Showing papers on "Silt published in 1982"

Settlements above tunnels in the united kingdom - their magnitude and prediction

Abstract: Settlements can be a problem with soft ground tunnelling in urban areas where buildings, both modern and ancient, can be put at risk; services, too, can be endangered and at times it has been deemed necessary to divert services before tunnelling is begun. These environmental considerations have led to a considerable research effort being devoted to the study of settlements caused by tunnelling through soft ground; much of the research work has been undertaken either directly by or under contract for the Transport and Road Research Laboratory. Measurements of settlement and ground movement made on tunnelling projects located, in the main, in built-up areas are reviewed. The ground conditions studied included stiff-fissured clays, glacial deposits and recently deposited silty clays, as well as cohesionless soils of low density, weak rocks and made ground. Many of the tunnels were driven in free air by use of shields, but compressed air was used in the weaker soils to maintain stability; the bentonite shield and chemical treatment of the ground were also used in loose sands. The data from these case studies are used to provide simple analytical tools that enable better prediction of the magnitude of settlements and ground movements caused by tunnelling through soft ground to be made.

Origin of desert loess from some experimental observations

Abstract: The aeolian attrition of angular quartz grains previously produced by weathering in deserts has been simulated, yielding abrasion products which show that both coarse and fine silt sizes are produced by this process These results suggest that desert aeolian processes can produce loess, and it is speculated that while much of this material from many deserts has been deposited in the sea, the Chinese loess could have been produced in the Gobi desert The finest of the particles produced by such attrition may serve as a source of silica for silcrete formation

The Role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments

Abstract: Storm related processes were a principle agent of stratification in Paleozoic shelf environments. During peak storm conditions the shallow sea floor was eroded, coarse materials including shells formed winnowed lags, while fine sand, silt and clays were put into suspension. As storms waned, the fine sand and silt settled out of suspension rapidly. Some infiltrated the upper portions of the coarse grained winnowed lag deposits and the bulk formed laminated, weakly graded beds with distinctive wave generated structures. Shell beds formed by these processes contain reworked but untransported fossils. Biases of preservation resulting from such activity include the destruction of preserved community succession, under-representation of originally aragonitic fossils and an enhanced preservation of whole, uncorroded fossils.

