Evolution of Marine Geotechnology
The necessity to design foundations for offshore structures has resulted in the remarkable and rapid development of Marine Geotechnology by Geotechnical Engineers. Originating so slowly in the late 1950s, Offshore Geotechnology developed briskly in the 1960s and 1970s and now has a noteworthy role in Geotechnical Engineering.
In contrast to the enormous information available on engineering properties of terrestrial deposits, few and far between is known about submarine soils. Some of the soils on the seafloor exhibit unusual behaviour which is abnormal in the case of land soils. Water depth and distance from the coastline determines the type and distribution of soil on the seabed.
Soils in marine environment
Sediments that are encountered in the sea environment can be classified into marine and submarine. Those deposits which have been removed by any means, say by uplift are known to be marine and those which continue to be submerged are submarine.
Submarine soils can further be classified into two types, one which is more inclined to properties of terrestrial soils, on the other hand, others are absolutely an output of the ocean environment. The former is known as terrigenous sediments and these are formed on-land and then transported to sea, the latter are called pelagic sediments and these are formed of materials, which settle down through water columns in the sea.
Calcareous ooze is the most plentiful one among pelagic soils and those kinds of soils are found in abundance in Indian and Atlantic oceans. Brown clay and siliceous ooze are huge in the Pacific Ocean on account of its greater water depths. Compared to pelagic soils, terrigenous soils are deposited at a much slower rate.
The flocculation of clay particles in water is usually explained using the double layer theory. If the net effect of the intraparticle electrical forces between two clay particles is attractive, they will tend to come close to each other to become flocs. On the other hand, if the net effect is repulsive, they would disperse.
As the submarine environment is rich in dissolved salts, the double layer surrounding the particles of the sediment gets depressed and particles in turn tend to flocculate. As flocs attain a certain size, they will begin to settle through the water column. Once flocs reach the ocean floor they constitute a relatively stable aggregation of particles with an open framework. This is much similar to the honeycomb structure proposed by Terzaghi.
Bonding between sediments & Marine Geotechnology
The rate of deposition of sediments must be slow enough and an impulsive deposition should be followed to have proper retention of the open fabric. If these open fabric structures have to be maintained even after the higher stresses caused by the deep burial, there should be very strong bonds that are often achieved by cementation caused by the action of chemical precipitates.
These cementation bonds can be due to the precipitation at particle contacts of inorganic materials such as oxides and carbonates of Carbon, Aluminium, and Iron. Cementation and Condensation due to calcium carbonate is the main reason for the strength development of marine deposits.
Engineering behaviour of marine sediments
Engineering study of deep-sea sediments can be enumerated as
- Marine sediments with high carbonate content have several intraparticle voids which result in high water content.
- Mass physical properties of sediments are substantially influenced by the environment of deposition.
- Bonding between particles in carbonate sediments is due to the presence of cementing material and relatively low rate of deposition. This kind of bonding due to the cementation is more effective in calcareous deposits than that due to the consolidation behaviour.
And that of continental margin deposits as
- The natural water content in the continental margin deposits will always be higher than the liquid limit.
- Unlike deep-sea deposits, these deposits are under overconsolidated, normally consolidated, or under consolidated states.
The over-consolidation may result from
- Stress in which the soil is subjected to in the past like glaciation, erosion or change in water level.
- Manner of computation of effective stress
- Secondary or delayed consolidation
- The slow rate of deposition and
- Cementation caused by various materials such as carbonates, organic matter, etc.
Marine Geotechnology & Carbonate sediments
Carbonate soils are found on over 35 percent of the sea bottom on the continental margin. They exist in enormous quantities in low lying areas or warm water zones which includes the east and west coast of India. In the deep sea, they occur extensively in the Indian and Atlantic oceans and to a lesser extent in the Pacific Ocean.
Calcareous soils are generally formed from the amassing of skeletal survives from living beings discharging calcium carbonate, similar to foraminifera, echinoderms, and also plants like banthenic calcareous green growth and planktonic calcareous green growth. These kinds of deposits are known to be skeletal deposits whereas non-skeletal deposits are also there which are accumulated by precipitation. Different studies from experts conclude that the presence of carbonate in the deposits can influence their engineering behaviour.
The variation in the engineering properties of such soils has usually been correlated with the amount of carbonate material present in the soil, without focusing attention on the nature of carbonate material present in these soils.
Carbonate materials can exist either as inert particles of various shapes and sizes, with or without intraparticle voids or as a cementing material in the form of filler or as a coating or as overgrowths.