Cement stabilization of soil is a useful method to improve the mechanical behaviors and engineering performance of soils in geotechnical design and construction projects involving weak or liquefiable ...soils. Among the factors affecting the strength of cement-stabilized soils, water content and water–cement ratio are important but less well understood because of controversial views. This paper presents a systematic laboratory study to investigate the effects of water content and water–cement ratio on the unconfined compressive strength, with good control of the packing density and void ratio of the tested specimens. The effects of void ratio and cement content are also investigated. The strength of the cement-stabilized sand continuously decreased with increasing water–cement ratio within the range of 0.5 to about 3. A general equation is suggested to evaluate the unconfined compressive strength of cement-stabilized soil. Finally, a new conceptual characterization chart is proposed with consideration of the effects of cement content, water content, and water–cement ratio.
Understanding direction-dependent friction anisotropy is necessary to optimize interface shear resistance across soil-structure. Previous studies estimated interface frictional anisotropy ...quantitatively using contractive sands. However, no studies have explored how sand with a high dilative tendency around the structural surface affects the interface shear response. In this study, a series of interface direct shear tests are conducted with selected French standard sand and snakeskin-inspired surfaces under three vertical stresses (50, 100, and 200 kPa) and two shearing directions (cranial → caudal or caudal → cranial). First, the sand-sand test observes a higher dilative response, and a significant difference between the peak and residual friction angles (ϕ
- ϕ
= 8°) is obtained at even a lower initial relative density D
= 40%. In addition, the interface test results show that (1) shearing against the scales (cranial shearing) mobilizes a larger shear resistance and produces a dilative response than shearing along the scales (caudal shearing), (2) a higher scale height or shorter scale length exhibits a higher dilative tendency and produces a higher interface friction angle, and (3) the interface anisotropy response is more pronounced during cranial shearing in all cases. Further analysis reveals that the interface friction angle and dilation angle are decreased with the scale geometry ratio (L/H). For L/H values between 16.67 and 60, the interface dilation angle varies between 9° and 4° for cranial first shearing and 3.9°-2.6° for caudal first shearing. However, the difference in dilation angle within the same shearing direction is less than 1°.
Naturally available sands are always found with finer particles of varying sizes and proportions which are generally not accounted for in the geotechnical design of a cemented soil system. This paper ...explores the behavior of cemented sand with fine particles in smaller proportions. Two types of cements: (1) ordinary portland cement (OPC), (2) calcium sulfoaluminate cement (CSA); three cement contents: 3%, 5%, 7% and four fine (kaolin powder) contents: 0%, 1%, 3%, 5% are considered in this study. The ultrasonic pulse velocity (UPV), shear wave velocity (
V
s
) and unconfined compressive strength (UCS) are measured to investigate the effects of fine particles on the cemented sand. The results show that fine particles do affect quite significantly the mechanical properties of cement-treated sand, even at negligent proportions. The strength and stiffness increase with fine content in both types of cement. The increase in strength and stiffness with increasing fine contents is attributed to the increased density with kaolin acting as a filler material facilitating more contact points among the particles. The results also show that the effect of fine particles on cemented sand depends not only on their relative volume and mineralogy but also on the type of the binding material.
Cement stabilization is a useful and widely adopted method to improve the engineering properties of soils. However, characterization of the unconfined compressive strength, a simple and useful design ...property, is not straightforward due to complex interactions of various influence factors. This study investigated the effects of water content on the unconfined compressive strength of cement-stabilized clayey sands. The results show that the strength of the cemented binary mixtures increases with water content and water-to-cement ratio until a threshold value is reached and then decreases with further increase in water content and water-to-cement ratio. The unconfined compressive strength is correlated with ultrasonic wave velocity and shear wave velocity, respectively, showing two nearly unique correlations. Microscale analysis based on the coated sphere model revealed that the strength of the sample is affected by the bonding area and the strength of the binder material (cement–clay mixture). An empirical equation is also proposed based on the microscale analysis so as to capture the effects of water content on the strength.
Cement-stabilization of soil is a common and useful ground improvement method to increase the strength and stiffness of weak soils. The engineering properties of cement-stabilized soil can be ...affected by the amount of fines in the soil, but there exist controversial views on the effects of fines in the literature, indicating that the underlying mechanism in such cemented soils is not fully understood yet. This paper presents a laboratory study to investigate the effects of fines content on the strength and the wave velocities of cement-stabilized soil using specimens with well-controlled packing densities. The effects of void ratio and cement content are also examined. Experimental results show an interesting finding that the strength and the wave velocities of the cement-stabilized sand first increase and then decrease as the fines content increases from 0 to 15%. The contact and bonding characteristics at the particle level indicate that the strength of the sample is affected by the bonding area and the bonding strength. For a given cement content, the addition of fines increases the bonding area but decreases the bonding strength. Such controversial effects of fines enable to better understand the observed trend of strength variation. A similar mechanism can also be applied to the effects of fines on the wave velocity. Based on the particle-level mechanism, an empirical equation is proposed to characterize the effects of fines. The unconfined compressive strength of the samples can be correlated with the ultra-sonic pulse velocity and shear wave velocity for field monitoring and quality control. The correlations are affected by the fines content.
•Autocorrelation method can turn irregular ambient noise to useful zero-offset signal.•Bedrock can be accurately mapped out and imaged with estimated P-wave reflectivity.•Realistic average P-wave ...velocity of soil sediments can be estimated.•The proposed method is convenient for implementation, even on uneven terrain.
