Sandy Pebble Research Articles

Soil Displacement of Slurry Shield Tunnelling in Sandy Pebble Soil Based on Field Monitoring and Numerical Simulation

Due to its inherent advantages, shield tunnelling has become the primary construction method for urban tunnels, such as high-speed railway and metro tunnels. However, there are numerous technical challenges to shield tunnelling in complex geological conditions. Under the disturbance induced by shield tunnelling, sandy pebble soil is highly susceptible to ground loss and disturbance, which may subsequently lead to the risk of surface collapse. In this paper, large-diameter slurry shield tunnelling in sandy pebble soil is the engineering background. A combination of field monitoring and numerical simulation is employed to analyze tunnelling parameters, surface settlement, and deep soil horizontal displacement. The patterns of ground disturbance induced by shield tunnelling in sandy pebble soil are explored. The findings reveal that slurry pressure, shield thrust, and cutterhead torque exhibit a strong correlation during shield tunnelling. In silty clay sections, surface settlement values fluctuate significantly, while in sandy pebble soil, the settlement remains relatively stable. The longitudinal horizontal displacement of deep soil is significantly greater than the transverse horizontal displacement. In order to improve the surface settlement troughs obtained by numerical simulation, a cross-anisotropic constitutive model is used to account for the anisotropy of the soil. A sensitivity analysis of the cross-anisotropy parameter α was performed, revealing that as α increases, the maximum vertical displacement of the ground surface gradually decreases, but the rate of decrease slows down and tends to level off. Conversely, as the cross-anisotropy parameter α decreases, the width of the settlement trough narrows, improving the settlement trough profile.

Novel interaction between an octopus and an introduced macroalgae

Numerous macroalgae facilitate other species, modifying the diversity patterns, reducing the environmental stress, or providing non-trophic resources. In the sandy–pebbly bottoms of San Antonio Bay (north Patagonia, Argentina), where shelters are an important but limiting resource for the small Patagonian octopus (Octopus tehuelchus), the recently introduced macroalgae Undaria pinnatifida has shown a rapid increase in abundance. Herein, we documented the use of U. pinnatifida as shelter for octopuses and describe the size and maturity condition of individuals using the macroalgae. Our results suggest that U. pinnatifida facilitates octopuses by providing shelter in an environment where these are scarce and there is competition for high-quality shelters. The presence of small octopuses and brooding females under U. pinnatifida suggest that the structure of the macroalgae constitute an adequate shelter for brooding and may facilitate a portion of the population that usually lose the competition for traditional shelters. Indeed, the holdfast of U. pinnatifida, full of potential food sources, may provide a larger positive effect on octopuses. The effects of U. pinnatifida in north Patagonia should be tested on the different components of the community since there seem to be no similar native macroalgae providing structural complexity in these sandy–pebbly bottom ecosystems.

Investigation on Dewatering of a Deep Shaft in Strong Permeable Sandy Pebble Strata on the Bank of the Yellow River

This paper reports the dewatering scheme of a deep excavation in sandy pebble strata. The excavation is in high permeability strata and is close to the Yellow River, making the dewatering difficult during construction. At present, few researchers have specially studied the dewatering scheme of deep excavations in strong permeable strata near the water resource. Field pumping test was conducted before the excavation activity, and the permeability coefficient of the strata was obtained by reverse analysis. According to the characteristics of the project, the dewatering scheme of “ waterproof curtain + base grouting + pumping ” was proposed. The influence of vertical waterproof curtain and base grouting on dewatering was analyzed by numerical simulation. In the construction process, the field water table and ground settlement were measured. The results show that (1) the groundwater table versus permeability coefficient curve shows three different stages and (2) the dewatering scheme of “ waterproof curtain + base grouting + pumping ” is effective for deep excavation in strong permeable strata.

