Vane Shear Strength Research Articles

Improved air-booster vacuum preloading method for newly dredged fills: Laboratory model study

Newly dredged fills feature a high moisture content and a high floating mud content. Based on the conventional vacuum preloading (CVP) method, this article presents a new method of air-booster vacuum preloading (AVP) to reinforce newly dredged fills for different air-booster periods (45, 60 and 90 min). Compared with other methods, this new method features no sand cushion and adopts a self-developed air-booster pipe and a water–air separation device. A series of laboratory model tests were performed to explore the effect of the improved AVP method for the enhancement of newly dredged fills. A comparison of variables monitored during reinforcement (vacuum pressure, surface settlement, water discharge and pore-water pressure) and after reinforcement (water content, vane shear strength, SEM and MIP) indicates that the reinforcement effect of the AVP method is superior to that of the CVP method; the former can effectively alleviate the problem of prefabricated vertical drain (PVD) clogging, greatly reducing engineering costs and significantly shortening construction periods.

Improving consolidation of dredged slurry by vacuum preloading using prefabricated vertical drains (PVDs) with varying filter pore sizes

Prefabricated vertical drains (PVDs) have been used extensively to accelerate the consolidation rate of dredged slurry. While some fine particles from dredged slurry can easily squeeze through the filter into the drainage channel, many cannot. As such, these soil particles deposit on the filter surface causing partial clogging of the drainage path. Although the pore size of filter is recognized as an important factor that influences PVD clogging, the standards for determining the pore size of the filter are lacking. To this end, the traditional gradient ratio tests with four different filter pore sizes were conducted, and the results show that the permeability of the filter at a given head increases with the increase in the pore size of the filter. To remove the effect of the difference between static hydraulic gradient and vacuum pressure, the vacuum preloading tests with varying pore sizes of filters were further conducted. Through these vacuum preloading tests, the degree of vacuum, settlement, pore-water pressure, water content, vane shear strength, and other parameters of PVDs with various filter pore sizes were obtained, and the optimal pore size of filter was determined.

Measurement of Peat Soil Shear Strength Using Wenner Four-Point Probes and Vane Shear Strength Methods

The general objective of this research was to measure the peat soil shear strength using Wenner four-point probes and vane shear strength methods. Specifically, the objective of this study was two-fold, namely, (a) investigating the relationship between laboratory soil resistivity and undrained shear strength and (b) determineing the relationship between in-situ soil resistivity and undrained shear strength. Data were randomly collected over six locations in Meranek, Sarawak, for in-situ test and three repetitions for each data were set based on three parameters. The selected parameters were soil density, moisture content, and salinity for both laboratory and in-situ test using Wenner four-point probes and vane shear method. The soil resistivity and vane shear strength readings for laboratory test were correlated with soil salinity, moisture content, and density. The R2values showed a good correlation for soil salinity (R2=0.8468) and density (R2=0.9475), respectively. However, a weak correlation of R2=0.1205 was observed for soil moisture. The R2value for in-situ correlation between soil resistivity and three parameters (soil salinity, moisture content, and density) was R2=0.8916. It can be concluded that the peat soil shear strengths of the study area using Wenner four-point probes from in-situ were (4.38 ohm.m) and laboratory was (2.47 ohm.m) and when using the vane shear strength method, in-situ was (23 kPA) and laboratory was (5 kPA). This study implies that the peat soil of the study area can be categorized as texture (soft loamy soil) and it is suitable for agriculture instead of construction. The relationship established between Wenner four-point probes and vane shear method can be beneficial for ground engineering design to enhance investigation on site suitability. Future work on DUALEM-421 technique should be emphasised for better subsurface exploration accuracy and resolve peat depth for an in-situ test.

