a theoretical envelope for internal instability of cohesionless soil the nature of seepage induced internal stability is characterised with reference both to effective stress in the finer fraction of the grain size distribution curve and also to hydraulic gradient within the soil the concept of a hydromechanical envelope is proposed in stress gradient space the onset of instability is triggered by a critical value of hydraulic gradient that is found consistent with the observations of terzaghi on piping in uniform sand and the observations of skempton and brogan on segregation piping in widely graded sandy gravels the envelope takes the form of a linear relation that for a particular soil is governed by the proportion of effective stress in the finer fraction of soil grains

the yerba buena mud record of the last interglacial predecessor of san francisco bay california the yerba buena mud member of the san antonio formation is an upper pleistocene estuarine deposit that records a marine transgression during the last interglacial into the valley today filled by san francisco bay these sediments now lie between 25 70 m below mean sea level and are not exposed subaerially foraminifers sand sized diatoms seeds fish elements molluscan shells and other microfossils preserved in core samples trace environmental changes that occurred as sea level rose this multitaxon approach permits biostratigraphic analysis of subsurface units even when core samples have undergone extended storage and consequent deterioration of calcareous microfossils foraminifers are preserved in only 22 of the yerba buena mud samples but more than 70 of the samples contain diatoms or other taxa useful for paleoenvironmental analyses in the yerba buena mud 32 species of foraminifers are present these species are predominantly estuarine and nearshore marine fauna seven species make up more than 98 of the population in all samples ammonia beccarii buccella frigida buliminella elegantissima elphidiella hannai elphidium excavatum elphidium gunteri and elphidium magellanicum foraminifers and sand sized diatoms define five biofacies that indicate changing environmental conditions during a single transgression into the yerba buena bay biofacies present in the lower portion of the unit indicate environments ranging from brackish such as those prevailing in the upper reaches of the present estuary to the higher salinity of the present central bay biofacies in the upper part of the unit indicate coastal marine salinity and lower water temperature conditions unlike those in the present estuary but prevalent along the outer central california coast today the transgressional record in the yerba buena mud is a useful example of environmental changes that might result from the effect of sea level rise in major holocene estuaries

experimental investigation of initiation of backward erosion piping in soils experiments were conducted on field and laboratory mixed soils in a new true triaxial test apparatus to assess factors that influence initiation of piping in non cohesive and cohesive soils three modes of piping behaviour were recognised concentrated leak erosion backward erosion and suffusion the amount and type of the fines present in the soils significantly influenced the initiation and mode of piping the presence of non plastic fines significantly reduced the required seepage velocity to initiate piping critical seepage velocity whereas the presence of plastic fines greatly increased the hydraulic gradient required to initiate piping concentrated leak erosion occurred along the soil apparatus interface in soils with plastic fines at very low seepage velocity but required very high hydraulic gradients backward erosion was the primary mode of failure in non cohesive soils and this mode required a seepage velocity of 0 8 1 1 cm s to initiate piping in uniform sand the critical seepage veloc

experiments on piping in sandy gravels tests on internally unstable sandy gravels show that a significant proportion of the sand content is washed out by piping at hydraulic gradients far lower than the critical gradient given by a classical theory the probable explanation is that a major part of the overburden load is carried on a framework of gravel particles leaving the sand under relatively small pressures by contrast piping of the sand from stable sandy gravels occurs at approximately the full theoretical gradient the tests broadly confirm kenney s criterion for the internal stability of granular materials des essais realises sur des graves sableuses a instabilite interne importante montrent qu une proportion non negligeable du sable est lessivee pour des gradients hydrauliques bien inferieurs au gradient critique donne par les theories classiques l explication la plus probable est que la majeure partie de la surcharge des couches subjacentes est transmise au reseau des particules graveleuses le sable n etant alors plus soumis qu

