification of the different phases of the seasonal freeze-thaw and respective rutting modes
A fundamental study has been performed to develop a sound but easy-to-use design approach that is based on simple laboratory and in-situ tests
See more details at: www.roadex.org
CONCLUSIONS
The most cost-effective method to improve the condition of a Low Volume Road is to ensure that it has a functioning drainage system
New solutions to the rutting problems are provided by:- non-traditional stabilizers- TPCS technology and special axle configurations- accurate timing of weight restrictions based on in-situ monitoring of spring thaw and new type ofinterplay between road users and owners
See more details at: www.roadex.org
CONCLUSIONS (2)