Discussion on Reinforcement Mechanism and Construction Technology of Pressurized Vacuum Preloading

Based on the basic theory of vacuum preloading, this paper proposes an good effect of pressurized vacuum preloading in which the high pressure gas results in tiny cracks between the air pressurized pipe and band drain. Therefore additional drainage paths are introduced to improve the permeability of soil and accelerate soil consolidation. But field tests show that the advantages of pressurized vacuum preloading are not obvious compared with conventional vacuum preloading, and the soil settlement, soil properties and vane shear strength are slightly lower than those by conventional vacuum preloading. The vacuum pressure transmission is analysed in the air pressurized pipe and band drain as well as the dissipation of pore water pressure in the soil during the pressurization process, it is concluded that the unsatisfactory reinforcement effect is mainly due to the sealing problem between the top of the air pressurized pipe and the sand cushion, and the overlap problem between the booster pipe and band drain.

Vane shear strength of bio-improved sand reinforced with natural fibre

In the recent developments, bio-improvement has considerably enhanced the geotechnical properties and reduced degradation of loose desert sand without the environmental concerns. However, the effect of bio-improvement on sand weakens due to water submergence. Therefore, it is necessary to explore the techniques for retaining the strength of bio-improved sand in water submerged state. So, this paper reports the geotechnical performance of submerged bio-improved sand reinforced by discrete natural fibres. The desert sand sampled from Al-Sharqia Desert of Oman, bio-improved by xanthan gum and reinforced by discrete natural fibres obtained from date palm tree. Date palm fibres having a diameter less than 0.2mm and length 1cm were employed for sand reinforcement. Vane shear tests and standard compaction tests were performed on specimens having different mix ratios of sand-xanthan gum with and without natural fibres. The qualitative description of bio-improvement and fibre reinforcement effect on the desert sand is also presented as the photomicrographs taken by field emission scanning electron microscope. According to the results of strength tests at higher moisture content state, the discrete fibre reinforcement has enhanced the geotechnical performance of sand improved at lower concentrations.

Consolidation Effect of Prefabricated Vertical Drains with Different Lengths for Soft Subsoil under Vacuum Preloading

The application of vacuum preloading to prefabricated vertical drains (PVDs) with different lengths is widely used in practical engineering to investigate their consolidation at the same depths of even and multilayer subsoils from the seabed. In a laboratory, model experiment was conducted using even subsoil and embedded PVDs with lengths of 0.6 and 1.2 m. The obtained results showed that in the even subsoil, the 1.2 m PVDs maintained a higher vacuum pressure in the shallow layer and demonstrated better consolidation behavior as compared to those of the 0.6 m PVDs. In the upper subsoil layer, the average vane shear strengths of these two systems increased to 18.2 and 22.6 kPa, respectively. The degree of consolidation of the upper subsoil layers in the two model experiments calculated from the pore water pressures under boundary drainage conditions were 51% and 68%, respectively. For practical verification purposes, similar experiments were conducted for multilayer subsoil by inserting PVDs with lengths of 6 and 15 m into different test sites. As a result, the vane shear strengths of the upper 6 m subsoil layers increased to 26.3 and 33 kPa, while the degree of consolidation were 72.1% and 80.9%, respectively, although some irregularities were observed at different depths.

Effect of surcharge loading rate and mobilized load ratio on the performance of vacuum–surcharge preloading with PVDs

The results from three laboratory model tests performed under various vacuum and surcharge loads with PVDs are reported. Different SLRs were adopted to investigate the effect on the consolidation of dredged soil. To measure the lateral displacement, a refitted inclinometer was developed and tested. In the tests, the settlement, lateral displacement, and vane shear strength were measured, and the degree of consolidation (DOC), horizontal coefficient of consolidation (Ch), and bearing capacity were calculated. The results indicate that larger SLR values promote consolidation. The largest vane shear strength, settlement, and Ch values were all obtained under the highest SLR, and the bearing capacity under this SLR was more than double that under the lowest SLR. The DOC was found to increase with the growth of the SLR. However, considering the vacuum pressure was higher in Case-III, the influence of SLR on reinforcement effect may not be so significant.