System Stability of Bored Piles with Steel Supports for Deep Foundation Pits in Sandy Pebble Stratum

Background: At present, there are a few research studies on the stability of the supporting structure of bored piles with steel supports for deep foundation pits, and it is even less common to use this support method for deep foundation pits in sandy pebble stratum. This paper studied the stability of the deep foundation pit support of the Southwest Jiaotong University Station in Chengdu, China, using the stability analysis theory of bored piles with steel supports for deep foundation pits. Objective: The study aims to show the stability of steel supports for deep foundation pits. Methods: The overall stability, anti-overturning stability and uplift stability of the foundation pit are important components of the stability analysis of bored piles with steel supports for deep foundation pits. Aiming at the designed method and parameters of the bored piles with steel support for the deep foundation pit, in-depth theoretical analysis, and calculation of the overall stability, antioverturning stability, and uplift stability of the deep foundation pit are conducted, respectively. Results: The results show that the stability of the deep foundation pit using bored piles with steel support meets the requirements, indicating that the design of this support form is safe and reasonable, thus ensuring the smooth progress of the project. Conclusion: The stability analysis of deep foundation pits in sandy pebble stratum provides ideas for the optimization of deep foundation pits and a reference for the stability analysis of deep foundation pits in sandy pebble stratum and other types of the stratum.

Numerical Simulation of the Type Selection of Cutterhead Selection and Tool Arrangement in Beijing Sandy Pebble Stratum Based on PFC3D

Numerical Simulation of the Type Selection of Cutterhead Selection and Tool Arrangement in Beijing Sandy Pebble Stratum Based on PFC3D

Design of Groundwater Extraction in Open Cut Foundation Pit and Simplified Calculation of Ground Subsidence due to Dewatering in Sandy Pebble Soil Strata

In order to study the design of incomplete well point for groundwater extraction in open cut foundation pit in which the inner and outer aquifers are not completely isolated and the change mechanism of ground subsidence due to dewatering in the foundation pit, an open cut foundation pit for a subway station on Chengdu Metro Line 6 is taken as an example; meanwhile, the typical sandy pebble soil strata are also considered as a research object in this paper. Firstly, a new method for designing groundwater extraction in the open cut foundation pit is presented and applied to the practical project. The dewatering funnel curve is derived based on Dupuit’s assumption, and the ground subsidence around the foundation pit due to groundwater extraction is calculated using the stratification summation method as well as considering the effect of seepage force. The finite difference software FLAC3D is employed to simulate the groundwater extraction process in the foundation pit, and the simulation of groundwater extraction by single well point and group well points is also carried out and the unapparent effect of group well points is obtained. The comparison among on‐site monitoring, theoretical calculation, and numerical simulation shows that these values have the same trend in indicating ground subsidence, and the conventional stratification summation method is conservative, and the algorithm considering the effect of seepage force is more accurate. Therefore, the ground subsidence curve caused by groundwater extraction in the foundation pit is presented. The above research methods and results can be applicable for practical engineering and be used to guide the design and construction of groundwater extraction in the foundation pit by using the open cut method in sandy pebble soil strata.

Research on Portland Cement Grouting Material in Sandy Pebble Soil

Research on Portland Cement Grouting Material in Sandy Pebble Soil

The Construction of Equivalent Particle Element Models for Conditioned Sandy Pebble

When a shield tunneling machine based on earth pressure balance (EPB) bores through the sandy pebble stratum, the conditioned sandy pebble inside the soil cabin of shield machine is an aggregation of numerous granules with pebble grains as skeleton. It is essential to construct a reasonable particle element model of the conditioned sandy pebble before carrying out discrete element simulation of the soil cabin system. Sandy pebble belongs to a kind of frictional material, the friction behavior of which is highly sensitive to the angularity of the grains. In order to take the shape effect into account, two particle element models—single sphere with rolling resistance and cluster of particles—were attempted in this paper. The undetermined contact parameters in two models were calibrated by virtue of least squares support vector regression machine (LS-SVR). With the purpose of making both the flow behavior and mechanical properties of the modeled soil consistent with reality, the calibration targets the result of laboratory test of slump test and large-scale triaxial test as goals. The presented comparative analysis indicates that the two established particle models both can well describe the strength property and fluidity of the actual soil due to properly calibrated parameters. So, the rolling resistance and cluster models are two effective ways to incorporate the shape effect. Besides, because of the angularity of the nonspherical grains, there exists strong interlocking between clusters. So, in the cluster model, relatively smaller rolling friction coefficient and surface energy are required. It is also concluded that the single sphere model is more computationally efficient than the cluster model.

