Procedure for predicting settlements of thawing bases for engineering calculations

It was shown that in order to compute the settlement of thawing cohesive soil under a load in time it is necessary to experimentally determine the consolidation coefficient. A procedure is suggested for determining this coefficient, as well as use of the classical solution of a differential equation for seepage consolidation to describe the settlement of soil, thawing of which can occur according to any law.State All-Union Scientific Research Institute of Roads. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 5–7, July–August, 1993.     

Deformational characteristics of weak sandstone and impact to tunnel deformation

In northern Taiwan, a tunnel under construction along a segment where weak sandstone, the Mushan sandstone, was encountered and an excess crown settlement (14–30 cm) has been reported. This paper studies the deformational characteristics of Mushan sandstone and its impact on tunnel deformation. To distinguish the volumetric and the shear deformation of the sandstone, experiments with controlled stress paths, including hydrostatic compression, pure shearing and conventional triaxial compression, were conducted. The measured deformations were then decomposed into elastic and plastic components further exploring the stress–strain behavior of weak sandstone. The results indicate that, similar to other soil-like geo-materials, this sandstone has plastic strain before the stress path reaches the failure envelope and significant shear dilation is induced, especially when approaching the failure envelope. Meanwhile, the distinct features of deformation have also been highlighted by comparing the experimental results to the prediction, derived from existing constitutive models that were originally developed for other geo-materials. These features include significant plastic volumetric strain at low levels of confining stress, suppression of plastic volumetric strain at higher levels of confining stress, and the fact that the actual amount of shear compression is less than that predicted by the model. Numerical analysis indicates that the weak rock leads to the greatest inward displacement, which results from the shear dilation prior to failure state.     

Experimental checking of stability of underground pedestrian crossings on heaving soils

Conclusions 1. Underground crossing structures on natural soil bases may be used also for heaving soils provided their strength and rigidity are increased, taking into account the nonuniform deformations. It is recommended that a value of 0.0015 be used for the limiting allowable coefficient of relative deformation nonuniformity of tunnels.2. If the tunnel is considered as a reinforced concrete underground frame buried in the soil, then for strength, crack-resistance, and deflection design under the normal frost heave force, this force should be assumed to be equal to the soil reaction in the tunnel base under all the permanent and short-duration loads, taken in accordance with SNiP II-D.7-62; also, account must be taken of the friction force caused by the backfill sand on the tunnel lateral surface.3. For freezing and thawing of the soils in the bases of underground crossings, the plastic deformations are insignificant, i.e., the frost heave is practically equal to the settlement under thawing.4. The weak correlation between the coefficient K_n and the heave indicates that measures intended to reduce the depth of freezing (heave) by heating the bottom or by partially replacing the heaving soils by nonheaving under deep seasonal freezing cannot, per se, ensure crack resistance of the crossing structures.5. During freezing of soils in the bases of crossings, processes of heaving and shrink-age occur. For hard and medium hard consistency, with W_PL, the shrinkage may be greater than the heaving, which should be taken into account for determining the depth of the foundations under deep seasonal freezing conditions.Omskgrazhdanproekt. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 10–12, May–June, 1981.     

石英砂砾破碎过程中粒径分布的分形行为研究

荷载作用下粒状土的颗粒破碎改变土的粒径分布,从而影响其力学特性。试验证据显示随着颗粒破碎的增加,任何初始分布的土粒都将趋向一种自相似的分形分布。为了揭示土的粒径分布的分形转变机制,利用侧限压缩试验研究高压应力下石英砂砾的粒径分布演化规律和颗粒破碎特性,基于分形模型和粒径分布实测数据研究破碎过程中粒径分布的分形行为。研究发现:颗粒破碎增长导致粒状土趋向分形分布的过程与颗粒破碎量密切相关,并可以通过增大的分形维数来描述。尽管石英砂砾的初始分布和粒径有所不同,分形维数大于2.2的粒径分布实测数据均展示了较为严格的自相似性,因而该数值可作为分形分布的分形维数下限值。研究还发现:相同破碎状态下Hardin相对破碎率小于Einav相对破碎率,但二者对应力和体应变的响应规律是一致的。颗粒破碎发展至粒径分布成为分形分布时,体应变与相对破碎率的比值将保持恒定,并且受初始分布的均匀性和颗粒大小的影响很小。这一特点可用于分形分布的识别,并意味着试验中如果粒径分布是分形的,则无须为了粒径分析而终止试验,只需测量到体应变就可估计相对破碎率。     