Dry Soil-Aggregate Stability: Energy-Based Index1

Abstract: Most methods used to estimate aggregate stability apply an unmeasured force, or a measured force without knowledge of transfer, to a single or a group of aggregates. A technique is needed to estimate aggregate stability based on quantitative transfer of energy. This paper presents an energy-based procedure for evaluating dry-aggregate stability. Soil aggregates were crushed by diametrically loading them between parallel plates. The energy of crushing was determined by integrating the area under the force against distance curve. The aggregate-surface area after comminution was calculated from aggregate-size distribution and aggregate density; the aggregates were assumed to be spherical. The results are expressed as energy per unit of surface area, joules per meter squared (J/nr). Example values obtained from field-sampled aggregates for several soil series ranged from 3.7 ± 0.7 for Hotlake silt loam (coarse-silty, mixed, mesic Aquic Haploxerolls) to 43.9 ± 7.5 for Bearden silt loam (fine-silty. frigid Aerie Calciaquolls). The wide range of aggregate stabilities among different soils made it possible to distinguish among them even though variability among aggregates of the same soil was relatively large. The results were relatively insensitive to initial aggregate size but sensitive to crushing end point. Additional Index Words: mechanical stability, soil structure, rupture stress. Skidmore, E. L., and D. H. Powers. 1982. Dry soil-aggregate stability^ energy-based index. Soil Sci. Soc. Am. J. 46:1274-1279. A PORTION of mineral soils consist of solid particles ranging in size from 1,000 jum. These primary individual particles associate together in various arrangements to form secondary or compound particles or aggregates. The particles are cemented by inorganic and organic bonding agents. The size distribution and stability of these aggregates influence greatly the soils' physical properties and the processes that occur in the soil. Additionally, the size and stability of aggregates determine the soil's susceptibility to wind and water erosion. Aggregates between 0.05 and 0.50 mm in diameter are most easily transported by wind (Chepil, 1958). Aggregates larger than 0.84 mm are resistant to wind transport. The 0.05to 0.25-mm size range dominates water sediments from high sand soils, and aggregates ranging in size from 0.02 to 0.20 mm are most erodible from high silt and clay soils (Young, 1980). Aggregate size is important in determining the dimensions of pore space in cultivated soils. The size of the pores in turn affects the movement and distribution of water and air (major factors affecting plant growth) in the soil. Many forces operating in the soil tend to cause a 1 Contribution from the ARS-USDA, in cooperation with the Kansas Agricultural Experiment Station. Department of Agronomy Contribution 81-396-J. Received 17 Nov. 1981. Approved 30 Aug. 1982. 2 Soil Scientist, ARS-USDA, and Research Assistant, Agronomy Department, respectively, Kansas State University, Manhattan, KS 66506. 3 Trade and company names are included for the benefit of the reader and do not imply endorsement or preferential treatment of the product listed by the USDA. change in aggregate-size distribution. Aggregates may break into smaller units or combine into larger units by such actions as wetting and drying, rainfall impact, freezing and thawing, animal and machine traffic, abrasion from saltating particles, and tillage. The resistance of soil aggregates to breakdown from physical forces is a measure of coherence or strength of cementation between or within the particles or soil aggregates. In the dry state, this resistance has been referred to as dry-aggregate stability or mechanical stability. Several procedures have been used to evaluate dryaggregate stability. Chepil (1953) determined a relative measure of coherence or strength of cementation between and within the soil aggregates in a dry state by rotary sieving and dividing the weight of soil material remaining on the sieve after sieving by the weight before sieving. Chepil (1951) also determined stability against collision. A 500-g soil sample obtained from the field and dried was sieved, and the portion retained on a 0.42mm sieve was placed in a metal cylinder 91.4 cm in length and 10.2 cm in diameter. The cylinder was inverted end-over-end 20 times. The aggregates were allowed to fall and strike the bottom on each inversion. The soil was then rotary sieved. The stability was expressed as percentage of the original weight of the soil retained on the 0.42-mm sieve (Chepil, 1951). Toogood (1978) used air-dried samples of aggregates 1 to 2 mm in diameter for estimating dry stability. A 5-g sample of the aggregates was sieved vigorously on a 1-mm sieve for 1 min; the sample was weighed, then sieved vigorously for 4 min. The weight of sample remaining after 5 min, expressed as a percentage of the weight remaining after 1 min, indicated the stability of the dry aggregates. Recognizing the desirability of knowing the work required to subdivide aggregates into smaller units, Marshall and Quirk (1950) used a drop-shatter method to determine the stability of natural aggregates. Airdry samples were shattered by dropping them onto a concrete floor from various heights. The kinetic energy was dissipated by impact with the hard surface. The small amount of energy lost to heat evolution and rebound was neglected. Others (Grossman et al., 1959; Farrell et al., 1967; Gill and McCreery, 1960) have used the drop-shatter technique to establish the relationship between the energy imparted to the soil and the degree of fragmentation. Rittinger, cited by Lowrison (1974), suggested that the energy consumed in size reduction of solids was proportional to the new surface area produced. Aggregate stability depends on the number and strength of bonds holding particles together. Logically, the work required to break those bonds and thus reduce aggregate size and create new external surface area would also be a measure of the stability of those aggregates. The purpose of this study was to develop and evaluate a method for measuring dry-aggregate stability

Origin of siltstones: glacial grinding versus weathering

Abstract: Siltstones are unusual rocks. They are mainly made up of quartz. Glacial grinding is considered by some authors to be the main, or even the only, process generating silt. This is supported by: (1) the presence of silt in basal tills from present ice-caps and, above all, (2) by the volumetric importance of Quaternary loess. However, ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites, or forming thin intercalations in pro- and periglacial deposits. These siltstones represent less than 5% of the total volume of Upper Proterozoic and Upper Ordovician glaciogenic sequences of West Africa. On the other hand, silt is abundant in weathering profiles of tropical, equatorial and mediterranean zones. This silt is generated by splitting of minerals, mainly quartz, inherited from the parent rock. Quartz grains are partly dissolved in situ and their fragments cemented by iron oxi-hydroxides in tropical and equatorial zones and by calcite in mediterranean zones. Silt is particularly abundant in tropical soils, comprising up to 50-75%. Secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water. Weathering processes, and especially those operating in tropical zones, are the main phenomena generating silt. Glacial grinding would generate only a little silt. A large part of the material of Quaternary loess may be derived from glacially reworked weathering profiles.