flume scale experiments on suffusion at the bottom of cutoff wall in sandy gravel alluvium this paper presents a series of flume scale experiments to investigate suffusion at the bottom of a cutoff wall in an internally unstable sandy gravel alluvium the initiation progression and potential failure of suffusion and the interactive effects of geomechanical and hydraulic conditions with the evolution of suffusion were investigated in this particular application temporal and spatial development of pore pressure earth pressure and settlement demonstrated suffusion was a multi phase involving pore water fine and coarse fractions and multi field involving seepage seepage induced fine fraction variation and stress deformation coupling phenomenon suffusion initiated at the downstream side of the tip of the cutoff wall and then generally progressed backward to the upstream side the monitored earth pressure provided an evidence of the heterogeneous stress distribution in internally unstable soil two linear empirical formulas for average hydraulic gradients at the initiation of suffusion a

spatial and temporal progression of internal erosion in cohesionless soil permeameter tests were performed on four widely graded cohesionless soils to study their susceptibility to in ternal erosion test specimens were reconstituted as a saturated slurry consolidated and then subjected to multi stage seepage flow under increasing hydraulic gradient the occurrence of internal instability is described qualitatively from vis ual observations through the wall of the permeameter during a test and from post test observations it is also described quantitatively from change of hydraulic gradient within the specimen and from axial displacement during a test the re sults provide a novel insight into the spatial and temporal progression of seepage induced internal instability this insight yields an improved characterization of suffusion and suffosion in cohesionless soils the progression of which appears gov erned by a critical combination of hydraulic gradient and effective stress resume des essais en permeametre ont eterealises sur quatre sols sans cohesion et agranulometrie etalee afin d etudier leur susceptibilitea l erosion interne les echantillons ont etereconstitues sous forme de pulpe saturee consolidee et en suite soumise aun ecoulement multi etapes sous un gradient hydraulique qui augmente l apparition de l instabiliteinterne est decrite de facon qualitative a partir d observations visuelles a travers la paroi du permeametre durant l essai ainsi que par des observations post essai l instabiliteinterne est aussi decrite quantitativement apartir des variations du gradient hydraulique al interieur des echantillons et du deplacement axial durant l essai les resultats offrent une vision nouvelle de la progression spatiale et temporelle de l instabiliteinterne causee par l infiltration ces resultats apportent une meil

critical hydraulic gradients of internal erosion under complex stress states abstractinternal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow which affects the hydraulic and mechanical behavior of the soil in this research extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths isotropic drained triaxial compression and triaxial extension stress paths these tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients the entire erosion process can be divided into four phases stable initiation development and failure accordingly three critical gradients termed as initiation skeleton deformation and failure hydraulic gradients can be defined these critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton the buckling of the strong force chains formed by the coarse particles and the soil failure re

comparison of two criteria for internal stability of granular soil internal instability describes the loss of soil particles as a result of seepage two criteria are commonly used to determine the potential for such instability within a granular soil that is subje

carbon isotope diatom and pollen evidence for late holocene salinity change in a brackish marsh in the san francisco estuary analysis of diatoms pollen and the carbon isotopic composition of a sediment core from a brackish marsh in the northern part of the san francisco estuary has provided a paleosalinity record that covers the past 3000 yr changes in marsh composition and diatom frequencies are assumed to represent variations in freshwater inflow to the estuary three periods of relatively high salinity low freshwater inflow are indicated 3000 to 2500 cal yr b p 1700 to 730 cal yr b p and ca a d 1930 to the present the most recent period of high salinity is primarily due to upstream storage and water diversion within the sacramento san joaquin watershed although drought may also have been a factor the two earlier high salinity periods are likely the result of reduced precipitation low salinity high freshwater flow is indicated for the period 750 cal yr b p to a d 1930 c 2001 university of washington