等向应力下原状黄土的压缩及增湿变形特性研究

 用非饱和土三轴剪切渗透仪,在等向应力条件下对原状黄土进行增湿–加载,加载–增湿及加载–增湿–加载3个系列加载增湿路径试验,分析吸力对压缩变形和加载屈服特性的影响,探讨增湿时应力对变形及屈服特性的影响,通过对单线法及双线法的试验结果对比,确认原状黄土的增湿体积变形与加载增湿路径有关,加载及增湿屈服线不具有唯一性,进而提出等向应力条件下原状黄土的弹塑性体变模型。研究结果表明：吸力及应力分别对屈服前压缩及增湿变形特性指标几乎没有影响,而对屈服后的指标皆有明显的影响;单线法与双线法确定的增湿体积变形皆随应力的增大而先增大后减小,峰值点处应力与吸力丧失程度及加载增湿路径无关,且近似等于天然状态土样的初始屈服应力;增湿变形与加载增湿路径有关,单线法的值比双线法确定的值小,差值随增湿程度增大而减小,增湿至饱和时2种方法确定的湿陷变形近似相等。对于相同的塑性体应变,吸力减小屈服线位于加载屈服线之下方,二者随塑性体应变的增大而耦合联动扩大。提出的弹塑性体变模型可以较好地预测不同吸力下的压缩变形,比采用唯一加载湿陷屈服线的模型更好地预测不同应力下增湿变形。     

Mechanical behavior of lightweight soil reinforced with waste fishing net

Lightweight soil is cement-treated and consists of dredged clayey soil, cement, and air-foam. Reinforced lightweight soil (RLS) contains waste fishing net to increase its shear strength. This paper investigates the strength characteristics and stress–strain behavior of reinforced and unreinforced lightweight soils. Test specimens were prepared with varying admixtures of cement content (8%, 12%, 16%, and 20% by the weight of untreated soil), initial water content (125%, 156%, 187%, 217%, and 250%), air-foam content (1%, 2%, 3%, 4%, and 5%), and waste fishing net (0%, 0.25%, 0.5%, 0.75%, and 1%). Then several series of unconfined compression tests and one-dimensional compression tests were conducted. The experiments with lightweight soil indicated that the unconfined compressive strength increased with an increase in cement content, but decreased with increasing water content and air-foam content. The stress–strain relationship and the unconfined compressive strength were influenced by the percentage of waste fishing net. In addition, the strength of RLS generally increased after adding waste fishing net due to the bond strength and the friction at the interface between waste fishing net and soil mixtures, but the amount of increase in compressive strength was not directly proportional to the percentage of waste fishing net. The results of testing indicated that the maximum increase in compressive strength was obtained for a waste fishing net content of about 0.25%. The bulk unit weight of lightweight soil was strongly dependent on the air-foam content. The compression characteristics of lightweight soil, including the yield stress and compression index, did not depend greatly on whether the samples were cured underwater or in air.     

Study on strength of artificially frozen soils in deep alluvium

This paper describes a series of stress-controlled uniaxial compressive tests performed on frozen loess and triaxial compressive tests performed on frozen/unfrozen loess, which experienced K₀ consolidated process before freezing, to study the stress–strain–strength behaviour of an artificially frozen soil in deep alluvium. The aim of subjecting the triaxial test samples to K₀ consolidation was to simulate the forming process of deep soils. These tests examined the influence of the initial confining pressure and the temperature of frozen soils on stress–strain–strength behaviour. An analysis of the mechanical behaviour of artificially frozen soil is performed from interpretation of results from the unconfined and triaxial compressive tests of frozen/unfrozen soils, in which the influence of both the degree of cementation arising from the interparticle bonding and the initial confining stress was investigated. For deep artificially frozen soils, it was concluded that the unconfined compressive strength is a direct measurement of the degree of cementation. Consequently, the triaxial compressive strength can be expressed as a function of only two variables: (1) the internal angle of the shearing resistance of the unfrozen soils; and (2) the unconfined compressive strength. Data from additional experiments performed later verified the validity of proposed relationship in evaluating the strength of deep artificially frozen soil.     

