2953
Design improvements for expansion of a roadway on a thick layer of soft soil
Un projet d'amélioration pour l'élargissement d'une autoroute sur une argile molle
Kim K.-H., Jung T.-M., Jung J.-H.
Korea Expressway Corporation, Korea
Kim T.-H.
Korea Maritime University, Busan, Korea (
Kim S.-R.
Dong-A University, Busan, Korea
You S.-H.
Samyoung Technology Corporation, Busan, Korea
ABSTRACT: Expansion of a roadway on a soft soil can cause settlement of the existing road during embankment construction due to
the consolidation characteristics of the soft soil. Many problems related to construction and maintenance, such as deterioration of the
surface, decreased safety, and decreased structural stability, could affect the existing road. This scenario is especially true if the
roadway foundation is a thick layer of soft soil. Therefore, the characteristics of the soil layer should be considered in the design and
construction of the roadway expansion. In this study, the expansion of the second branch of the Namhae Expressway was selected as
the target site because the expressway is constructed on a soft soil layer approximately 53 m thick. The original design plans were
reviewed, problems were discussed and solutions to the problems were proposed.
RÉSUMÉ : L'élargissement d'une autoroute sur un sol d'argile sensible risque d'entraîner des tassements pour les voies actuelles
pendant le remblayage sous l'effet de la compression des sous-sols argileux. D'importants problèmes liés à la construction et à la
maintenance sont posés : dégradation de la surface, instabilité structurelle, insécurité des usagers. Le scénario présenté s'avère
particulièrement important lorsque le sous-sol de l'autoroute en question est constitué d'une couche épaisse de sol argileux. Il faut
donc tenir compte des caractéristiques du sol pour le projet d'élargissement de l'autoroute. Dans cette étude, nous prendrons comme
exemple le cas de l'autoroute Namhae dont la seconde voie repose sur une couche argileuse atteignant 53m d'épaisseur. En examinant
son plan de construction original, on étudiera les problèmes posés et on suggérera diverses solutions.
KEYWORDS: deep thick soft soil, roadway expansion, settlement, construction, design.
1 INTRODUCTION
Stress can exist at a point in a soil not only because of loading
above the point by a superstructure but also by loading on
adjacent points. For saturated clay, this stress can result in
consolidation due to dissipation of excess pore water pressure.
Localized nonuniform settlement can cause unevenness and
cracking in the road and cracks in a superstructure. To minimize
the likelihood of this problem occurring, it is best to apply a
uniform embankment preload to the entire roadway area. In a
road-widening project, however, it is difficult to accomplish this
because of the need to divert traffic and comply with the
construction schedule.
Recently, the traffic on the second branch of the Namhae
Expressway has been approaching the traffic capacity of the
roadway because of the increase in national income and
increased desire for leisure. Severe traffic jams have diminished
the comfort of drivers and the operating speeds of vehicles on
the expressway. Expansion of the roadway from 4 lanes to 8
lanes is in progress because of the expansion of SOC (social
overhead capital) and the economic growth of the country
(Korea Expressway Corporation, 2008a; 2008b; 2008c).
As part of the original design for this project, embankment
construction was planned after removal of the existing road and
installation of vertical drains. Differential settlement might not
be a problem if sufficient time were available for consolidation
of the soil after construction of the embankment. However,
differential
settlement
could occur
because the
overconsolidation ratio of the soil and the load history of the
expanded and existing portions of the roadway are different. In
addition, PBD (plastic board drain) are to be installed after
removing the soil of the existing road. This installation may be
difficult because of the resistance of granite material, such as
crushed stone and gravel, used in the exiting roadway
construction. The strength of the soil could also be reduced by
disturbance during the installation of the PBD.
The original design plans call for expanding the roadway
from 4 to 8 lanes by banking soil on the existing road and
raising the longitudinal grade of the roadway. However,
settlement of more than 2 m, and in some locations up to 5.4 m,
was expected because the construction section of the second
branch is located on soft soil up to 53 m thick. As an alternative,
embankment construction after removal of the existing road and
improvement of the underlying soil was planned. However, this
strategy was also expected to be complicated and time-
consuming. Moreover, this construction was expected to cause
settlement in the surrounding area due to disturbance to the soil
under the existing roadway that had been stable for more than
30 years. Consequently, complaints about damage to nearby
farmlands, factories and houses were anticipated.
Expansion of a roadway typically involves diversion of
traffic. Expansion of a roadway on a soft soil could cause
settlement of the existing road during embankment construction
due to the consolidation characteristics of the soft soil. Many
problems related to construction and maintenance, such as
deterioration of the surface, decreased safety, and decreased
structural stability, could affect the existing road. This scenario
is especially true if the roadway foundation is a thick layer of
soft soil.
Accordingly, design and construction methods need to be
identified to minimize the problems that can occur during
construction of the expansion and minimize the expenditures
