The growth of Indian port city’s over the last few decades has been phenomenal
and Cochin is no exception with the increase of population, housing and construction of
various facilities have been a problem with urbanization. Having exhausted all the trouble
free hand, man is now on the look out for techniques to improve areas which were
originally considered uninhabitable. Thus studies on the nature and engineering behavior
soft clay’s covering long stretches of coastal line and methods to improve there geotechnical properties have been of great relevance, and is explained in the following pages.
2. REVIEW OF COCHIN MARINE CLAY
According to King (1884), 2500 years back, the sea washed up to the high ranges
of Western Ghats. The land between the Western Ghats and the present coastal line was
once well above the sea and was subsequently submerged. It was uplifted again by volcanic
action and again partially covered by sea water. The uplifted area includes the coastal belt
(10 to 30Km wide and 150 Km long) starting from Crangannore at the north to Quilon
at the south. Most of the Cochin city is situated in this belt.
3. PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Marine clay is formed by the sedimentation of clay soils in marine environments
exhibit unusual physical properties. They posses very high liquid limit and their natural
water content is close to the liquid limit values. The most striking feature of the physical
properties is the phenomenal changes that are caused by drying of sample. Some physical
properties of the Cochin marine clay is shown in the following table.
PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Sl No Physical properties (%) Moist soil Air dried soil Oven dried soil
1 Liquid limit 131.5 94.5 62.0
2 Plastic limit 53.9 45.2 40.5
3 Plasticity index 77.6 49.3 21.5
4 Shrinkage limit 18.1 18.8 19.2
5 Grain size distribution
4. SHEAR STRENGTH
In greater Cochin area the layers of shallow depth have their natural moisture
content quite close to their liquid limit. The clay deposits below vary widely in consisting up
to 40 to 50m where good load bearing stratum can be met with.
The shear strength of clayey soils is dependent on its water content. Any
increase in water content is necessarily accompanied by the reduction in cohesion or shear
strength. But at the same water content, different soils have different shear strength. From
the test results that were conducted at different places in Cochin, it was found that, the shear strength is very low and it is around 0.01Kg/cm² and 0.02Kg/cm².
5. CONSOLIDATION CHARACTERISTICS OF COCHIN
Several series of consolidation tests were carried out on samples of Cochin arine clay varying different parameters like duration of loading, effect of pore fluid, effect
of washing. In the consolidation test which served as the reference test. It was found that
about 2 days were required for a complete dissipation of pore pressure and for reaching
an equilibrium void ratio for a particular loading stage. The clay deposits are highly
compressive with Cc value around 1.5 and natural moisture content
very close to liquid limit. It has been observed that almost all the structures, founded on
spread footings or raft footings invariably show actual settlement far below the computed
6. PROBLEMS DUE TO THICK MARINE CLAY
Cochin City and the surroundings are covered with thick marine clay deposit
extending to a large depth (30 to 40m) below the ground level. Due to the presence of this
soft clay layer, following problems are observed.
Low bearing capacity
High seepage losses
Liquefaction during earthquake
Instability of foundation excavations
Higher earth pressure on retaining structures
IS: 1904(1966) permits a maximum settlement of 40mm for isolated foundations on
sand and 65mmfor those for clay. The allowable settlement is higher for clay because
progressive settlement on clayey soils permits better strain adjustments in the structural
members. The maximum settlement for raft foundations on sand is 40mm to 65mm and that
on clay is 65mm to 100mm.
For typical industry following issues are to be addressed:
Large area is covered with thick vegetation. If the area is reclaimed without removal of the vegetation. It will result in a fill with high organic content and high compressibility. On the other hand, if the vegetation is to be removed, it requires dewatering of the area so that powered vehicle can operate on the surface and clear slush and vegetation before selected soil is placed for reclamation.
While reclaiming the area, particularly for industry requiring very large area, for example refinery, oil terminal, gas bottling plant, etc. the drainage pattern of the area needs to be planned carefully. The flow surface water which was taking place through the area prior to reclamation has to be diversified through special drains/culverts such a way that it does not create any inundation.
The area avail is low-laying and often inundated with water. Unless the area is raised by soil fill (3 to 4m thick) it is difficult to carryout any construction.