冻融循环作用下混凝土受压本构特征研究

采用混凝土棱柱体试件,通过快速冻融试验方法,对经受冻融损伤的混凝土受压性能进行了试验研究,分析了冻融循环次数、混凝土等级、相对动弹性模量对混凝土受压性能的影响,通过碟簧耗能装置,得到了完整的应力-应变关系试验结果.将冻融次数和混凝土等级作为参数,回归试验结果,提出了冻融循环后受压性能的计算公式以及适用于立方体抗压强度为20～50 MPa的冻融循环作用下混凝土的应力-应变全曲线方程,并将曲线控制参数与相对动弹性模量和混凝土等级建立关系.结果表明:碟簧耗能装置起到了吸收机械能量的作用,是应力-应变关系试验成功的重要保证.随着冻融循环次数的增加,受压应力-应变曲线趋于扁平;受压峰值应力降低,受压峰值应变、受压极限应变增大,三者随冻融循环的变化关系均近似线性;随着混凝土等级的提高,冻融对于混凝土材料的影响下降.此外,结合所建立的破坏准则所得到的数值分析结果与试验曲线有较好的一致性.     

Frictional effects on the volumetric strain of sandstone

The contributions of frictional slip on the nonlinear, hysteretic deformation of sandstone in the reversible regime (i.e., prior to the onset of permanent deformation) for uniaxial strain compression are investigated through an analysis of a Hertz–Mindlin face-centered cubic sphere pack model and laboratory stress–strain tests on Berea sandstone. The analysis demonstrates that the dynamic¹ moduli are path-independent functions of the strain. The analysis also reveals that for uniaxial strain consolidation it is possible to decompose the volumetric strain into a path-independent contribution from nonlinear grain contact deformation and a path-dependent contribution from frictional (slip) compaction. Laboratory stress–strain measurements on Berea sandstone support these findings and, in addition, reveal that frictional compaction accounts for a significant portion of the volumetric strain of Berea sandstone.     

Quantifying topography and closure deformation of rock joints

This paper presents a study for quantifying both the joint topography characteristics and the load–closure deformation of a rock joint under normal compressive loading condition. The study covers (1) laboratory measurements of rock joint surface profiles using a profilometer designed and fabricated by the research team, (2) development of a mathematical method to identify the waviness and unevenness components in joint surface profiles and the associated composite topography, (3) development of a general load–closure deformation model by using both the waviness and unevenness components in the composite topography, (4) unconfined compressive testing of rock samples with joints for the experimental load–closure deformation of joints and, (5) verification of the general load–closure deformation model by the experimental load–closure deformation results. The study leads to the following four findings: (a) the mathematical method can be used to identify the waviness and unevenness components for joint surface profiles and its composite topography. The height characteristic parameters of the complete surface topography of a joint are mainly determined by the waviness component. The texture characteristic parameters of the complete surface topography of a joint are mainly determined by the unevenness component, (b) joints can be classified into the three contact state cases using the waviness and unevenness components for both the joint surface profiles and the associated composite topography. The load–closure deformation behavior of a joint is determined by the waviness and unevenness components of the composite topography for a specific contact state, (c) the general load–closure deformation model developed in this paper is applicable to the three contact state cases. The general load–closure deformation model uses the composite topography of a joint and can take into account the effects of the contact states, the initial aperture and the waviness and unevenness components; (d) parametric studies and verifications with the uniaxial compression test results show that the general load–closure deformation model gives reasonable estimations of the load–closure deformation behavior of rock joint under compressive loading and can be reduced to those given by other researchers in the relevant literature.     

Results of field tests of piles in permanently frozen ground

Conclusions 1. To estimate the bearing capacity of piles and to determine the deformation characteristics of soils it is necessary to conduct comprehensive pile tests on plots with the same frozen-ground conditions.2. Pile tests should be conducted before the appearance of the second region of continuous deformations. For structures allowing considerable deformations this will permit increasing the bearing capacity, taking it to be equal to the value at which the jog of the soil consolidation under the pile point ends.3. Calculation of pile foundations with respect to the second limiting state permits a substantial increase of the design bearing capacity of piles, which ultimately leads to a reduction of construction costs.4. The use of the accelerated test method with the obtainment of the experimental values of the conversion factor to long-time strength permits a considerable reduction of the test period.5. The data obtained in pile tests permits developing more economical designs.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 35–36, September–October, 1977.     