Laboratory Investigation on Quasi-Static Penetration Testing Using SPT Sampler in Soft Clay Bed

Abstract The standard penetration test (SPT) is one of the most widely used in-situ penetration tests for subsurface exploration. Its suitability for assessing and predicting the design parameters of granular materials has been well appreciated by the geotechnical engineering community over several decades. However, in the case of soft to very soft clays, the correlation between the standard penetration number (N) and the undrained shear strength is poor. In the recent past, tests conducted at a constant rate of penetration, such as the cone penetration test (CPT), ball penetration test, and T-bar penetration test, have become popular in soft clay regions. These tests also have a distinct advantage over SPT in terms of test setup because, in the case of SPT, the entire weight of the sampler and the rod assembly is directly applied on the subsoil, whereas in the case of CPT, the weight of the assembly is supported externally prior to the penetration. This can cause significant operational advantage in case of soft clay deposits. Despite the shortcomings, SPT is still one of the most widely available site investigation tools in the field, as samples are available for inspection and index tests. The operational disadvantages in the case of soft clay deposits can be overcome by altering the penetration process of SPT. A quasi-static penetration at a constant rate rather than a dynamic penetration will help establish a relationship between the penetration resistance and the strength parameters of soft soils. In this article, a quasi-static penetration test using the standard SPT sampler is conducted on reconstituted clay samples in order to develop an empirical correlation with the laboratory vane shear strength. The particle image velocimetry technique is used for detecting the influence zone and plugging during penetration.

Relating planter downforce and soil strength

Soil strength is a property that contributes to agricultural outcomes and describing related soil attributes. It is typically measured at low throughput with manual implements or approximated on-the-go with unique systems, limiting the robustness and broader applicability of data sets. This work assessed the viability of planter downforce as an on-the-go and relatively standardized means to collect soil strength data. Manually sampled soil mechanical and physical property data was collected and compared with planter downforce. Medium to high relationships were observed between planter downforce and vane shear strength (r2 = 0.76), cone penetration resistance (r2 = 0.67), and bulk density (r2 = 0.78). These indicate planter downforce – a dataset extensively collected by modern planters – has the potential to meaningfully describe distributions of soil strength in agricultural fields, and thereby serve as a useful soil informatics dataset to improve modeling of crop development and productivity.

Strength and hydraulics characteristics variations within a tropical Alfisol in Southwestern Nigeria under different land use management

This study investigated the hydraulics and strength characteristics of an Alfisol under long-term land use in Akure, southwestern Nigeria. The three agricultural land uses under study are Cropland, Grazing land, and Oil palm plantation. Hydraulic conductivity tests were randomly conducted at 60 sampling points per location (10 m apart), to make a total of 180 samples from the three land uses, while the unconfined compressive strength (quf) and vane shear strength measurement were conducted in the laboratory and insitu, respectively on soil at depths up to 45 cm (i.e. 0–15, 15 – 30, and 30 – 45 cm). The hydraulic conductivity and soil water sorptivity were determined using mini disk infiltrometer at a steady-state flow of – 0.5, – 1 and – 2 cm water suction rates. Land use and land cover change significantly affected the hydraulic and mechanical properties of the soil (p ≤ 0.001, p ≤ 0.01 and p ≤ 0.05). The hydraulic conductivity value showed an increasing trend from the grazing land to cropland and oil palm plantation in the order: grazing land (4.72 cm h−1) < cropland (10.80 cm h−1) < oil palm plantation (13.21 cm h−1). Grazing land recorded the highest mean soil water sorptivity value, while oil palm plantation had the least value. Grazing land had the highest average quf and shear strength among the land use types at depths 0–15 cm, 15–30 cm and 30–45 cm of soil. Depth – wise consideration showed that in all the land uses, unconfined compression strength (quf), shear strength and vane shear strength significantly increased with depth. Soil properties such as organic matter content, bulk density, and aggregate sizes influence the infiltration and strength characteristics of soils of the study area. Land uses influenced the hydraulic properties and strength characteristics of agricultural soils and therefore, assessment of changes in soil properties under land use types should be conducted periodically to effectively monitor the soil status over time. This will guide decisions on measures adoptable to ensure the sustainability of soil structure and texture for optimum crop productivity.

