Silt Strata Research Articles

Mechanical properties and micro-mechanism of improved shield tunnel muck with phosphogypsum and lime

Massive shield tunnel muck is being landfilled without effective use due to high moisture contents and poor engineering properties, which not only leads to soil resources waste, but also occupies a large amounts of land sources. In this study, industrial waste phosphogypsum (PG) was recycled to improve the tunnel muck with lime, and the improved muck is aiming to be used in subgrade filling. A series of laboratory tests, including compaction tests, unconfined compressive strength (UCS) tests, california bearing ratio (CBR) tests, resilient modulus tests and microscopic tests were carried out. The effects of PG, lime, carboxymethyl cellulose (CMC) and moisture contents on physical, mechanical and microscopic properties of improved muck were examined, respectively. Results indicate that the incorporation of lime and PG significantly improves the mechanical properties and water stability of shield tunnel muck generated in silt stratum. The performance of improved tunnel muck meets the requirement for subgrade filling. Notably, for a fixed total contents of lime and PG, the proportion of 3:1 lime/PG results in the best mechanical properties and water stability. The tunnel muck generated in silt stratum mainly consists of lamellar and layered structures, which contributes to stronger cohesion among soil particles and the hydration products. The CMC has the ability to create cementation bonds among soil particles and the hydration products of lime and PG, resulting in a denser gel network microstructure.

Liquefaction-Induced Lateral Spreading Mitigation Using Pervious Concrete Column Inclusion in Sloping Strata

ABSTRACT The effectiveness of pervious concrete column remediation in homogeneous and sandwiched soil strata for mitigating liquefaction-induced lateral spreading is being investigated in this study. The seismic performance of pervious concrete column improved ground is compared with stone column improved ground. The efficacy of pervious concrete column on three types of soil strata in mitigating liquefaction along with the parameters influencing ground lateral deformation such as thickness of sandwiched liquefiable soil layer, permeability of surrounding soil, ground surface inclination, peak ground acceleration and surcharge load are reported. Three-dimensional nonlinear finite element software OpenSeesPL is used to analyze remediated ground with stone column and pervious concrete column inclusions. Liquefaction-induced lateral deformation is found to be lesser in pervious concrete column improved ground in comparison with stone column improved ground. The lateral deformation of pervious concrete column remediated ground is found to be independent of surrounding soil permeability. The pervious concrete column inclusion is found to be a better alternative to stone column in mitigating liquefaction in susceptible soils like loose sand, medium-dense sand, silt strata and sandwiched liquefiable soil deposits.

Propagation characteristics of vibrations due to rock blasting in silt-rock strata

The geological effects of rock blasting in silt-rock strata are complex, and the vibrations caused by the blasting threaten the safety of inhabitable areas. Based on measured data for the vibrations at the site and dynamic finite element simulations, the propagation characteristics of vibrations in silt-rock strata were studied. The propagation of the vibrations of the surface, the propagation characteristics of the vertical vibration velocities from the surface to the tunnel, and the attenuation of the blasting vibrations along the tunnel’s axis were analyzed. The results of this study are as followings: in the direction of the tunnel’s axis and in the direction vertical to the tunnel’s axis, the peak particle velocities (PPVs) of the surface decreases with increasing horizontal distance from the explosion source. The magnitudes of the PPVs of the surface under five working conditions are the same when the surface particle is right above the center of the explosion source and 10 m away from the center. In the vertical direction from the surface to the explosion source, the vertical direction of the silt-rock strata and the vertical direction of the silt stratum, the PPVs increases gradually with the increasing distance from the surface in the vertical direction. The change in the thickness of the artificial fill and silt in the shield interval strata has little effect on the PPVs along the tunnel’s axis. The results of this study provide a reference for identifying the safety distance around the inhabitable areas of rock blasting under similar geological conditions.

A Case Study on the Deformation of Metro Foundation Pit in Silt Stratum in North China

Geological conditions of urban subway foundation pits are controllable factors in determining the deformation of pits. In this paper, the monitoring data and statistical data of a subway deep foundation pit in North China are presented and compared with those of Tianjin subway. The deformation characteristics of the proposed pit, open excavated with triple-layer steel supports, are introduced in detail. Based on the aforementioned information, the energy conservation equation of the mobilized strength design (MSD) method in which the compression deformation energy of internal support is considered is applied to predict the maximum lateral movement. The maximum lateral movement turns out to be 22.2 mm according to the improved MSD method, which is very close to the measured value.