Simulation of a fully coupled thermo–hydro–mechanical system in freezing and thawing rock

This paper describes the application of a numerical model of thermal–mechanical–fluid flow coupling system to the simulation of laboratory freezing and thawing experiments on rocks. A theoretical formulation that accommodates a linear stress–strain constitutive relationship is presented and a two-dimensional (plane stress) numerical modeling is performed based on the finite element method applied to thermo–poro–elasticity. As the primary objective of this research is to simulate the freezing and thawing process, the developed code takes into account the phase change of pore water during freezing. It is found from the numerical simulation that a relatively good prediction can be made of temperature transfer and deformation behavior within the elastic deformation limit. In some cases, however, large deformation results from the freezing processes, leading to material failure. Future research should therefore take into account irreversible and plastic effects.     

Investigation of the bearing capacity of piles driven in plastic frozen soil

Conclusions The bearing capacity of a pile lowered into a borehole whose cross section exceeds the cross section of the pile for the possibility of filling in with slurry (frozen-in, friction, bored-sunk piles) is close to the calculated, but has a greater settlement under a standard load in comparison with a bored-drive pile.The bearing capacity of piles driven by the VMS-1 vibratory hammer into plastic frozen soil (bored-drive piles) is on the average 1.6 times greater than the calculated. Such an excess of the bearing capacity of the piles can be explained by the formation of a new structure and frost texture of the soil within 30–50 mm around the pile skin. A characteristic feature of the newly formed zone of soil is a considerably greater homogeneity and density of the soil than under natural conditions and absence of ice lenses and interlayers directly contacting the pile.The settlements of the pile tested under a standard load were less than the allowable deformations for the bases of the majority of buildings and structures.The cross-sectional area of the pilot hole should be equal to 0.65–0.75 of the pile section in the case of driving it at the time of the maximum depth of thawing of the soil and 0.95 at the time of seasonal freezing of the soil.Restoration of the contact bonds between the soil and pile during its freezing-in after driving into soil with a temperature of -0.5°C and higher can continue for several months.Deceased.Central Scientific-Research Institute of Transport Construction (TsNIIS). Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 19–20, January–February, 1979.     

Settlements and deformations of industrial structure built on weak soils

Conclusions 1. In engineering-geologic explorations, it is necessary to investigate the engineering-geologic morphology to a depth of 15–18 m: lenses and interlayers of weak soils - unevenly compacted silty-sand fills, peat, slime, etc.2. Open unwatering of pits in the vicinity (10–15 m) of existing structures is impermissible, since it leads to removal of fine soil fractions because of piping, and to large and impermissible deformations of the load-bearing elements of the structures. In such cases, deep groundwater lowering by means of wellpoints should be used.In the case described in this article, the industrial structure underwent a mean settlement of about 20 cm, a tilt of 0.015, and a wall deflection of 0.002. By using deep groundwater lowering, such deformations could have been prevented.3. The development of settlements with time was due to the presence of weak soils - peat lenses and thin slime interlayers - in the foundation bed of the shop.4. An effective means for stopping the settlement of the industrial structure was the construction, under the walls, of two rows of cast-in-place piles with cantilever beams to which the wall loads were transmitted.Scientific-Research Institute of Foundations. GIPROLESKhIM. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 5–7, January–February, 1975.     

Experimental thawing of permafrost soils in bases of reconstructed main building of the Chita-I Thermoelectric Power Plant

Conclusions 1. Compaction of thawed soils takes place directly during the thawing process or in the first 10–35 days after it. The thawed soils do not require subsequent compaction and, as regards the load-carrying properties, they are comparable to similar rocks in a naturally thawed state.2. The specific electric energy consumption for thawing 1 m³ of permafrost soils in bases is 25.2 kWh, and it is a fundamental characteristic in the design of industrial thawing of soil bases for determining the spacing between the heaters their capacity, and the thawing period.3. In test thawing, the number of ground mark vertical lines and the number of marks in each vertical line should be determined from the degree of heterogeneity of the frozen soils as regards the collapsibility in plan and section. For permafrost soil conditions similar to those of the construction site of the Chita-I plant, the optimal solution is to lay one vertical line per 100–120 m² of thawing area, and to use in each vertical line not less than one mark per 5 m of section of thawed permafrost soils."Atomteploelektroproekt" Institute, Novosibirsk. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 9–10, September–October, 1986.     