Experimental study on a dredged fill ground improved by a two-stage vacuum preloading method

The vacuum preloading method has been wisely chosen among many ground-improvement methods considering the time limit of many projects and the characteristics of reclaimed soil. However, the loss in vacuum with soil depth, the clogging around prefabricated vertical drains (PVDs), and the deteriorative consolidation of the deep soil layer, among other factors, create a large challenge to vacuum preloading for dredged marine clay fill. Thus, this study proposes a two-stage vacuum preloading method and focuses on its feasibility and effectiveness. Contrasting laboratory tests are performed in two identical experimental tanks with dredged marine clay fill from the Wenzhou land reclamation site in China. In one tank, the one-stage vacuum preloading method is used to serve as a baseline for this study. In the other tank, use of the two-stage vacuum preloading method is proposed for consolidation; it comprises two stages. In the first stage, the dredged marine clay fill is conditioned by vacuum preloading using half of the PVDs, where the dissipation of the excess pore water pressure tends to be steady. In the second stage, vacuum preloading is activated using all the PVDs. The results show that a better consolidation effect is achieved with the proposed method in terms of the settlement, vacuum pressure, pore water pressure, water content, vane shear strength, and soil particle microstructure after soil consolidation.

Facultative commensalism of a free-burrowing urothoid amphipod with a deep burrow-dwelling callianassid shrimp in intertidal sand

Species of the free-burrowing amphipod genus, Urothoe, are common on open sandy beaches. On intertidal sandflats, some species are associated with burrows or tubes of large infauna. How this link is formed and persisting under sheltered conditions was examined. On an intertidal sandflat in mid-western Kyushu, Japan, U. carda co-occurred with the deep burrow-dwelling callianassid shrimp, Nihonotrypaea harmandi, along a 300-m transect between tide marks. Amphipods resided in the surface 5-cm sediment outside shrimp burrows, as confirmed by sediment coring and burrow casting. In summers 1980 and 1981, the shrimp and amphipod populations were confined to the upper shore at mean densities of 182 and 701 inds m−2, respectively. In winter to spring, when the sediment surface mixing was caused by seasonal wind-induced waves, the amphipod but not the shrimp expanded down to the lowest shore. Later, by 1983, the shrimp increased mean density by 2.5 times and now also ranged to the lowest shore. In the summers of 1984, 2010, and 2015, the amphipod expanded to the lowest shore as well, with small variations in population size. Three marked changes in substrate properties were associated with shrimp inhabitation: thicker oxidized layer (proxy for oxygenated layer) in the sediment column; looser surface sediment, as evaluated with vane shear strength; and coarser and better-sorted surface sediment with less mud content. At least the former two changes were attributable to shrimp bioturbation, which could provide the amphipod with more permeable and softer substrates, leading to the formation of facultative commensalism.

Effect of electrochemical treatment on consolidation of soft clay

In this study, a method of electrochemical consolidation is applied. This method utilizes electro-osmosis, which is an effective ground improvement technique for soft clays, and soil treatment using lime, which is the oldest traditional soil stabilizer. The mechanism of lime treatment for soil involves cation exchange, which leads to the flocculation and agglomeration. Five representative laboratory tests-an electro-osmotic test and four electrochemical tests with various proportions of lime-were performed on dredged marine clay. The objectives of this study are to investigate the effect of electrochemical treatment and to determine the optimum dose for optimal consolidation performance of dredged marine clay. The results show that a better consolidation effect was achieved in terms of current, temperature, and vane shear strength by using electrochemical treatment. The best results were observed for the electrochemical test using 4% lime content.