Geology, soils and vegetation of Blackhawk Island, Wisconsin.

Abstract: Curtis (1959) showed that both climate and soils influence the composition of Wisconsin forests but he could not determine the relative effects of each. To determine the effects of soil properties on species composition, a wide variety of soil and community types is needed in a single climatic regime. Blackhawk Island is a 70-ha island in the Wisconsin River in southwestern Wisconsin which supports a wide variety of forest communities on several soil types, including some communities and soils usually found only in northern Wisconsin. The soils form a gradient of moisture and N availability. Pines (Pinus strobus L., P. resinosa Ait.) are dominant on sandy soils with low N mineralization and nitrification; oaks (Quercus borealis Michx., Q. alba L.) are dominant on sandy clay-loams and silt loams with moderate N mineralization and nitrification, and sugar maple (Acer saccharum Marsh.) is dominant on silty clay-loams with high N mineralization and nitrification. Species replacement along this gradient results in an increase in continuum index that is strongly related to the increase in N mineralization, nitrification, silt + clay content and P content of the soils, but less strongly related to other soil properties. Two communities not on this gradient are: (1) aspen (Populus grandidentata Michx.) on plowed silt loam with low N mineralization and nitrification and (2) hemlock (Tsuga canadensis (L.) Carr.) on organic soil with low N mineralization and nitrification. The soil textures are a result of the geologic history of the island. The distribution of communities is therefore a result of ecological processes which are working within the framework of a soil texture gradient determined by the geologic history of the island. INTRODUCTION In a gradient analysis of the vegetation of Wisconsin, Curtis and his co-workers (Curtis, 1959; see Literature Cited for further references) showed that the geographical distribution of species and communities throughout the state was related to both climate and soil. Because of the large geographic area covered and because of the relationship between climate and soils, it was difficult to determine which had more influence on the distributions and community relationships of species. Many factors varied along a climatic gradient from southwestern to northeastern Wisconsin. For example, the distribution of communities was similar along both a pH gradient and a mean July temperature gradient because of coincident but independent changes in bedrock chemistry and July temperature. In addition, the importance of several species in the communities, such as sugar maple, increased with both decreasing temperature and increasing soil moisture. More recently, Peet and Loucks (1977) showed that the composition of southern Wisconsin forests is related to a moisture-nutrient gradient and to a successional gradient, but did not specify the important nutrient(s) involved. In order to determine more precisely the effect of soils or climate on vegetation distribution, it is desirable to find sites in which one of the factors is held constant, or nearly so, while the other varies. The purpose of this paper is to examine changes in species composition of several forests along a gradient of soil types under equivalent climatic conditions.

Characteristics of cryogenic weathering in the permafrost zone of the european ussr

Abstract: Changes in the distribution of rock forming minerals, such as quartz, feldspars, amphibolines, and pyroxenes, with respect to grain size, are examined within the context of loessic sediments of the northern European USSR. Contrary to trends in temperate or warm climates, quartz is characterized by a high degree of instability under cryogenic conditions and is the cause of the dominant silt fraction (0.05 to 0.01 mm). The yield of heavy minerals is also unusual in that the 0.05 to 0.01-mm fraction is lower than that in the 0.05 to 0.1-mm fraction. An attempt is made to classify soils according to their grain size and mineralogical composition, and to distinguish those formed predominantly by cryogenic processes.