Numerical simulation of stress-strain state in frozen wall during thawing

Blasting and drilling operations during shaft sinking assume switching off the freezing columns which can lead to stability loss of a frozen wall and inrush of thawed soil into the stope. This work assesses safety of shaft sinking when freezing columns are turned off for a period of four days. It is assumed that the frozen wall loses its bearing capacity when displacement of its unfixed section exceeds 10 cm. Calculations are carried out for silt and sand stratums. Simulation was performed in finite-element software Comsol Multiphysics. Displacements of internal wall were obtained for two initial thicknesses of the frozen wall. In the first case the soil was frozen to the boundary of the future shaft, in the second case initial thickness corresponded to project documentation. It has been shown that frozen wall in silt stratum does not lose its bearing capacity even for complete defrosting. For sand stratum calculated time was equal to 85 days for the first thickness and 24 days for the second thickness of frozen wall.

Measurement and Analysis of Settlement Induced by Rectangular Pipe Jacking in Silt Stratum

The large‐scale rectangular pipe jacking technology has attracted more and more attention in the construction of urban underground pipe galleries, especially the problem of prediction and control of the surface settlement. Taking the pipe gallery project of Chengbei Road in Suzhou as an example, the settlement values of three groups of monitoring sections under the conditions of natural settlement and synchronous grouting are monitored, and the measured settlement values are compared with Peck formula and random medium theory. The results show that the Peck formula is more accurate in predicting the natural settlement within 1.5 times the pipe jacking width, while the stochastic medium theory is more accurate in predicting the settlement beyond the width. By comparing the field test and theory, the suggestions on the prediction of ground settlement of rectangular pipe jacking of large scale are put forward. The results of this study provide a basis for subsequent related research work and engineering applications.

Experimental Study on Vacuum Grouting in Silty Sand Stratum

Vacuum grouting is a new grouting method to control the groundwater in geotechnical engineering, which is often used in the strata where penetration grouting is difficult to reach an expected slurry diffusion radius, such as sand and silt strata. There are some theoretical studies about the influence of the vacuum field on the law of the slurry diffusion. However, the research on soil properties is rare. In this paper, through a self-designed grouting device, grouting in the silty sand after vacuum field action is studied. The test results indicate that with the increase in vacuum degree, the grouting capacity has an obvious improvement. When the vacuum degree is −15 kPa, the grouting capacity increased by 45.9% compared with the blank control group (0 kPa). Then, through the thermogravimetric analyzer and zeta potential meter the change of bound water and electric double layer in the silty sand after the effect of the vacuum field through a small device is studied. The results show that the effect of the vacuum field could make an influence on free water and loosely bound water, but the change of firmly bound water is unnoticeable. With the change of the vacuum degree, the variation of the water is increased. When the vacuum degree is −15 kPa, compared with the blank control group, the variable weight of free water is 5.79% and loosely bound water is 2.86%. As the bound water has a relationship with the effective void ratio, it can be inferred that the vacuum field may have an effect on the permeability coefficient of silty sand. These conclusions provide a reference for the theoretical study of vacuum grouting.

交通荷载对淤泥地层中地下管线的扰动影响

交通荷载对淤泥地层中地下管线的扰动影响

Experimental Study of the Influence of Excavation Surface Stability and Sand Flowability Caused by Dense Slurry–Earth Pressure Balance Shield Tunneling in Silty Sand Stratum

Silty sand is highly permeable and has a large internal friction angle. To ensure the stable and efficient construction of the Earth pressure balance (EPB) shield in silt stratum, it is necessary to reduce the disturbance to the stratum during tunneling and reduce the risk of spewing and blocking during soil discharge. Thus, a new type of dense slurry‐EPB shield construction technology is proposed. Using a custom‐made test device, the consolidation tests were conducted on sand after mixing it with different amounts of slurry to study the change in the pore pressure in the mixed soil and sand stratum and analyze the stress transmission rule. In addition, slump tests were conducted on the mixed soil to study the change in the fluidity. Based on the laboratory test results, a field investigation was carried out to study the effects of the amount of slurry addition on the pore pressure in the stratum, the driving parameters, and the sand’s flowability during shield tunneling. The results show that when the amount of slurry reaches a certain value, the filter cake effect occurs. The greater the amount of slurry added during the shield tunneling, the smaller the rate of change of the pore pressure in the stratum. When the amount of BC2 slurry added reached 8 m3 per ring, the rate of change was reduced by 57%, and the slump value improved by 93%. The type of filter cake of the DS‐EPB shield was only internal filter cake during tunneling, which reduced the disturbance of the excavation face and improved the flowability of the sand. These results are of great significance to the application and promotion of DS‐EPB shields in sandy cobble strata.