Characteristics and Causes of Water Seepage on Riverside Tunnel Structure and Control Measures in Sandy Pebble Stratus

The water content and permeability of silt and silt strata are significant in cities of Southwest China, and the seepage of underground passages in cities is unavoidable, which affects normal operation. More serious situation may occur when large-scale water gushing phenomenon and obvious structural deformation appears. After long-term and repeated repairs, water cannot be sealed, and even causes surface subsidence and deformation, voids in ground soil, which cause severe social and economic impacts. Loss. Therefore, it is necessary to formulate effective water shutoff measures for specific seepage projects according to local conditions to ensure that groundwater seepage can be effectively solved or controlled. Taking an underground passage in a city as an example, this paper analyses the causes and characteristics of seepage diseases, explores the specific stratum conditions of seepage location and puts forward control measures. Through detection the effectiveness of the implementation of control measures are verified, which provides reference for the treatment of underground seepage in similar strata.

Macro-Micromechanical Properties of Sandy Pebble Soil of Different Coarse-Grained Content

Sandy pebble stratum is a typical discrete particle unstable stratum, mainly consisting of sand and pebble. However, the effect of coarse-grained content on the stability of stratum is not clear. This paper defined the sandy pebble soil of different coarse-grained content in Chengdu City, Sichuan Province, China as the research object. Research on macro-mesomechanical properties of sandy pebble soil of different coarse-grained content was carried out using the method combining the indoor large-scale triaxial test of coarse-grained soil with the discrete element numerical triaxial test. The research results showed that the stress-strain curve of sandy pebble soil exhibited strain softening with the increase of coarse-grained content; when the confining pressure was the same, the stress peak increased and the strain when the peak was reached decreased gradually with the increase of coarse-grained content. It revealed the functional relationship between coarse-grained content and mechanical indexes of sandy pebble soil such as internal friction angle and cohesion. The internal friction angle and cohesion of sandy pebble soil linearly increased with the rise of coarse-grained material; it proposed the particle discrete element micro parameters of sandy pebble soil of different coarse-grained content, including contact modulus, friction coefficient, particle stiffness ratio, contact bond strength. The research results provided the theoretical support for the new design and construction of sandy pebble stratum project.

Sediment sequences and palynology of outer South Bay, Manitoulin Island, Ontario: Connections to Lake Huron paleohydrologic phases and upstream Lake Agassiz events

South Bay on the southern coast of Manitoulin Island is a fjord-like embayment connected to Lake Huron by a natural narrow gap in the bay's outer sill 6.5–14 m above the lake. A seismic profile, pollen, plant macrofossil, grain size analyses, and other sediment properties of two piston cores from a shallow outer basin of the bay document a 9 m-thick sediment section comprising rhythmically laminated clay under silty clay containing zones with small molluscan shells and marsh detritus. A sandy pebbly layer under soft silty clay mud overlies these sediments. This stratigraphy represents inundation by deep glacial Lake Algonquin followed by the shallowing Post Algonquin series of lakes, and exposure in the early Holocene by 5 Lake Stanley lowstands in the Lake Huron basin separated by 4 Lake Mattawa highstands. Overflow from South Bay in the first lowstand is thought to have eroded the outer sill gap. Marsh environments are inferred to have formed in the bay during subsequent lowstands. The Lake Mattawa highstands are attributed to outburst floods mainly from glacial Lake Agassiz. Palynological evidence of increased spruce occurrence, an apparent regional climate reversal, during the dry pine period is attributed to cold northwest winds from the Lake Superior basin and a lake effect from the Mattawa highstands in the Lake Huron basin. Lake waters transgressed South Bay following the pine period to form the Nipissing shore on Manitoulin Island. Transfer of Lake Huron basin drainage to southern outlets and continued glacioisostatic uplift of the region led to the present configuration of South Bay and Lake Huron.

Study on Sandy Pebble Reach Riverbed Erosion and Deposition Basic Law after the Three Gorges Reservoir

After the Three Gorges Reservoir, sandy pebble Reach incoming water and sediment conditions has change, the changes of incoming water and sediment conditions might make sandy pebble reach riverbed scouring-erosion variation. This paper study on sandy pebble reach riverbed erosion and deposition basic law after the Three Gorges Reservoir, and provide the decision basis for large-scale system renovation in the middle Yangtze River.