what have natural and human changes wrought on the foraminifera of san francisco bay late quaternary estuaries abstract in this study we compare the foraminifera of modern south san francisco bay with fossils from sediments of a previous estuary at 125 ka to provide a basis for interpreting the impact of natural and human change on the benthic ecosystem all the species found in the pleistocene sediments of this study are estuarine and or shallow water species occurring commonly in san francisco bay today except for the introduced foraminifer trochammina hadai a native of japan that was not found in samples taken in san francisco bay before 1983 the biodiversity and species composition of the fossil and modern assemblages before the introduction of t hadai are nearly identical suggesting that the environmental and physical changes in the 125 000 year old and modern estuaries have not had a significant effect on the meiofauna of the bay in contrast modern anthropogenic change in the form of species introductions has impacted the modern foraminiferal assemblage t hadai began to dominate the modern assemblage a decade after its introduction similar to the recorded impacts of introductions of marine metazoan invertebrate species the dominance of t hadai changed species proportions in the post 1980s foraminiferal assemblage however no known extinctions in the native foraminiferal fauna occurred

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

strontium isotope ages of the marine merced formation near san francisco california abstract the strontium isotopic compositions of foraminifers elphidiella hannaiwere determined to refine the age of coastal sediments of the merced formation and to evaluate the relation between transgressive regressive cycles and pleistocene glacio eustatic sea level fluctuations relatively good age resolution 0 1 myr is possible in the upper part of the section the sr isotope ages provide strong supporting evidence for the fission track ages of an ash layer at the top of the formation 0 45 myr as well as recent age estimates for the mammoth fossil of 0 45 myr sr isotope ages in the upper part of the section also confirm that marine nonmarine cyclic sedimentation in the merced formation is controlled by glacial interglacial cycles good age resolution for the lower part of the section is not possible due to little variation of the87sr 86sr ratio in global sea water between 2 4 and 4 3 myr the age of the base of the formation is constrained only between 2 4 and 4 8 myr sediment accumulation rates in the upper portion of the merced formation between 0 and 720 m are significantly higher than those calculated for the basal portion between 720 and 1670 m the change in accumulation rate may be due to increased continental weathering rates and or increased tectonic subsidence beginning about 0 8 myr ago

temporal changes in foraminiferal faunas and impact of pollution in sandebukta oslo fjord abstract sandebukta oslo fjord norway is an inlet with depths up to 70 m characterised by estuarine water stratification an influx of pollutants and a mudflat in the inner part the distribution of its foraminiferal fauna displays essentially an estuarine pattern but also reveals significant subrecent changes as demonstrated by means of 9 sediment cores supplemented by a larger number of sea bed surface samples the faunal changes have taken place during the last 100 years and are summarized as follows 1 upwards increase in size of the total population shown by the sediment cores 2 reduction in the relative frequency of calcareous foraminifera upwards in the cores so that the recent fauna of the inlet consists mainly of arenaceous species 3 alterations in the species composition of populations most pronounced in deeper waters faunal comparisons with outer parts of the fjord system suggest that these changes are at least partially induced by pollution originating from human activity in addition post mortem destruction of foraminiferal tests is considered as a possibly important modifying factor requiring further study an important additional reason for the faunal turnover found in the mudflat seems to be successive shallowing during progradation

differences in radiocarbon age between shell and charcoal from a holocene shellmound in northern california abstract the west berkeley shellmound the oldest well dated archaeological site in the san francisco bay region contains shell and charcoal ranging in age from ca 1200 to 5700 cal yr b p radiocarbon ages of marine shell and charcoal collected from fifteen stratigraphic levels in the west berkeley shellmound suggest changes in the 14 c content of san francisco bay surface waters relative to the atmosphere the oceanic reservoir age over the past 5000 yr the reservoir age of san francisco bay waters fluctuated between 870 and 170 14 c yr over the past 5000 yr with the lowest values occurring 2900 to 3800 cal yr b p and the highest values between 1200 and 2000 cal yr b p changes in the radiocarbon reservoir age may be due to changes in the strength of seasonal wind driven upwelling off coastal california where upwelling brings 14 c depleted waters to the surface the period of lowest r values at 3500 to 3900 cal yr b p is coincident with relatively low salinity in san francisco bay indicating high freshwater inflow and wet climate in california based on lake level records the period of high r values 1200 2000 cal yr b p is coincident with one of the driest periods in california during the late holocene these data suggest a link between coastal upwelling and precipitation over central california the age of the top of west berkeley mound and several other mounds in the san francisco bay region 1100 to 1300 cal yr b p coincides with a prolonged dry period in california and low river inflow to san francisco bay perhaps the sites were abandoned because of the drought

