The oceanographic way in calculations of dynamic soil properties for offshore engineering surveys

KOROTKOV S.V. JSC " AMIGE ", Russia

In the modem scientific literature the litodynamical researches are referred to as geophysical [the Geological dictionary. V. 1, 1978], geological [Engineering hydrometeorological surveys. 1993] and hydrometeorological [Engineering survey for construction basic principles 11-02-96. 1996; Engineering hydrometeorological survey for construction 11-103-97. 1997].

In our opinion, ambiguous reference of the term "litodynamics" to various areas of natural sciences characterizes close communication of research problems between its various tasks.

Thus, it is possible to ascertain, that the general subject of the litodynamics researches for problems of engineering surveys is a studying transportation of sediments, deformations of seabed relief and shoreline as well as determination of thickness of active soil layers on the study area of seabed.

The results of such surveys should contain qualitative and quantitative characteristics of the litodynamical processes necessary for designing constructions, evaluating reliability of their work and their impact to the environment due to change of soil transportation ways.

Practice of litodynamical researches as a part of marine engineering surveys earned out by Joint-Stock company (JSC) " АМИГЭ " shows that clients are most interested in the data on deformations of seabed surface for the periods of separate storms and during shipping season ("the hydrographic way"). At present there is no comprehensive standard procedure to evaluate deformations of seabed composed of cohesive and loose soils. In fact, this complex multipurpose and multiparameter problem always breaks down into separate independent fragments which depending upon clients requirements have its own procedure, way of processing and interpretation.

Reliable methods of physical simulation of a litodynamical processes are not developed so far and it is not clear enough how to scale up the results of stimulations to natural conditions [Devis M.H., Mishchenko S.M.. 2000].

When using litliodynamical calculations in engineering survey it should be noted that numerical estimation of seabed deformations can be applied only to big bottom areas, whereas even relatively homogeneous sandy or silty soils within these areas have different particle size composition and their dynamic properties may vary.

Oceanographic approach to the problem in question lies in considering sediments transportation and seabed deformations as a result of dynamic and hydrophysical behavior of near-bottom flows.

For sites with seabed composed of loose (sandy) and cohesive (silty or clayey) soils the following calculations and constructions are carried out:
• Estimating threshold speeds of water flows at which transportation of sediments begins and plotting the graphs of their distribution (based on particle size analysis of seabed soil and relationship between current speeds and weighted mean diameter of soil particles).
• Matching the estimated threshold speeds graphs with those of particle size distributions to determine most active zones of soil transportation.
• Coefficient of overburden stability is a value that is used for evaluating probable deformations of seabed composed of sandy soil [Romanovsky, 1977: Schuljak, 1971]. This coefficient is applied as a generalized characteristic of flow effect to seabed. Inverse value of the coefficient is used as an input parameter when calculating dependence of simulated ultimate wash-out upon this coefficient [Devis M.H.. Mishchenko S.M., 2000]. Change over to real wash-out is made by multiplying the result of calculation by 70 (scale-up ratio adopted in the simulation) [Devis M.H., Mishchenko S.M., 2000]. We evaluate the relative error to be no more than 30% of the actual depth of seabed wash-out.
• If seabed is composed of cohesive soils the intensity of deformation processes depends upon kinetics of turbidity flows and pliability of soil to wash-out. This condition should be complied with when analyzing probable seabed deformations.

These methods of analysis were applied in JSC AMIGE during lithodynamic surveys off the Varandey Island and on various detailed sites in the Ob Bay.
It should be noted that the results of such analyses allow to quantitatively estimate special and temporal seabed deformations and their rates in various study areas, but cannot comprehensively solve the problem of lithodynamic calculation as a part of offshore engineering survey.

References:
The geological dictionary. V.l. //M.: Publishing house " Nedra "э 1978.486р.
Devis M.H., Mishcheiiko S.M. Experimental research of local washouts of a bottom at the basis of sea hydraulic engineering constructions. //News VNIIG.-2000-V.236. -P. 140-151.
Engineering - hydrometeorological surveys on a continental shelf. //M.: Gidrometeoizdat, 1993.-277p.
Rowaiiovsky S.I. The sedimentological of a basis of a lithology. //L.: Publishing house "Nedra ", 1977.-407c.
Construction standards and rules "Engineering survey for construction basic principles 11-02-96. //M.: Gosstroy of Russia, 1996. - 47p.
Code of practice "Engineering - hydrometeorological surveys for construction 11-103-97".//M.: PRISC.1997.- 33p.
Shculjak В A. The physics of waves on a surface of the loose environment and a liquid. M.: Publishing house " Nauka ", 1971.- 400p.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004

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