Thixotropic aging and its effect on 1-D compression behavior of soft reconstituted clays

Abstract The present study investigates the moisture content at which the thixotropic effect is maximum, as well as the effect of such aging on compression. For this research, samples were prepared at moisture contents ranging from 0.55–1.25 times their corresponding liquid limits, and fall cone and vane shear tests were administered to determine the shear strength. Oedometer tests were performed on both thixotropically aged and unaged samples to observe the effects of thixotropic aging on 1-D compression characteristics. It was observed that the maximum thixotropic strength was obtained at a moisture content close to 0.75 times the corresponding liquid limit for all samples, and thixotropic strength recovery was negligible at a moisture content of 0.55 times the corresponding liquid limit. Vane shear strength was found to be lower than fall cone strength, and an increase in the rate of strain resulted in lower estimation of thixotropic strength recovery. The thixotropic strength ratio was observed to be dependent on the plasticity index and activity of the clay sample. Of all of the clays examined, the thixotropically aged sample provided the highest yield stress. The study suggests that in situations where the effective vertical stress is less than 100 kPa, the settlement calculation, using the conventional effective stress principle, needs to be rigorously revised. A scheme for considering both strength and compressibility behavior in design practice is also provided.

Pilot tests on vacuum preloading method combined with short and long PVDs

Dredged marine clay has been widely used as a filling material for land reclamation in China. The difficulty of using the vacuum preloading method to improve the dredged marine clay together with the bottom sediment clay is the different spacing requirement of the PVDs. To solve this problem, the Vacuum Preloading method combined with the Short and Long PVDs (VPSL) is proposed in this paper. The short PVDs are installed only into the dredged marine clay layer in-between the long PVDs which are installed through the whole clay layer. Pilot tests are also conducted at a land reclamation site in Tianjin, China, to investigate the performance of the proposed method. The ground settlement, the applied vacuum pressure and the pore water pressure in the soil are monitored during the pilot tests. The average degrees of consolidation are calculated based on the monitored settlement and pore pressure data. It is found that the proposed VPSL method is more effective for improving top dredged clay together with the bottom sediment clay than the conventional vacuum preloading method. The vane shear strength profiles of soil layers after ground improvement also show that the VPSL method is more effective to achieve a uniform soil strength profile.

Two Possible New Techniques for Determining the Early-Age Shear Strength of Shotcrete: Auger Penetrometer and Vane Shear Tester

Shotcrete or sprayed concrete is widely used as ground support in the construction of tunnels. The early-age strength development of shotcrete is very important because it influences the determination of safe re-entry times beneath the tunnel heading. This study has examined the possible application of a handheld soil vane shear tester and a handheld auger penetrometer for testing the early-age strength of shotcrete. This is achieved by measuring the shear strength because previous research in this area indicates that the shear failure dominates the early-age resistance of shotcrete. The investigation involved testing of three techniques, which resulted in readings of both compressive and shear strengths. Shear testing involved a combination of the vane shear tester and the auger penetrometer, which obtains its readings through auger penetration and rotation. The compression testing involved the use of the Meyco needle penetrometer, which was tested on the same cast specimens as the shear tests and at the same time intervals. In this paper, the term shotcrete has been used for the special normal concrete, with a simple shotcrete mix design, used in this investigation. Strong correlations were found among the vane shear strength, auger penetrometer shear strength, Meyco penetrometer compressive strength, and time after pouring, for up to 4 h of testing. The shear strength results from the auger penetrometer and the vane shear tester are very similar, and there is almost a perfect linear correlation between the results from these two methods.

Evaluation of Uniformity of Soil Specimens Prepared in Large Tanks by Slurry Consolidation

This paper evaluates the degree of uniformity achieved in clay beds prepared for model tests using slurry consolidation method. Clay Specimens were prepared in a cylindrical tank split into five horizontal rings of equal height. This set-up facilitated the sampling of clay for quality checks viz., oedometer tests, gradation analyses, vane shear strength and water content determination in horizontal and vertical directions throughout the specimen. The results from the study show that while the spatial variation of water contents conforms to the acceptance criterion, the same is not true for that of undrained shear strength which is the main parameter of interest for model tests. It is proposed, therefore, to make the acceptance criterion for water content stricter by the factor bs, which is the characteristic of the soil type, depicting slope of the linear function of water content, w, versus undrained shear strength, su, on log–log scale. Oedometer tests showed the reduction of consolidation pressure with depth, which indicates the loss of applied pressure while being transmitted to the lower layers. Gradation analyses suggested no particle segregation in the consolidated specimen. The relation between water content and consolidation pressure was utilised to plot pressure distribution contours throughout the specimen. The uniformity in the specimen was also compared with that prepared through centrifuge consolidation.