collision induced tectonism along the northwestern margin of the indian subcontinent as recorded in the upper paleocene to middle eocene strata of central pakistan kirthar and sulaiman ranges abstract outcrop data from the upper paleocene to middle eocene ghazij formation of central pakistan provide information about the depositional environments source areas and paleogeographic and tectonic settings along the northwestern margin of the indian subcontinent during the closing of the tethys ocean in this region in the lower part of the exposed stratigraphic sequence are various marine carbonate shelf deposits jurassic to upper paleocene overlying these strata is the ghazij which consists of marine mudstone lower part paralic sandstone and mudstone middle part and terrestrial mudstone and conglomerate upper part petrographic examination of sandstone samples from the middle and upper parts reveals that rock fragments of the underlying carbonate shelf deposits are dominant also present are volcanic rock fragments and chromite grains paleocurrent measurements from the middle and upper parts suggest that source areas were located northwest of the study area we postulate that the source areas were uplifted by the collision of the subcontinent with a landmass during the final stages of the closing of the tethys ocean middle eocene carbonate shelf deposits that overlie the ghazij record a return to marine conditions prior to the miocene to pleistocene sediment influx denoting the main collision with eurasia

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

distribution of biologic anthropogenic and volcanic constituents as a proxy for sediment transport in the san francisco bay coastal system abstract although conventional sediment parameters mean grain size sorting and skewness and provenance have typically been used to infer sediment transport pathways most freshwater brackish and marine environments are also characterized by abundant sediment constituents of biological and possibly anthropogenic and volcanic origin that can provide additional insight into local sedimentary processes the biota will be spatially distributed according to its response to environmental parameters such as water temperature salinity dissolved oxygen organic carbon content grain size and intensity of currents and tidal flow whereas the presence of anthropogenic and volcanic constituents will reflect proximity to source areas and whether they are fluvially or aerially transported because each of these constituents have a unique environmental signature they are a more precise proxy for that source area than the conventional sedimentary process indicators this san francisco bay coastal system study demonstrates that by applying a multi proxy approach the primary sites of sediment transport can be identified many of these sites are far from where the constituents originated showing that sediment transport is widespread in the region although not often used identifying and interpreting the distribution of naturally occurring and allochthonous biologic anthropogenic and volcanic sediment constituents is a powerful tool to aid in the investigation of sediment transport pathways in other coastal systems

assessing the potential of internal instability and suffusion in embankment dams and their foundations suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces soils susceptible to suffusion are described as internally unstable this technical note describes a method for assessing the potential of internal instability of silt sand gravel or clay silt sand gravel soils based on their particle size distribution based on laboratory tests carried out by the writers and results published by others it is shown that some commonly used methods are conservative for these soils

experimental parametric study of suffusion and backward erosion within hydraulic earth structures dikes levees or dams internal seepage flows can generate the entrainment of the soil grains grain transportation affects both particle size distributions and porosity and changes the mechanical and hydraulic characteristics of the earth s structure the occurrence of failures in new earth structures due to internal erosion demonstrates the urgency of improving our knowledge of these phenomena of erosion with this intention a new experimental device has been developed that can apply hydraulic stresses to reconstituted consolidated cohesive soils without cracks in order to characterize the erosion evolution processes that might be present a parametric study was conducted to examine the influence of three critical parameters on clay and sand erosion mechanisms when the hydraulic gradient was low it was concluded that the erosion of the structure s clay fraction was due to suffusion when the hydraulic gradient increased it was concluded that the sand fraction erosion initiation was due to backward erosion the extent of the erosion was dependent on the clay content the study underlines the complexity of confinement stress effects on both erosion phenomena

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr