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material density but undisturbed samples are required in order to test the shear strength parameters. Specialnote should be taken of the relative density of the material. Fill and reclamation are also discussed inAppendix D.3.3.3.1.3 Characteristics of Alluvium, Littoral and Marine DepositsIn the study area, these natural materials occur as thin, flat-lying recent deposits that have poorly developedor no weathering profile. They form complex coastal and submarine stratigraphies due to the fluctuations insea level during the last 10 000 years. In geological terms they are immature.There is a wide range in the particle size distribution in this group.The alluvial material is predominantly a cobbly sand with poor stratification and is usually in a loosecondition. The littoral deposits consist of well-sorted coarse sands that are usually well-compacted due towave action, but are subject to marine erosion. Marine deposits in the study area are variable with some areasof sand that have been commercially extracted, and other areas of highly compressible marine mud. Many ofthe marine deposits have plasticity indices that plot above the 'A' line with values in excess of 50% and are,therefore, highly plastic. No incidence of erosion was evident on the terrestrial deposits but alluvium iserodible if hydrological conditions are adversely altered by construction activity.All steep-sided excavations require strutting as these superficial materials have little cohesion. There is littlenatural instability in these materials due mainly to their intrinsically low slope angle. These various depositsexhibit a wide range of shear strengths; the lowest values correspond to the marine muds and the highestvalues to alluvial horizons. Consolidation is rapid in the alluvium and littoral deposits, but may be very slowand of a high magnitude in the marine deposits. The absolute magnitude of settlement is largely dependenton the imposed load, local groundwater conditions and the local stratigraphy. Undisturbed samples arerequired for laboratory tests to determine the material strength characteristics applicable to individual sites.Site investigations in alluvium may be enhanced by the application of geophysical techniques such asresistivity or shallow seismic refraction. Offshore, marine seismic techniques are useful in obtaining profiles ofmarine sediments.None of the materials in this group have high bearing capacities and all large loads need to be transferred tounderlying bedrock. Low to moderate loads can be accepted on raft foundations but problems of differentialsettlement may be experienced. The appropriate piling technique used to transfer high loads will bedependent on the overall stratigraphy but nearly all members of this group are amenable to driven piles. Thematerials in this group are easily excavated by machine methods. Marine deposits of sand, such as those inTide Cove, have been extracted or covered by reclamation and the marine silts and clays are generallyunsuitable as hydraulic fill. The siting of villages and proposed large scale developments such as the Ma OnShan New Town on areas of alluvium generally precludes the use of these areas as sources of fill.From a planning point of view, this group of materials, although not free of problems, is generally suitablefor development, with the exception of littoral deposits which are subject to marine erosion.3,1.4 Characteristics of ColluviumColluvium is a complex heterogeneous material which is highly variable in its engineering character. Thedistribution is described in Sections 2.3 and 2.4 and in Appendix C.1.3.As well as being derived from a range of rock types, generally colluvium is deposited intermittently over aperiod of time. This intermittent deposition results in considerable variation in the degree of weathering of itsconstituent boulders and detrital fragments.Relict colluvium may be considerably weathered but recent colluvial deposits, especially those occurring inthe beds of fast flowing streams, may contain very little weathered material. Relict colluvium, in a lobe nearWu Kai Sha, is considerably weathered and has significantly different geotechnical properties compared tothe fresh bouldery colluvium found in streambeds on Tsing Yi Island.Colluvial materials occur offshore and beneath areas of reclamation, this indicates a response of thedepositional environment to past changes in sea level. This colluvial deposit can be traced offshore ininvestigation records for the Sha Tin Area B reclamation. A relict deposit of weathered colluvium also occursbeneath the Sha Tin Sewage Treatment Works.From an examination of Tables B10 and B12 it appears that colluvium has a higher incidence of erosioncompared to insitu materials. This may reflect the mode of origin of these materials and the fact that colluvialdeposits frequently occur in drainage lines, where they are subject to erosion by streams and are generallysubject to a high water table. There does not appear to be any difference in the susceptibility to erosion ofgranitic colluvium as compared to colluvium derived from volcanic rocks.28