Empirical modeling for predicting excavation performance of EPB TBM based on soil properties

This study was conducted in order to determine the effects of soil properties on the excavation performance of an earth pressure balance tunnel boring machine (EPB TBM) used in the Istanbul Ayvali Waste Water Tunnel Project in the development of empirical performance prediction models. The tunnel alignment was divided into three general sections in terms of geological and geotechnical conditions. Soil samples were collected from 14 different locations along these sections of approximately 60 segment rings, and subjected to laboratory tests in order to determine the particle size distribution, natural water content, consistency limits (liquid limit, plastic limit, plasticity index, and consistency index), vane shear strength, fall cone penetration depth, unified soil classification, and clogging potential. The field performance data were obtained from the EPB TBM data logger, which operated in open mode without any face pressure being applied. Data analysis was carried out by statistically linking the net cutterhead torque, TBM thrust force, instantaneous cutting rate, and field-specific energy with the geotechnical properties of the excavated clayey soil by using single and linear multi-variable regression methods. The results indicate that, while the performance parameters are similar for the silty sand and lean (low plasticity) clay, the cutterhead torque and field-specific energy are higher, and the thrust and instantaneous cutting rate are lower, for high plasticity clays. The vane shear strength and consistency index may be used for predicting the cutterhead torque, fall cone penetration depth for the thrust force, plastic limit for the instantaneous cutting rate, and plastic limit and plasticity index, as well as the consistency index, for field-specific energy predictions. Additional studies are required to provide more reliable and generalized prediction models, by increasing the number of laboratory and field tests for different soil types and performance data for different EPB TBMs.

The difference analysis of physical-mechanical properties of sediments in the central South Yellow Sea and Zhe-Min coastal area in China

The difference analysis of physical-mechanical properties of muddy sediments is made in the central South Yellow Sea and the Zhe-Min (Zhejiang Province to Fujian Province of China) coastal area. The results show that sediments in the two regions are both dominated by mud. There are perfect negative power function correlations between the water content and the density, the compression coefficient and the compression modulus; a good positive power function correlation between the liquid limit and the plastic limit, a perfect positive linear correlation between the water content and the void ratio, and a perfect polynomial function correlation between the miniature vane shear strength and the pocket penetration resistance. In general, compared with sediments in the Zhe-Min coastal area, sediments in the central South Yellow Sea possess high water content, high void ratio, low density, high plasticity, high compressibility, low shear strength. The causes of the differences between physical-mechanical properties of sediments are analyzed from the topographic features, material sources, hydrodynamic conditions, deposition rate, and material composition. Compared with the Zhe-Min coastal area, the central South Yellow Sea is far from the Mainland and low-lying; has poor hydrodynamic condition; the materials diffused to the area are less and dominated by fine clay, have the high content of smectite and organic matters. These factors lead to sediments of the central South Yellow Sea has the higher water content, the higher plasticity, the lower density, and the lower strength than sediments in the Zhe-Min coastal area.

Reinforcement Effect of Polyacrylamide on Dredger Fill Improved by Vacuum Preloading

A series of laboratory tests were carried out to evaluate the effectiveness of polyacrylamide (PAM) on dredger fill combined with the vacuum preloading method. Five groups of vacuum preloading model tests were conducted, and the groups were mixed with 0, 15, 30, 60, or 90 mg/L PAM. The test results indicated that the optimal dosage of PAM is 30mg/L for the Tianjin Binhai dredger fill with a water content of approximately 300%. The optimal dosage of PAM can not only considerably shorten the consolidation time of the dredger fill but can also notably increase the rate of dissipation of excess pore pressure. An approximately 70% increase in the vane shear strength was also observed. Mercury intrusion porosimetry was performed to analyze the pore characteristic parameters of the soil samples after vacuum preloading combined with PAM, and the results revealed that an excessive dosage of PAM can lead to relatively bigger pore diameter in the soil samples.