淤泥地层大直径盾构始发掌子面支护压力研究

淤泥地层大直径盾构始发掌子面支护压力研究

Geological environment problems during metro shield tunnelling in Shenzhen, China

Shenzhen has abundant river systems, intense tectonic activity and complex geological conditions. The depth of Quaternary deposits is thin and uneven, and the primary bedrock comprises granite with various degrees of weathering, which covers more than 50% of the area of Shenzhen. Karst strata have developed in the eastern part of Shenzhen, and there are five groups of fracture zones in the area. Shenzhen also has some problematic soils, including granite residual soil, and muddy clay and silt. Metro tunnels are constructed using the shield tunnelling method. In Shenzhen, different strata are encountered during metro shield tunnelling, including upper-soft and lower-hard strata, hard rock strata, soft–hard alternating strata, under-crossing river or reservoir strata, muddy clay and silt strata and granite residual soil strata. The various strata encountered during shield tunnelling may result in a series of problems with the geological environment. Environmental geological problems during tunnelling are different for different strata, among which the major issues include serious cutter wear, difficulty in controlling the trajectory of the shield machine, shield machine jam, roof fall and slurry spewing. Different countermeasures should be implemented according to the characteristics of the strata to ensure the safe construction of metro tunnels. In this study, a case study of the Shenzhen Metro Line 10 is introduced, where tunnels pass through three specific strata: hard rock strata, boulder strata and under-crossing reservoir strata. In the future, more engineering cases that pass through different strata can be summarized according to the tunnel construction project in Shenzhen.

Deformation Characteristics Analysis of Supporting Structure Caused by Deep Excavation of Large-Span Subway Parking Lot in Deep Silt Stratum

With the rapid development of China’s economy and the continuous improvements in urban modernization, the high-rise buildings, subways, civil defense fortifications, underground substations, underground storage, and other projects have increased, and the subsequent foundation pit projects, especially deep foundation pit engineering, have grown as well. Concurrently, the technical challenges and construction risks faced by deep foundation pit engineering have increased, especially in the coastal soft soil areas along rivers, due to the extremely fragile and sensitive geological environment. For this purpose, the ABAQUS finite element simulation software was used for numerical simulation analysis to study the influencing factors and the deformation rate of the diaphragm wall and aimed at different failure mechanisms of instability of deep foundation pit projects in the Yitian Parking Lot of Shenzhen Metro Line 10. Moreover, preventive measures for the deformation of supporting structures were proposed. The results showed that: (1) In the engineering mechanical properties of marine soft soil, deformation of the diaphragm wall is sensitive to the changes in the e-lnp rebound curve slope $$ \upkappa $$ and Poisson’s ratio v; moreover, the maximum surface settlement behind the wall is sensitive to changes in the critical state parameter M and Poisson’s ratio v. (2) A mechanical analysis model for diaphragm wall was established. All the external conditions are regarded as generalized loads or foundations. Furthermore, the interaction between the diaphragm wall and the soil is considered, which is closer to the actual situation. (3) The elasto-viscoplastic model of soft soil by Borja was established, and the deformation rate formula of the diaphragm wall was proposed.

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.

A 130,000-year-old archaeological site in southern California, USA.

