2949
Design and Performance of Highway Embankments Constructed Over Sri Lankan
Peaty Soils
Conception et performance de remblais d'autoroute construits sur sols tourbeux au Sri Lanka
Karunawardena A.
National Building Research Organization, Sri Lanka
Toki M.
Oriental Consultants Co., Ltd., Japan
ABSTRACT: The construction of the Southern Expressway in Sri Lanka involved extensive ground improvement work as many parts
of the Expressway traverses through flood plains and marshy ground consisting of very soft peat, organic soils, and clays. Depending
on the ground conditions, various ground improvement methods including remove and replacement, preloading, preloading with
vertical drains, dynamic compaction and vacuum consolidation were applied to improve the soft soil to build the embankments with
heights varying from 2m to 12m. The performance of the ground improvement was evaluated in terms of the degree of consolidation,
improvement of the physical and engineering properties, increase in preconsolidation pressure and gain in shear strength of the peaty
soil. The results indicate that the properties of the peaty soil have been improved significantly, providing the required control over
future settlements while ensuring embankment stability. The results of the post construction surface settlement monitoring of the
expressway carried out up to date reconfirm that the ground improvement work was very successful and the expected residual
settlements are well below the allowable limit of the contract.
RÉSUMÉ : La construction de la Southern Expressway au Sri Lanka a nécessité un important travail d'amélioration des sols. En effet
plusieurs sections de la voie rapide traversent des zones inondables et du sol marécageux constitué de tourbes très meubles, de sols
organiques et d'argiles. Selon les conditions du sol, différentes méthodes d'amélioration (excavation et remplacement, préchargement,
préchargement avec drains verticaux, compactage dynamique et consolidation sous vide) ont été utilisées pour renforcer le sol mou et
construire des remblais de tailles allant de 2 à 12m. La performance de l'amélioration des sols a été évaluée par rapport au degré de
consolidation, à l'amélioration des propriétés techniques et physiques, à l'augmentation de la pression de préconsolidation et au gain
de résistance au cisaillement de la tourbe. Les résultats indiquent que les propriétés de la tourbe ont été améliorées significativement,
permettant le contrôle nécessaire des tassements à venir tout en assurant la stabilité des remblais. Les résultats du suivi après
construction des tassements en surface sur la voie rapide confirment que l'amélioration des sols a été une réussite et les tassements
résiduels prévus sont très inférieurs aux limites imposées contractuellement.
KEYWORDS: peat, embankment, monitoring, secondary consolidation, over consolidation ratio
1 INTRODUCTION
The Southern Highway is Sri Lanka's first E Class highway that
links the Sri Lankan capital Colombo with Matara, a major city
in the south of the island. The 96 km long section from
Colombo to Galle was completed and opened to traffic in
November 2011. Many parts of the highway traverses through
flood plains and marshy ground consisting of very soft peat,
organic soils, and clays. Especially, in the major flood plains of
Welipenna river, Bentota river and Gingaga river areas, thick
peat and organic clay deposits were found. The construction of
road embankments over peat deposits is quite problematic, and
thus, it is often done after first improving the properties of the
peaty soil through the utilization of appropriate ground-
improvement techniques.
This paper presents the ground improvement methods
applied in the Southern Expressway between Ch.0.000 km to
Ch.66.500 km to improve the peaty soil, with some background
information on the design methodology. In the first 34.5 km of
the highway, about 50% of the area is covered in soft ground
and from 34.5 km to 66.5 km, the area covered by soft ground is
around 12 km. In this project, embankments of about 4 km in
length were constructed by improving the peaty soil mainly
through the application of the heavy tamping method. The
length of the embankments that were built by improving the
peaty soil by vacuum assisted surcharging was around 2.5 km.
The problems encountered during ground improvement and
embankment construction work and the solutions given for the
same are highlighted and discussed. The details of the
laboratory and field investigations carried out before and after
ground improvement, field instrumentation program and field
monitoring program that was carried out during and after the
construction of highway embankment to assess the soft ground
improvement are presented.
2 TYPICAL SUBSOIL CONDITION OF SOFT GROUND
Many geotechnical investigations have been carried out since
the inception of the project in order to assess the condition of
the soft ground. At the preliminary stage, to provide information
to bidders and to facilitate initial designs, boreholes were
carried out at 500 m intervals. After commencement, boreholes
were carried out at about every 50 m intervals in order to
provide the necessary information for the detailed design.
Site investigation consisted of bore holes with Standard
Penetration Test (SPT), hand augering, Cone Penetration Test
with pore pressure measurement (CPTu) as in-situ testing and a
series of laboratory tests such as index property tests,
unconsolidated undrained triaxial compression tests and
conventional consolidation tests.
The investigation identified that the soft ground area of the
highway mainly consisted of peat, organic clay, alluvial clay
and lose sand deposits. The distribution of soft soil deposits
along the highway trace from Kottawa to Kurundugahahetekma
is shown in Figure 1. Silty clay and silty sand were found as a
top soil in most of the lowland areas up to a depth of 1.5 m to
3.0 m. This was followed by the sand to lateritic soil and the
thickness of the layers varied from 1 m to 5 m.
