Proceedings of Indian Geotechnical Conference December 13-15,2012, Delhi (Paper No B 251) FIELD OBSERVATIONS ON GENERATION AND DISSIPATION OF PORE PRESSURE DURING LIME-CEMENT STABILIZATION OF A LAYERED COASTAL CLAY DEPOSIT V. Thakur, Adjunct Associate Professor at Aalesund University College, and Senior Geotechnical Principal at Norwegian Directorate of Roads, Norway vikas.thakur@vegvesen.no ABSTRACT: Lime cement stabilisation using a dry mixing technique is often practiced for costal slopes as the soils generally have high water content. The pore pressure increase during the stabilisation process also increases the stress condition in the soil mass towards the failure state. In other words, stability of already marginally stable slopes can be at risk if the pore pressure is increased to a very high level for a longer period. This paper present field results of pore pressure measurements, during the stabilisation phase, taken from seven different locations of a stabilisation project in the west-coast of Norway where 150 x 120 m area was stabilised to an average depth of 10-12 m. The presented data is original, rarely recorded in practice, and thought to be a useful resource for the engineers dealing with such type of stabilisation projects. INTRODUCTION The use of lime-cement piles is one of the most applicable stabilisation methods in soft and sensitive clays. There are sites that stabilization components have the most impact because lime reacts more with clay than sand, gravel, silt or peat. The chemical reaction creates a hardening process that creates a much higher strength than the original soil. Curing process depends on ground material to be built in, the choice of binder, binder amount, how well it blends in and the temperature of the area. On one hand, lime-cement stabilisation guarantees significant increase in the shear strength of the stabilised material; on the other hand, the stabilisation process itself is very critical. Great care needs to be exercised while choosing mixing techniques, procedures and ground conditions of the area to be stabilized. A dry mixing technique is often used for costal slopes as the soils generally have high initial water content. Usually the stability of costal slopes is marginal due to poor undrained shear strength. When such slopes need to be stabilised, an air pressure of 3-4 bar is commonly used in the dry mixing process which builds extremely high pore pressure. Hence, this will further decrease the stability of the slope in the short duration until the slope gets the benefit of the stabilization and result in higher soil strength. The pore pressure increase at constant total stress will also increase the stress ratio in the soil mass towards the failure state. In other words, stability of already marginally stable slopes can be at risk if the pore pressure is increased to a very high level for a longer period. Therefore, it becomes important to monitor pore pressure generation and dissipation during the stabilisation period. This paper presents field results of pore pressure measurements, during the stabilisation phase, taken from several locations. The field data is taken from a stabilisation project in the west-coast of Norway where 150 x 100 m area was stabilised to an average 10-12 m depth, see Figure 1. More information about the project as well as numerical studies can be found in NPRA (2010a, 2010b, 2010c). THE HIGHWAY PROJECT The project site Hoegset is located in the west-coast of Norway. The coastal terrain is sloping about 1:10 towards the main land whereas the terrain is inclined to 1:20 as it approaches towards the sea. This coastal area mainly consists of sand and soft marine clay layers to a depth up to 12 m. A new road section was constructed in 2011 on this area. The road fill was between 2.5 to 5.0 m high above the ground. In addition, a bridge and a subway were also constructed. Lime cement stabilisation of the area prior to these constructions was recommended to achieve the required factor of safety. Coastal deposit Sea Area stabilized with lime cement ribs Fig. 1 Areal view of the construction site Hoegset 0 5 10 20 Sensitive clay Sand Moraine Embankment Depth [m] Total sounding Resistance (kN) 5m Peat Fig. 2 Coastal deposition and the road fill (NPRA 2010a)