In many projects, the height of the fill required is 2 to 3m. This will require boundary wall which has a high retaining wall plus a compound wall. In the absence of retaining wall the fill material can encroach in the neighboring areas beyond the property line which may not be acceptable in many cases. Alternately, fill is placed with toe of the slop within the property line .in this case, large area with a width of 6 to 7m all along the periphery of the area remain unutilized.
The placement of fill can lead to the following foundation problems.
Negative drag force on pile foundation.
Lateral flow under surface loading leading to settlement and lateral force on the adjacent foundations.
Deep-seated slop failure for construction of embankment and heavy surface loading.
Difficulty in executing foundations requiring deep excavation.
7. GROUND IMPROVEMENT
The clay deposit also needs to be improved to meet the foundation requirements. In early days, areas having clay deposit was avoided for construction. But with scarcity of land in urban areas, we do not have choice and structures have to be built on weak deposits. Pile foundation is of course a possible approach as it by pass the weak deposit and transfer the load on next component layer. Where thickness is very large, pile foundation is uneconomical and time consuming.
The methods of ground improvement that can be adopted depend basically on nature of strata and purpose of improvement. The methods available are as follows.
8 METHODS OF GROUND IMPROVEMENT
8.1 VERTICAL DRAINS
This method is suitable for deep deposit of soft clay. The natural moisture content in this strata can brought down substantially by installing vertical drains with preloading. The pressures of vertical drains reduce the drainage path of water in the pores of the soil and thereby reduce the time required for consolidation. The spacing of drains depends upon the speed at which required improvement is to be achieved.
In earlier days, such vertical drains were installed by driving a close ended steep pipe o 100 to 200mm diameter up to the full thickness of such clay deposit. The pipe is then filled with sand and withdrawn in stage to form a vertical drain. The pipe is generally refused for installing other drains.
In the recent past, there has been number of different materials developed to replace the sand drains. These are basically plastic sections having thickness varying from 5 to 10mm and width from 100 to 150mm. The sections have channels to permit flow of water. The perimeter of the section is covered with a layer of geo-textile to prevent the entry of soil particles in to the channel. The advantages of such drain are that it results in minimum remoulding of surrounding soil during installation. The process of installation is very fast. The machine is mounted on a crane and typical drain up to 10m depth can be installed in the period of varying 1 to2 minutes including the time for shifting the machine to a new location.
The percentage of consolidation, which can be achieved by such vertical drains, can be theoretically predicted from the design charts developed based on the theory of three-dimensional consolidation. Typically, a deposit, which requires a period of over ten years for 95% consolidation, can complete the same consolidation within a short period of 3 to 6 months when vertical drains are installed.
After the drains are installed, the magnitude of preload to be placed on the required shear strength of the layer after improvement. Depending on the time available for improvement, the degree of consolidation is worked out and the effective over burden pressure (p) is computed. From the plasticity index I\of the soil, the ratio Su/p is determined. The ratios normally remain constant and therefore with increase in value of p the shear strength after treatment increases.
8.2 IN-SITU DEEP MIXING
In- site deep mixing using hydraulically operated helical augers penetrate the ground to the required depth. The hollow stem of the auger is used to inject cement / lime / any other stabilizing compound into the ground. A pair of 2 or 3 augers is operated simultaneously. After injecting the stabilizer in to the ground, the augers are rotated such that the soil along with the stabilizer is churned in-situ and mixed the stabilizer thoroughly with the soil without necessity of taking out the soil. The auger configuration can be chosen such that either a stabilized wall can be formed (to act as a barrier) or the entire area can be stabilized.
8.3 STONE COLUMN
Stone columns are cylindrical columns made below ground level which comprises of granular material of size varying from 25 to 100mm. The hole is made in the soft clay deposit by different techniques and then filled with stones in layer and compacted to form the complete column. When the structure is placed over the area treated by stone column, majority of the load (80 to 90%) is transmitted to the stone columns because of their higher stiffness. Balance 10 to 20%of the load is taken by the clay deposit. With the help of this 10%of surcharge load, the soft clay able to provide adequate confinement to the cylindrical column.
The area treated stone columns can be used to support only flexible structures such as embankment, oil storage tank, etc. because the settlement even after treatment with stone column can be large (50 to200mm). Without stone column the settlement could have been 3 to 4times higher and also the bearing capacity would have been much less.