The earliest dispersal of humans into North America is a contentious subject, and proposed early sites are required to meet the following criteria for acceptance: (1) archaeological evidence is found in a clearly defined and undisturbed geologic context; (2) age is determined by reliable radiometric dating; (3) multiple lines of evidence from interdisciplinary studies provide consistent results; and (4) unquestionable artefacts are found in primary context. Here we describe the Cerutti Mastodon (CM) site, an archaeological site from the early late Pleistocene epoch, where in situ hammerstones and stone anvils occur in spatio-temporal association with fragmentary remains of a single mastodon (Mammut americanum). The CM site contains spiral-fractured bone and molar fragments, indicating that breakage occured while fresh. Several of these fragments also preserve evidence of percussion. The occurrence and distribution of bone, molar and stone refits suggest that breakage occurred at the site of burial. Five large cobbles (hammerstones and anvils) in the CM bone bed display use-wear and impact marks, and are hydraulically anomalous relative to the low-energy context of the enclosing sandy silt stratum. 230Th/U radiometric analysis of multiple bone specimens using diffusion-adsorption-decay dating models indicates a burial date of 130.7 ± 9.4 thousand years ago. These findings confirm the presence of an unidentified species of Homo at the CM site during the last interglacial period (MIS 5e; early late Pleistocene), indicating that humans with manual dexterity and the experiential knowledge to use hammerstones and anvils processed mastodon limb bones for marrow extraction and/or raw material for tool production. Systematic proboscidean bone reduction, evident at the CM site, fits within a broader pattern of Palaeolithic bone percussion technology in Africa, Eurasia and North America. The CM site is, to our knowledge, the oldest in situ, well-documented archaeological site in North America and, as such, substantially revises the timing of arrival of Homo into the Americas.

Model Test on Negative Skin Friction for Super-Long Pile under Surcharge Load Considering Time Effect

Super-long pile bridges and high-rise buildings are commonly used in the form of pile foundation. In the flood area, due mainly silt stratum, as well as the decline in groundwater mound pile under the conditions set out in the consolidation, there will be a negative pile skin friction and negative friction obvious time effect, which bearing capacity of the pile has a great influence. Current research on negative friction for super-long pile considering time effect is relatively little. In the model box, on the base of the super-long pile silt model test, set out in a pile mound under consolidation, soil testing flour gradually over time preloading consolidation settlement, pile-soil relative displacement, so as to arrive negative friction pile variation with time, tip resistance variation with time from the pile-axial force curve deduce the location of the neutral point and changes in circumstances, the pile foundation engineering practice has a certain significance.

Analysis of Face Stability during Excavation of Double-O-Tube Shield Tunnel

This paper focuses on the face stability analysis of Double-O-Tube shield tunnel. This kind of analysis is significant to ensure the safety of workers and reduce the influence on the surrounding environment. The key point of the stability analysis is to determine the supporting pressure applied to the face by the shield. A collapse failure will occur when the supporting pressure is not sufficient to prevent the movement of the soil mass towards the tunnel. A three-dimensional collapse failure mechanism was presented in this paper. Based on the mechanism of a single circular shield tunnel, the mechanism of Double-O-Tube shield tunnel was established by using the fact that both of the mechanisms are symmetrical. Then by means of the kinematic theorem of limit analysis, the numerical results were obtained, and a design chart was provided. The finite difference software FLAC3D was applied to investigate the face failure mechanism of DOT shield tunnel established in this paper; the critical supporting pressures of the collapse failure mechanism in different strata (sand and silt) were calculated. Through comparative analysis, the theoretical values were very close to the numerical values. This shows that the face failure mechanism of DOT shield tunnel is reasonable, and it can be applied to the sand and silt strata.

Experimental study on the effect of injecting slurry inside a jacking pipe tunnel in silt stratum

Through a three-dimensional model test, the variations of jacking force and ground settlement inside a jacking pipe tunnel in silt stratum, under testing conditions which include non-injection and different synchronized injections of slurry, are simulated in order to study the effect of traditional thixotropic slurry (consisting of bentonite, CMC (Carboxymethyl Cellulose) and soda ash) and HL compound slurry to decrease frictional resistance and ground settlement. And then their mechanical properties are investigated according to the different slurry micro-structure features which are captured by an electron microscope. The tests and analysis demonstrate that: (1) injecting thixotropic slurry inside a jacking pipe tunnel has conspicuous effects in reducing frictional resistance, and (2) viscosity and condensation force of the slurry have a dominant effect on decreasing frictional resistance and ground settlement, respectively. To reach optimal jacking distance and efficiently control ground settlement, more attention should be given to slurry parameters in the actual engineering processes. The research results indicated excellent effects when they were applied to the jacking pipe of Shanghai Lingang New City Sewage Conduit.