The acquisition of post-depositional detrital remanent magnetization in a variety of natural sediments

Abstract: Summary. Post-depositional detrital remanent magnetization (pDRM) is the primary means whereby many sediments acquire their palaeomagnetic signal. We have studied the acquisition of this magnetization in a variety of natural sediments. Our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field. Most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments. Grain sizes were measured by the settling tube method which led to the determination of the clay, silt and sand fractions in each sediment. Isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite. In the pDRM acquisition studies two distinct modes of behaviour were found. For sediments with a sand content less than 60 per cent, the original direction of magnetization was preserved regardless of the water content. Such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid-filled voids and hence are able to realign along a new magnetic field direction. For sediments with a sand content in excess of 60%, remagnetization along a new magnetic field direction occurred as expected, provided the sediments were sufficiently wet. Studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older, desiccated Sediments.

Sedimentation in a Glacier-Fed Lake: The Role of Pelletization on Deposition of Fine-Grained Suspensates

Abstract: Bow Lake is a small alpine lake in western Alberta, Canada, mainly fed by glacial meltwater. Suspended sediment loads, high only during summer months, are transported through the lake dominantly by interflows within the epilimnion. Consideration of interflow velocities and particle settling rates suggests that most clay supplied by stream inflow should be transported out of the lake during summer when thermal stratification is best developed. Field observations, however, show that clay comprises a persistent and significant component of lake-bottom sediment deposited during summer. This apparently anomalous clay-settling behavior can be attributed to production of fecal pellets found in both suspended sediment traps and bottom surficial deposits. The pellets, probably produced by pelagic copepods, are ovoidal with lengths typically ranging between 100 and 250 µm and are composed of clay and silt. Measured settling rates in water at 5°C ranged between 80 and 400 m/day. Pellet production, then, accelerates the downward transfer of suspended detritus, resulting in deposition of much of the fine sediment that would otherwise escape the lake in interflows. This confuses patterns of bottom-sediment texture and mineralogy which would ordinarily be expected from settling behavior of unpelletized suspensates.

The Pedogenesis of Desert Loam Soils in the Barrier Range, Western New South Wales. I Soil Parent Materials

Abstract: The particle-size distribution, silt (63-20 µm) mineralogy and micro-morphological properties of three desert loam soils developed over contrasting rock types are examined. The silt mineral assemblages in the soils contain more weatherable minerals and are more varied than those of the underlying rocks. Round to subround clayey pellets constitute the majority of the soil materials. Such pellets are probably aeolian in origin and are similar to clayey pellets originating in dried-out, saline lake beds elsewhere in southern Australia. The combination of aeolian silts and clays in the desert loam soils indicate that the soil parent materials are primarily transported aeolian deposits.

Pedogenic zones in postglacial sediment of lake manitoba, Canada

Abstract: The postglacial sediment record of Lake Manitoba is composed dominantly of silty clay to clayey silt with little variation in most physical, chemical, and mineralogical properties. Distinct in the sedimentary sequence of this large lake, however, are four zones that have a low moisture content, blocky to pelletal structure, and gleyed colour. All of these zones formed during the warmest and driest postglacial period in the region, 9500 to 4500 years ago. Although several possible hypotheses can be formulated regarding the origin of these zones including changes in sedimentation rate, clay mineralogy, lake chemistry, and depth of winter freezing, the most likely explanation is that they represent pedogenic horizons. Formation by pedogenesis during dry or extremely low water conditions is not mutually exclusive of the other hypotheses, and several are favoured by low water conditions.

Middle Devensian deposits beneath the ‘Upper Floodplain‘ terrace of the River Thames at Kempton Park, Sunbury, England

Abstract: Gravel pit sections in the ‘Upper Floodplain’ terrace exposed current-bedded gravel and sand containing a grey clayey silt lens. This silt has yielded fossil macroscopic plant remains, molluscs, ostracods and insects. Sedimentological and palaeontological results indicate that the silts were deposited in a slowly flowing river channel. The gravel and sand accumulated under cool, seasonally contrasting climatic conditions and represent braided stream deposits. A new lithostratigraphical unit, the Kempton Park Gravel, is proposed and its relation to the local terrace stratigraphy of the Thames is discussed. A radio-carbon date of 35,230 ± 185 BP indicates that the Kempton Park Gravel unit aggraded during the Middle Devensian substage.