8.4 VACUUM DEWATERING
In this technique also the layer is provided with vertical drains as mentioned above. The surface is then covered with a layer of a sand blanket 150 to 300mm thick and covered with a thin plastic sheet which is suitably anchored along the perimeter. Vacuum is then applied with a sand blanket which creates a suction pressure up to 50 to60mm height of mercury and forced the pore water out from the deposit through vertical drains. The area is also subjected to atmospheric pressure which then acts as preload. The vacuum pressure is maintained till the required improvement is achieved.
Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid pattern. Vibro replacement is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increase the amount of densification, but provides a degree of reinforcement and potentially effective means of drainage.
8.6 DYNAMIC COMPACTION
In dynamic compaction, a heavy weight of 15 to 20ton is raised to a height about 20m above the ground level and freely dropped on the ground surface. The impact on the ground surface transmits compression waves, which spreads radially from the point of impact. The compression waves travels fast and creates Liquefaction of the strata, which is saturated. It is followed by shear waves reorients the soil particle which are liquefied state to denser state and compacts the configuration. The spacing between the treating point, number of impact at each point, etc. can be varied depending on the required improvement.
Figure : 2
8.7 COMPACTION BY DEEP BLASTING
In this technique, a borehole is made to a depth approximately 3/4 or 2/3 of the total depth. Suitable explosive is lowered to the bottom of the borehole and the space above is back filled with compacted soil (known as stemming). The explosive is attached with cordex, which is taken up to the surface and connected to electric detonator. When the explosive is detonated, it creates a cavity, which expands instantaneously. The cavity expansion generates shock waves. Propagation of these waves densifies the soil to certain extent. After blast, a large conical depression is observed at the place of blast. The water within the pores of the deposit gets expelled out and comes up to the ground and form a large pool in the cone of depression.
The quantity of charge and spacing between the explosive can be suitably selected to achieve the required densification. As a thumb rule, about 15 to 45 grams of explosive is required for densification of every 3m of the deposit. Initial loose deposit which has relative density of about 50% and susceptible to liquefaction can be densified to a relative density of over 60% which is adequate to prevent liquefaction. The vibrations created during the blast are within permissible limits for construction purpose.
8.8 COMPACTION GROUTING
Compaction grouting is a technique where by a slow-flowing water /sand /cement mix is injected under pressure into a granular soil. The grout forms a bulb that and densifies the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.
9. CASE STUDY
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
The new bridge connecting Ernakulam town with Bolgatti Island and Vypeen Island is supported on 1m diameter bored cast in-situ pile. The design load for each pile is 450tones. The strata comprise of soft marine clay of 20 to 30m. This is followed by alternative layers of dense sand and stiffing clay which contains decayed wood within the deposit. The SPTN value of clay with decayed wood is generally >100 and the pile length were estimated as 45 to 50m.
During excavation of the work, it was noticed that even pile length 45m was not adequate to meet the loading requirement. Due to the large depth of pile, stabilization of the borehole and cleaning of the bore before concreting was found to be difficult. The load test results revealed that the end boring resistance offered by the strata was not significant and particularly all the load was resisted by side friction. To meet the requirement of loading, the pile length had to be increased from 45 to as high as 65 to 70m depending on the strata condition. For the certain piers, before the load test results were available, the piles were terminated with depth of 45m. Additional piles were provided at these locations to meet the loading requirements. It is very rare that large diameter piles have been installed to meet large depth.
The variations in the strata conditions and uncertainty about the behavior of stiff clay with the decayed wood made the designers to adopt a conservative approach which has resulted in large depth of pile. Apart from construction of piles, the strata condition posed followed technical problems.
Reclamation of large area by dredged material on Vypeen Island.
Construction of 6 to 8m high approach embankments on soft clay deposit created problems of settlement and negative drag of the pile. To reduce this 2 structural span have been added at the Ernakulam end so that the height of the embankment get reduced and approach embankment can be supported on the available weak strata.
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
Due to the increase in construction activities and urbanization the availability of suitable soils are decreasing. Cochin City and its surroundings are covered with marine clay to large depth. Which posses very low shear strength, low physical and engineering properties. Whenever the soft clay deposits are very loose sand or soft clay and are unsuitable for resting the foundation, the following options can be considered.
Adopt deep foundation such as pile foundation so that the top soil can be bypassed.
Treat the existing soil to improve the bearing capacity and reduce the settlement and place the foundation directly over the top soil.