Soil/rock interface detection is an essential and critical task in geological site investigation for underground and tunnel projects. The required accuracy is within a few meters both vertically and horizontally. The required resolution poses big challenges to geophysical surveys to be applied in near-surface engineering works. This paper presents a new approach of non-invasive geophysical survey, namely autocorrelation. It is originally derived from seismic interferometry cross-correlation, which can convert a physical geophone to a virtual source. To the extreme is autocorrelation. It turns a single geophone into a self-source receiver. With this concept, the proposed approach includes two methods aiming to reduce the reliance on reference boreholes. A field testing carried out in Singapore is presented to demonstrate the concept and results step by step. The case study also demonstrates that the proposed method can fine-tune the autocorrelograms to achieve the required resolution both laterally and vertically. A by-product of realistic average Vp of the soil layers above soil/rock interface can be estimated reasonably by the proposed method. Two more case studies are followed to illustrate the proposed methods can produce reasonable 2D and 3D soil/rock interface profiles. Due to the self-source feature, this approach is convenient to implement on site. The proposed approach can be an attractive option for geotechnical and geological site investigation especially for delineating the soil/rock interface topography.
AbstractThe at-rest lateral stress coefficient (K0) is an important soil parameter in geotechnical design problems and yet it is quite elusive in our ability to assess its value, either by laboratory ...or in situ tests. In this study, an innovative geophysics approach toward the evaluation of the in situ K0 profile with depth is obtained by using small-strain stiffness anisotropy ratio (G0,HH/G0,VH) in soils. The newly proposed K0 equation is derived from a database compiled from 12 test sites which have direct K0 measurements, as procured from field tests, either self-boring pressuremeters (SBP) or total stress cells (TSC), and/or laboratory triaxial tests, instrumented consolidometers, and/or suction measurements. The data show a strong relationship between the overconsolidation difference (OCD=σp′−σvo′) and stiffness anisotropy ratio, thus enabling a K0 assessment in clays, silts, and sands.
In densely populated countries, underground construction and land reclamation could be possible options to solve the demand for land space, thus securing sustainable long-term development of the ...nation. For example, in Singapore, land reclamation has been widely conducted using excavated materials from underground development. The excavated materials are commonly marine clays that contain sandy soils. To improve the mechanical properties of these soft soils, cement-treated soil stabilization is popularly adopted. In fact, many researchers have investigated the properties of pure cemented clay or pure cemented sand using conventional design parameters such as water content (water/solids) and cement content (cement/dry soil). However, can these terminologies be still used to accurately examine the role of sand in cemented sandy clay mixtures? Through unconfined compression testing, it is herein shown that the use of existing mix design approaches in the literature cannot properly explain the variation of strength with sand content for cemented sandy clay mixtures. A new mix design approach is thus proposed in this study, which ensures that the role of sand in a cemented clay matrix can be quantified.
Determining the location and boundary of underground obstructions and/or anomalies is a common problem and often a great challenge for tunneling and underground construction. In this study, ...geotechnical investigations (penetration tests and borehole drilling/sampling) and geophysical investigations (surface wave method and cross-hole seismic method) were conducted with the aim of identifying the location and boundary of rock obstructions in Changi East, Singapore. The surface wave method is frequently used in the sites with lateral homogeneity in previous studies, but its application in the sites with rock obstructions is rare. The experimental results of this study indicate that the surface wave method is also able to determine the upper surface of rock obstructions, but difficult to identify the lateral and bottom boundaries of rock obstructions. To improve the precision of detection, the full waveform inversion (FWI) method was used to process the data from the cross-hole seismic survey. The results indicate that the inversion precision of P-wave is higher than that of S-wave. The horizontal and vertical ranges of rock obstructions in the P-wave inversion results are 14–26 m and 7.5–11.0 m respectively, roughly consistent with the results of penetration tests (about 15–25 m) and borehole log (8.85–10.80 m). This result proves that the sequential application of first-arrival time analysis and FWI can effectively delineate the boundary of rock obstructions. Finally, the results of various detection methods were analyzed and compared in this study. Considering the advantages of various methods, we propose a cost-effective and high-precision workflow containing both geotechnical and geophysical investigations.
The field measurement of shear wave velocity (Vs) is essential for geotechnical design practices, as it directly provides the initial tangent shear modulus at very small strain levels (γs < 10-6) in ...geo-materials. The small strain shear stiffness is a fundamental soil property for assessing dynamic loading responses, ground vibrations, and static deformation problems related to shallow and deep foundations. In addition, the Vs is one of the critical elements in evaluating seismic ground hazards such as site amplification and liquefaction potential. Various field and laboratory geotechnical site investigation programs in Kazakhstan have been conducted to understand basic soil behavior. However, in-situ geophysical seismic surveys such as surface reflection and refraction tests and down-hole and cross-hole tests were generally not included in the site investigation program in Kazakhstan, and a few limited seismic surveys have been carried out for specific projects. In most prior construction projects, the small strain shear stiffness was assessed by limited data using general empirical correlations from other in-situ measurements or selective laboratory testing programs that may result in significant uncertainties. In this study, in-situ dynamic soil characteristics of loam soils using active MASW (multi-channel analysis of surface waves) testing are evaluated to obtain comprehensive insights for geotechnical boundary value problems. The resulting Vs profiles are in good agreement with a-priory known geotechnical information (e.g., borehole logs) of sites. Thus, to minimize potential uncertainties of dynamic soil properties estimation via in-situ tests, MASW methods are suggested for construction works in Kazakhstan.
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