bearing capacity at the pavement side - the nvf method, hvtt12 granlund et al
DESCRIPTION
Traditional pavement bearing capacity analysis is based on an assumption of infinite geometry without any road edge; ”geometric compatibility”. This simplification leads to systematic under-dimensioned pavements at roads with narrow shoulders. The need for a model for analysis of bearing capacity at the road edge was pointed out by Nils Odemark already in 1956. As showed in the WAASHO Road Test in 1955, roads with weak edges exhibit fast deterioration, resulting in damages such as rutting, cracking and roughness in the outer (truck) wheel path. In Sweden, some 10 % of budgets for maintenance of paved roads are spent on repair of road edge deformations. For truckers, road edge damages on narrow roads bring serious risk for loss-of-control crashes. The crash type where most truck drivers are injured is the rollover. A high CoG makes heavy trucks prone to lateral forces, such as from roll-vibration at non-uniform road edge deformations. In the Nordic countries, over 1 000 truck rollovers occur every year. Roll-related lateral buffeting is particularly hazardous on ice-slippery surfaces, as it may cause the truck (or driver, by improper manoeuvring) to skid. Sustainable road edges require a relevant method for analysis of bearing capacity. Such a method has been in use in Denmark for decades. This method is based on classic geo-technological analysis as established by Terzaghi, and it has now been updated. The model shows that with regular 0.25 m shoulder, the outer wheel path may have as little as 45 % of the bearing capacity at the road centre. Sensitivity analysis shows that key factors are slope towards ditch, shoulder width, depth of ditch and pavement bearing capacity at the road centre. The method is currently being reviewed in the Norwegian Durable Roads programme, where achieving stable road edges have been given top priority. The insight from the sensitivity analysis has also made an immediate impact on the largest road reinforcement project ever in Sweden, the 157 km Pajala iron ore express road, a road that will be serving a 90 ton high-capacity truck every 7´th minute starting in 2013. The method for analysis of bearing capacity at the road edge is published as Nordic Road Association´s Report 04/2012.TRANSCRIPT
BEARING CAPACITY AT THE PAVEMENT SIDE
- THE NVF/NRA ANALYSIS METHOD
Presenter: Johan Granlund, Vectura
Co-authors: Jan M Jansen, Danish Road Directorate
Sebastian Pettersson, Oden Enterprise
Outline
Unstable road edges is a serious heavy truck crash risk.
Road edge deformation is common and repair is costly.
Low bearing capacity at edge neglected by road design codes.
Updating the Danish method.
Impact on national design codes and road projects.
Harmonizing into a Nordic method in NVF report 04/2012.
Heavy vehicles
are extra susceptible to lateral forces
The crash type where
most truck drivers are
injured is the rollover.
Higher C.o.G. makes the
vehicle prone to lateral
forces. [NVF report 02/11].
[Source: Volvo Trucks crash investigation]
Crashes with
severely injured truck
drivers / passengers
Typical number of truck rollovers:
• Norway: 200 per year
• Finland: 200 per year
• Sweden: 650 per year [Source: Insurance-companies IF & LF]
Photo: Volvo Trucks
Roll-related lateral buffeting
at road edge deformations
This is what lateral buffeting looks like, Rd 331 in Sweden:
Photo: J Granlund
Lateral buffeting is extremely hazardous on ice-slippery roads,
as it can trigger vehicle (or driver) to develop a skid.
Roll-related lateral buffeting
at road edge deformations
Roadex IV project (Finland):
Truck ride sensors recorded
lateral buffeting up to 0.5 – 0.7 G
at many edge damages.
Reference: Cargo latches are
designed for 0.7 G.
Photo: J Granlund
Road edge deformation is common
and the repair is costly
On roads with narrow shoulders, deterioration typically
starts in the outer wheelpath and at the pavement edge.
Damage modes include rutting, cracking and
roughness/warpiness.
Edge slump is a significant problem on the road networks
in the Nordic countries (less of a problem in Denmark).
The problem of weak edges has been given much focus in
Finland, Sweden and Norway [Gullberg, Lehtipuu,
Lampinen, Mork, Aksnes, Weydahl, Granlund and others].
Approximately 10 % of the budget for maintenance of
paved roads is spent on repair of edge deformations.
Analysis of bearing capacity
close to the road edge
Denmark implemented an edge analysis method decades ago.
Based on classic geotechnology; Terzaghi and Vesic.
With regular 0.25 m shoulder, the outer wheelpath may have as little as 45 % of the b.c. at the road center.
Sweden has updated the method:
• References to Eurocode 7 ”Geotechnical Design”.
• New figures.
• Parameter sensitivity test.
• Application cases.
Road edge bearing capacity
Key parameters:
• Slope towards ditch.
• Shoulder width.
• Depth of ditch / Embankment.
• Pavement bearing capacity (at the road center).
Access road provides lateral support:
No highway edge deformation;
Compare with the 7 cm deep depression just in
front of the access road (Note exploded truck tyre).
Denmark: Wide shoulder provides lateral support. Photo: J Granlund
Impact on design codes and road projects
The method is derived from the Danish ”Vejregler” codes.
C/B-analysis of widening roads being made in the Norwegian Durable Roads programme.
Immediate impact on the design of Pajala Iron Ore Express road in Sweden; A 90 ton high-capacity truck every 7´th min.
Finland and Iceland recognize common problems with weak road edges, and lack analysis method up until now.
E6 at Stortjønna, Norway:
The narrow road section forced
oncoming truck to park on the tiny
shoulder, to avoid mirror crash.
Photo: Johan Granlund
Summary
Unstable road edges is a serious heavy truck crash risk.
Road edge deformation is common and repair is costly.
Edge bearing capacity neglected by road design codes.
Updating the Danish method.
Impact on national design codes and road projects;
Harmonizing into a Nordic method in NVF report 04/2012.