A case study of a chemical spill: Polychlorinated biphenyls (PCBs): 2. Hydrogeological conditions and contaminant migration

Abstract: The purpose of the second paper in this pair is to describe the hydrogeology and to account for the observed patterns of PCB contamination of the Federal Pioneer site in Regina, Canada. Six major hydrostratigraphic units were identified from detailed test drilling and sampling. From the surface downward these are a thin surface fill, the Regina clay, the Condie silt, the till zone, the interglacial silt, and the Regina aquifer system. This latter unit is an important source of potable groundwater. At present the nearest wells are hundreds of meters from the site. The permanent water table is situated below the Condie silt in sandy glacial till. An active perched groundwater system has developed within the thin fill layer beneath the floor of the plant. The extensive lateral flow of ground-water in this zone has contributed significantly to the spreading of PCBs to at the site. The PCBs have also moved downward through fractures in the Regina clay and in the upper part of the Condie silt. It can be expected that capillary forces will halt the downward movement of Inerteen 70-30 as fractures narrow or stop. However, the possible existence of fractures connecting the contaminated zone to the Reginamore » aquifer system or the potential for transport as a dissolved phase in the groundwaters may permit PCBs migrate downward to the aquifer or to flow laterally off-site. The most significant hazard represented by the spill is the potential for contamination of future water supply wells that may be placed near the site or aquifer contamination that could result if future drilling provides pathways for contaminants through poorly sealed wells or test holes.« less

Deep-Sea Bottom-Currents Differentiated from Texture of Underlying Sediment

Abstract: Forty-five core-top samples taken in the Vema Channel (southwest Atlantic) within the deduced boundaries of two important bottom-currents have been analyzed for the particle size distribution of the noncarbonate silt-size fraction. This was done in order to characterize the sediment beneath each water-mass. Using statistical and graphical methods it was found that sediment beneath the high-velocity Antarctic Bottom Water in the bottom of the channel exhibits the coarsest mean size, poor sorting, and a large percent of coarse silt left as a lag deposit; sediment beneath the overlying low-velocity North Atlantic Deep Water is marked by fine mean size, good sorting, no coarse silt, and a large percent of fine silt; sediment beneath the intervening transition zone is characterized by fine t mean size, moderate sorting, largest percent fine silt, and a small lag component of coarse silt. The methods developed for delineating bottom-currents in the Vema Channel may be applied to a wide range of benthic oceanographic settings as well as the geologic record in order to examine relative paleovelocity fluctuations of major bottom-currents.

Volumes, Types, and Distribution of Clay Minerals in Reservoir Rocks Based on Well Logs

Abstract: Discussed in this paper, is an innovative digital shaly sand evaluation approach which provides information on total and effective reservoir porosity, total and effective fluid distribution based on the Waxman-Smits equation, reservoir productivity, silt volume, and volumes, types and distribution modes of clay minerals present in subsurface ford. 52 refs.

Structure and chemistry of sediments in Belfast Lough, a semi-enclosed marine bay

Abstract: In a study of the benthic environment of Belfast Lough the structure of the subtidal sediment was examined along with its content of organic matter and selected metals (Cr, Mn, Ni, Cu, Zn, Pb). A wide range of sediment types occurred within the lough but the mean grain sizes for individual sites lay predominantly within the size classes of fine sand, very fine sand and silt. The content of organic matter and metals in the sediments showed strong positive correlations with each other and with the content of silt/clay. Both organic matter and metals decreased in concentration in a seawards direction. The content of organic matter lay generally within the range of 1–5% which is normally associated with shelf sediments, but constituted up to 12% of the sediment in the docks area. Likewise, metals were usually within the range reported within other nearshore coastal waters of the U.K., but sediments in the docks area and inner lough were enriched with several metals. Cluster analysis identified four major subdivisions in the lough which were distinctly different in their contents of silt/clay, organic matter and metals.