Efficacy of preloading combined with prefabricated vertical band drains in improving a clayey silt stratum in situ

This paper presents a detailed field study of a soft clayey silt stratum improved in situ with preloading combined with prefabricated vertical band drains (PVDs). The clayey silt stratum was to support a warehouse in a chemical plant. As the bearing capacity of the soil was much lower than the load it was to support, the subsoil condition was improved by installing PVDs in combination with preloading. Thus the rate of consolidation of the clay stratum was increased. The band drains were installed up to a depth of 14·2 m at a spacing of 1·6 m. The intensity of preloading applied was 52 kPa. For preloading, the surcharge fill was placed up to a height of 3·10 m from the natural ground level. The area of both preloading and band drains was 15% greater than that of the floor of the warehouse for more effective improvement of the ground. Preloading was done for 60 days. Settlements were monitored using settlement gauges. The settlements observed across the entire area indicated that the preloading operation was uniform all through. The amount of settlement was large up to the point of filling the surcharge, but thereafter it did not show a significant increase. Cet exposé présente l'étude de terrain détaillée d'une strate tendre de limon argileux améliorée par précharge et drains en bande préfabriqués verticaux (PVDs). La strate de limon argileux devait porter un entrepzô t dans une usine chimique. Comme la capacité porteuse du sol était bien plus basse que la charge qu'elle devait recevoir, la condition du sous-sol a été améliorée en installant des PVD avec la précharge. Ainsi le taux de consolidation de la strate argileuse a augmenté. Des drains en bande ont été installés jusqu'à une profondeur de 14,2 m avec un espacement de 1,6 m. L'intensité de la précharge appliquée était de 52 kPa. Pour la précharge, un remplissage de surcharge a été placé jusqu'à une hauteur de 3,10 m à partir du niveau du sol naturel. Pour avoir une amélioration plus efficace du sol, la surface de la zone de précharge et de drains é tait de 15% plus é tendue que celle du sol de l'entrepô t. La précharge a été appliquée pendant 60 jours. Les affaissements ont été contrôlés en utilisant des jauges de tassement. Les tassements observés à travers toute la zone ont indiqué que l'opération de précharge était uniforme. Le volume de tassement é tait important jusqu'au point de remplissage mais après, il n'a montré aucune augmentation significative.

Stone columns as liquefaction countermeasure in non-plastic silty soils

In many cases densification with vibro-stone columns cannot be obtained in non-plastic silty soils. Shear stress re-distribution concepts [1] have been previously proposed as means to assess stone columns as a liquefaction countermeasure in such non-plastic silty soils. In this study, centrifuge testing is conducted to assess the performance of this liquefaction countermeasure. Attention is focused on exploring the overall site stiffening effects due to the stone column placement rather than the drainage effects. The response of a saturated silt stratum is analyzed under base dynamic excitation conditions. In a series of four separate model tests, this stratum is studied first without, then with stone columns, as a free-field situation, and with a surface foundation surcharge. The underlying mechanism and effectiveness of the stone columns are discussed based on the recorded dynamic responses. Effect of the installed columns on excess pore pressures and deformations is analyzed and compared. The test results demonstrate that stone columns can be an effective technique in the remediation of liquefaction induced settlement of non-plastic silty deposits particularly under shallow foundations, or vertical effective stresses larger than about 45 kPa (1000 psf) in free field conditions.

Crag aquifer characteristics and water balance for the Thurne catchment, northeast Norfolk

Abstract A study of the hydrogeological controls on the Pleistocene Crag aquifer in northeast Norfolk is reported. Geophysical methods, in particular multichannel seismic reflection profiling, have provided useful information on the internal structure of the aquifer. The Crag represents a layered aquifer, with horizons of clay and silt strata able to limit the overall vertical permeability of the aquifer. A laterally continuous clay horizon is identified at around -25 mOD and is mapped as far as the coast. Shallow wells and borehotes in the Crag are prone to contamination. Nitrate is widely recorded, with concentrations of typically up to 150 mg l -1 in shallow wells, derived from mainly agricultural sources, although deeper boreholes are less affected given the restricted downward movement of ground-water. The land drainage systems within this lowland area dominate the hydrology and hydrogeology of the catchment. Surface flows into the River Thurne are controlled by the operation of drainage pumps and a catchment water balance demonstrates that groundwater principally discharges to the drainage ditches within the extensive arable and grazing marshes in the catchment.