Relations of calculated and actual settlement of a thawing foundation bed

Conclusions 1. The dependence of relative compression of thawing soil on pressure is nonlinear, and it is in agreement with views on the nature of deformation of thawing soil. A dominance of natural pressure in the total load on a foundation bed permits one to use the method of unit summation in calculating settlement of nonlinearly deformable soil. Such calculations, as compared with the linear formulas employed in SNiP, give better agreement with actual settlement.2. In calculating settlement of thawing foundation beds, we need differential consideration of soil compressibility with depth, since the combination of genetic features of frozen ground and the increasing natural pressure with depth may create a different inhomogeneity in the foundation bed.Yakutniproalmaz, Mirnyi. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 18–21, January–February, 1975.     

Effect of dynamic action on compressibility of thawing sands

Conclusions 1. The compressibility of thawing sands under dynamic impact action exceeds the compressibility of thawed sands having the same initial density.2. As the ice content decreases and the density and grain size increase, the compressibility of thawing soils under impact load action decreases.3. The relation between the compressibility of a thawing sand and the vibration acceleration under impact action is nonlinear and is best approximated by a logarithmic function.4. The method developed here for analysis of the settlements of thawing bases, taking into account the dynamic action is approximate and it can be recommended only for bases consisting of sands of massive cryogenic texture without appreciable ice lenses or inclusions.Leningrad Economy-Engineering Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 14–17, November–December, 1986.     

The effect of water content on the mechanical response of a high-porosity chalk

The paper deals with the effect of water content on the stress–strain response and compressive strength of a high-porosity chalk. Specimens were tested in three different testing configurations: uniaxial compression of solid cylinders, radial compression of hollow cylinders, and hydrostatic compression of hollow cylinders. Special tools for the local measurement of axial and lateral deformations were developed and proven effective in the testing of the chalk even at water contents as high as 45%.The data illustrate the transition from linear to nonlinear stress–strain response when water content is below about 8%. The small strain modulus of deformation within the plane of material symmetry was found to be 30% larger than its counterpart perpendicular to the plane of material isotropy, regardless of water content. These aspects of chalk response are not yet properly integrated into constitutive models aimed at describing the deformation of chalk units in the field.A Lade-type failure criterion was developed for the compressive strength of the chalk within the plane of material isotropy. It was found that results of radial compression of hollow cylinders correlate well with uniaxial compression data in the construction of the failure criterion.     

Vertical dynamic interactions of poroelastic soils and embedded piles considering the effects of pile-soil radial deformations

The paper presents an analytical solution for the vertical dynamic interaction analysis of a poroelastic soil layer and an embedded pile with the consideration of pile-soil radial deformations. The soil is treated a three-dimensional porous continuum and described by the Boer’s poroelastic model, while the pile is treated as a two-dimensional rod with both radial and vertical deformations of which the equation of motion is derived by the Hamilton’s variational principle. Without the introduction of potential functions, first take the volumetric strain of soil skeleton and pore fluid pressure as intermediate variables to deal with the equations of motion for the soil and then use the separation of variables to solve the equations of motion for the soil and the pile. By imposing the boundary and continuity conditions of the pile-soil system, the dynamic impedance in frequency domain and the velocity response in time domain of the pile top are obtained. The present solution is then verified by comparing with the corresponding finite element model computation results and the existing solutions. The effects of the pile-soil parameters on the dynamic characteristic of the pile-soil system are also analyzed. Some significant conclusions are drawn, which can provide useful reference for related engineering practice.     

Compressive characteristics of methane hydrate-bearing sands under isotropic consolidation

Depressurization is an effective method to produce methane gas from methane hydrate reservoirs. However, during gas production, sediments consolidate due to increasing effective stress. Revealing the compressive characteristics of methane hydrate-bearing sands during consolidation is essential for an accurate understanding of sediment properties and for the development of a constitutive model. Therefore, a series of isotropic consolidation tests was performed on sand in which methane hydrate was artificially generated, and its compressibility characteristics were evaluated. Furthermore, to assess prolonged production, creep compressive behavior was investigated. The experimental results showed volumetric strain due to increasing confining stress decreased with increasing initial methane hydrate saturation. Particle crushing during consolidation was inhibited by the presence of methane hydrate. It was confirmed that the increase in the effects of methane hydrate on soil compressibility followed a power function with the increase in methane hydrate saturation. Creep deformation was observed during the stress holding period regardless of the presence of methane hydrate. Creep behavior during the stress holding period was related to the extent to which the creep component had dissipated before the stress holding period in the past. A theoretical concept for creep strain was proposed based on the experimental results.