Gschliefgraben mudslide (Austria): hazard evaluation and risk mitigation

A rock slide on top of the clayey–silty–sandy–pebbly masses in the Gschliefgraben (Upper Austria province, Lake Traunsee) having occurred in 2006, together with the humid autumn of 2007, triggered a mudslide comprising a volume up to 4 million m³ and moving with a maximum displacement velocity of 5 m/day during the winter of 2007–2008. The possible damage was estimated up to 60 million € due to the possible destruction of houses and of a road to a settlement with intense tourism. The movement front ran ahead in the creek bed. Therefore, it was assumed that water played an important role. Inclinometer measurements showed that a less permeable layer was sliding on a thin, more permeable layer. During the last centuries, mudslides had already pushed farms into the lake, as reported by chronicles. Thus, the inhabitants of 46 houses had to be evacuated for safety reasons. They could return to their homes after displacement velocities had decreased. It was decided (a) to prevent soaking of water into the uppermost, less permeable layer by transversal drainages, (b) to lower the pore water pressures by longitudinal trenches filled with blocky material, (c) to pump water out of the more permeable layer by well drillings upslope of the houses in order to create a stable block below the houses and (d) to remove material thrust over the stable blocks in order to avoid damage to the houses. These mitigation measures costing 11.5 million € led to a deceleration of the process to displacement velocities of some cm/year up to now. The houses and the road were not damaged.

Experimental Study on Static Characteristics of Sandy Pebble Soil

Through the consolidation drained triaxial shear test of sandy pebble soil, the stress-strain curve of sandy pebble soil is obtained, analysis manifest that in the shearing process, shear dilatancy deformation and shear contraction deformation of sandy pebble soil are intercoupling, making asymptote of the curve after peak stress nearly a horizontal line. Meanwhile, based on the Duncan-Chang model, elastic modulus and Poisson ratio features of sandy pebble soil are analyzed, and Duncan-Chang model parameters are obtained.

Experimental Study on Dynamic Characteristics of Sandy Pebble Soil

Through the laboratory dynamic triaxial test of sandy pebble soil, evolution law of influence of confining pressure and vibration frequency on dynamic strength is analyzed. Analysis indicates that dynamic stress of sandy pebble soil improves in a large degree with accretion of vibration frequency, dynamic strength of which increases significantly with confining pressure rising, and relation of dynamic stress and dynamic strain can also be symbolized with the R.L.Kondner hyperbola curve. Meanwhile, dynamic elastic modulus of sandy pebble soil decreases with growth of dynamic strain, while increases with growth of consolidation pressure.

Experimental Investigation on Dynamic Constitutive Model of Saturated Sandy Pebble Soil

The dynamic constitutive model could show the dynamic characteristics of soil, so it was essential in ground motion in earthquake safety evaluation and seismic response analysis of soil, and it had great influence on research and analysis of seismic performance of soil and the stability of the foundation. Sandy pebble soil distributed abroad in Sichuan, so it was widely used in engineering construction. Based on the test results of dynamic triaxial non-drain consolidation test，this paper studied the dynamic constitutive model of saturated sandy pebble soil to provide reference for the future use.

Evidence from the northwestern Venezuelan Andes for extraterrestrial impact: The black mat enigma