A preliminary discussion on the source of loessic material in China — A study of the surface textures of silt quartz grains by transmission electron microscope

Abstract: Loess of China, regardless of its age, exhibits similarity in the surface texture of silt quartz grains, all of them having conchoidal fractures, flat cleavage planes (or plates), and mechanical cracks, and having been re-worked by silica precipitation and chemical dissolution. Silica precipitation, different-degree chemica etching and dissolution are commonly found on these grains, indicating that they would be separated from source rocks, such as granites, gneisses, etc.; or they may have been subjected to relatively long weatheringin-situ before transported to loess-accumulating areas, or they probably underwent diagenesis when brought into the sedimentation system, followed by weathering and erosion, and then transported to loess-accumulating areas. It is evident from the relationship between loess of China, deserts and Gobis, and loessic composition and its regional distribution that Mesozoic-Cenozoic land detrital sediments widely spread in the Ordos and Alashan regions are the principal source of loessic material in China.

Illuviated clay in sandy podzolic soils of new brunswick

Abstract: Satisfactory uniformity with depth was established in sandy materials of marine, till and alluvial origin and a Podzolic soil was sampled in each of the materials. Marked maxima of clay, organic C, and pyrophosphate-extractable Fe and Al occurred within the podzolic B horizon of each soil. These maxima were least pronounced in the youngest soil developed in alluvium. In general, organic C and pyrophosphate Fe maxima occurred above clay and pyrophosphate Al maxima. Calculations of gains and losses of constituents showed marked gains of clay, organic C and extractable Fe and Al in the podzolic B horizons. Though weakly oriented, silt and clay cutans occurred on some grains in the B horizons of these soils, micromorphological evidence was not conclusive in indicating clay illuviation. The results are interpreted as indicating that clay was probably translocated and deposited with organic complexes of Fe and Al in these acid, sandy soils.

Sedimentation in Nadis in the Indian arid zone

Abstract: “Nadis” which serve as principal drinking water sources in the Indian Arid Zone are subjected to heavy sedimentation due to erratic and torrential rainfall in sandy and eroded rocky basins. Sediment yield increases with increasing rainfall and drainage basin slope and its magnitude depends upon the physiography and lithology of the region. Sediment yield can be reduced by promoting vegetation growth in the drainage basins and by constructing silt traps.

Genesis of a So-called Ferrolysed Soil of Bangladesh

Abstract: It was postulated by Brinkman (1970) that low clay contents in the surface horizon of paddy soils (Planosols) are caused by clay destruction through ferrolysis. In an effort to test this, a rice profile from the site of Brinkman's Eutric Planosol (Salna, Bangladesh) and a neighbouring forest soil (Dystric Cambisol) of the same parent material and particle-size distribution (Bhawal National Park) were investigated. The micro-structure, particle-size distribution, mineral contents, pedogenic oxides, contents of Zr, Ti and K of the fine earth and individual fractions and also the exchangeable ions were studied. The clay fraction of both soils were chloritised by interlaying of hydroxy-Al in the smectite interlayers in the surface horizons and got stabilized. As a result CEC showed some decrease. The forest profile has a loose and uniform brown-yellow surface horizon and is relatively rich in coarse sand at 62–75 cm depth which might be caused by a strong termite activity as observed during sampling. On the contrary, the frequent and alternate reducing and oxidizing conditions have resulted in gleying the top soil and forming iron-oxide films on the surface of aggregates during dry periods in the paddy soil. An evaluation of all results shows that a little clay destruction and an enrichment of silt in the top soil through vertisol formation and also through primary parent material stratification were the reasons for low clay content in epipedons. In forest soil there was a little clay migration, while the epipedon of the paddy soil had low clay in addition, because of selective erosion during heavy rains following the destruction of structure by puddling. The ferrolysis as a dominant factor for a clay-low epipedon is improbable, because the clay loss of the neighbouring forest soil can not be explained by ferrolysis.