A carbon-rich black layer encrusted on a sandy pebbly bed of outwash in the northern Venezuelan Andes, previously considered the result of an alpine grass fire, is now recognized as a ‘black mat’ candidate correlative with Clovis Age sites in North America, falling within the range of ‘black mat’ dated sites (~ 12.9 ka cal BP). As such, the bed at site MUM7B, which dates to < 11.8 ka 14C years BP (raw dates) and appears to be contemporaneous with the Younger Dryas (YD) cooling event, marks a possibly much more extensive occurrence than previously identified. No fossils (megafauna) or tool assemblages were observed at this newly identified candidate site (3800 a.m.s.l.), as in the case of the North American sites. Here, evidence is presented for an extraterrestrial impact event at ~ 12.9 ka. The impact-related Andean bed, located ~ 20 cm above 13.7–13.3 ka cal BP alluvial and glaciolacustrine deposits, falls within the sediment characteristics and age range of ‘black mat’ dated sites (~ 12.9 ka cal BP) in North America. Site sediment characteristics include: carbon, glassy spherules, magnetic microspherules, carbon mat ‘welded’ onto coarse granular material, occasional presence of platinum group metals (Rh and Ru), planar deformation features (pdfs) in fine silt-size fragmental grains of quartz, as well as orthoclase, and monazite (with an abundance of Rare Earth Elements—REEs). If the candidate site is ‘black mat’, correlative with the ‘black mat’ sites of North America, such an extensive occurrence may support the hypothesized airburst/impact over the Laurentide Glacier, which led to a reversal of Allerød warming and the onset of YD cooling and readvance of glaciers. While this finding does not confirm such, it merits further investigation, which includes the reconnaissance for additional sites in South America. Furthermore, if confirmed, such an extensive occurrence may corroborate an impact origin.

Outburst Flood Sedimentation on the Proglacial Tuttle Canyon Alluvial Fan, Owens Valley, California, U.S.A.

ABSTRACT Surface deposits of the Tuttle Canyon alluvial fan, eastern Sierra Nevada piedmont, California, mainly accumulated during a catastrophic outburst flood probably triggered by the failure of a moraine-dammed lake in the steep (16.4°), high-relief (2361 m), and high-elevation (to 4275 m) catchment. Moraines of the catchment and upper fan (Facies A) comprise unsorted sand to blocks derived from the granitic bedrock. The main flood facies (B) is texturally like the moraine, consisting of unsorted, matrix-supported, sandy pebbly cobbly blocky boulder gravel in a single unit 3 to 8+ m thick distributed across most of the 10.5 km long and 28.1 km2 fan. Facies B was deposited during the initial outburst-flood phase as a high-volume, noncohesive sediment gravity flow (NCSGF) instigated by water rapidly descending over moraine. It differs from the moraine by a planar unit geometry and a slope-transverse long-axis alignment of boulders. NCSGF deposition was followed by a water-flood phase caused by continued lake drainage subsequent to removal of loose sediment in the flood path. This discharge carved channels 2-22 m deep and 5-60 m wide around and across fan NCSGF deposits, with channels departing from various points along Tuttle Creek. Horseshoe separation scours also formed around bedrock spurs, and laminated pebbly sand accumulated on the channel beds (Facies D). Flood discharge expanded on the distal fan into a sheetflood that first deposited a boulder unit (Facies E), and then 4+ m of cobbly sandy pebble gravel in planar beds 5-20 cm thick (Facies F). The final flood phase entailed incision of Tuttle Creek, wherein bouldery cobble gravel (Facies C) was concentrated. Post-flood modification of these deposits has been limited to secondary processes such as eolian deposition (Facies G) and bioturbation. Outburst floods on fans of the Sierra Nevada piedmont ultimately are caused by the glacial transfer of colluvium from the upper catchment slopes into moraines on the valley floors that dam drainage to form lakes. Outburst floods differ from meteorological floods in the valley by the high water and sediment discharge achieved during a single event, by deposition of massive NCSGF beds, and by causing major changes to the landscape such as the carving of channels. These differences produce fans with distinctive facies and hazards, the former allowing their recognition in either modern or ancient settings.

Log-Grounds and the Ichnofossil Teredolites in Transgressive Deposits of the Clayton Formation (Lower Paleocene), Western Alabama

A thin (∼1.5 m) transgressive systems tract within the Lower Paleocene Clayton Formation of western Alabama, composed of, in ascending order, a sandy pebbly marlstone (transgressive lag), calcareous muds, and a chalk (condensed section), contains an unusual abundance of allochthonous logs with Teredolites, a clavate boring produced within xylic substrates. Borings are characterised by diameters ranging from 1 to 15 mm, high length/width ratios, contorted axes oriented primarily parallel to the grain of substrates, calcite linings, and rare terminal bioglyphs. Although no body fossil evidence for the affinity of the boring organisms is preserved, these morphological characteristics, which are most allied with the ichnotaxon T. longissimus, reflect the boring activities of wood-digesting bivalves of the family Teredinidae