Estuarine ecosystem of India

Abstract: An estuary is considered as a partly enclosed body of water that is formed when a river joins the sea. The water of the former is salinated by sea water. Such an estuarine environment is obviously to be found at the mouth of a river and will extend to as far as sea water can reach. The river, flowing over a considerable distance, brings a large amount of silt with it which is deposited on the banks of the river and, which is at the same time responsible for island formations in the river. These islands, which are barren at first, are soon subjected to the process of succession and develop into stable ecosystems.

A depositional process of reverse graded bedding in flood deposits of the sakura river, ibaraki prefecture, japan

Abstract: The Sakura River flows in a narrow alluvial channel with gentle slope, draining the southeast area of Ibaraki Prefecture (Figs. 1 and 2). The bed consists of arcosic sand and gravel which reflect the geological condition of the upper drainage basin of the river. The investigation of flood deposits in the upper reaches of the river shows that the flood deposits have reverse grading structure (becoming coarser grain from bottom to top of a bed) and that a sedimentary unit, the lower mud layer and the upper sand layer, is formed during a single flood. It is the purpose of this paper to explain the depositional process of the reverse graded bed from the characteristics of sediment transport in the Sakura. Main results are as follows. The flood deposits are thick only on the cut-sides of the river channel, which are attacked by the main flow of the flood (Fig. 6). Levees are better developed in these areas. Medium and fine sand make up the thick flood deposits on the natural levee (Fig. 7). It begins with the deposition of a thin silt layer and is followed by a thick sand layer which displays no internal structures. Sand sizes, φ1 as well as φ50, gradually increase toward upward. Fig. 8 shows the repeated cycles of coarsening upward sequence of flood deposits on the natural levee. The maximum diameter of each flood deposit has the upper limit of about 10(0. 5mm) . The floods which left the thick flood deposits are shown in the time-stage curves of Fig. 9. Reverse grading is also developed among the thin flood deposits on the lower part of the flood plain (Fig. 7). The lowest clayey silt layer with a thickness of a few milimeters suddenly changes the upper sand layer which usually has fine lamination. Fig. 9 shows the observational results on the hydraulic regimes and the suspended sediment transports in some floods in the Sakura River. It is the common fact to all observations that the maximum concentration of suspended load precedes the peak discharge. Furthermore, it is clear from Fig. 10 that the amount of the finer sediment load of silt-clay and very fine sand is extremely high at the leginning of each flood and rapidly decreases with the rising of waterlevel. The depositional process of reverse graded bedding in flood deposits can be explained as follows and Fig. 12 schematized them. 1. Deposition of the mud layer (Fig. 12-I) It is very likely that the lower muddy part of reverse graded bed is deposited during the beginning of the flood when the concentration of finer grained suspended sediments, the socalled “wash load”, is extremely high. Flood water with large quantities of wash load is diverted into the flood plain and the stagnation of flows on the flood plain causes the deposition of this finer sediments. It can be considered that the reason why mud layers on the natural levee are very thin and rather indistinct (Figs. 7 and 8) is that the natural levee is not covered by the inundated water at this stage. Boils along the bank begin to occur in the mean while. 2. Deposition of the sand layer (Fig, 12-II) The upper sandy part is presumed to be produced by the deposition of suspended sand after the rapid decrease of wash load concentration. The upward coarsening structure in this sand layer may be produced under the condition of gradual increase of flow intensity. The sand make up the natural levee is coarser than that of suspended up to the bankfull level at the center of the channel (Figs. 7, 8 and 11). This fact is explained as follows. Boils over the bank become more and more violent. This suggests that the upward flows (kolks) near the bank become violent with increases of discharge. It is supposed that these kolks cause the coarser sediment grains to suspend near the bank. 3. Preservation of the reverse graded bed (Fig. 12-III) With the beginning of water level falling, the inundated flow turns back from the flood plain to the channel.

Percentage Silt Content in Sands and Its Effect on Liquefaction Potential

Abstract: One of the most detrimental hazards caused by earthquakes is the ground failure resulting from sand liquefaction. Previous research effort in liquefaction potential has been focused on the liquefaction of saturated, uniform, clean sands under cyclic loading. However, natural sand deposits are rarely uniform and clean. Instead, soils found at previous liquefaction sites were either silty sand or sandy silt.