Consolidation behavior of soil–cement column improved ground

Columnar inclusion is one of the effective and widely used methods for improving the engineering properties of soft clay ground. This article investigates the consolidation behavior of composite soft clay ground using both physical model tests under an axial-symmetry condition and finite element simulations using the PLAXIS 2D program. It was determined that the final settlement and the rate of consolidation of the composite ground depended on the stress state. For an applied stress that is much lower than the failure stress, the final settlement of the composite ground was lower, and the consolidation was rapid. When the soil–cement column failed, the stress on the column suddenly decreased (due to strain-softening); meanwhile, the stress on the soil increased to maintain the force equilibrium. Consequently, the excess pore pressure in the surrounding clay increased immediately. The cracked soil–cement column acted as a drain, which accelerated the dissipation of the excess pore pressure. The consolidation of the composite ground was mainly observed in the vertical direction and was controlled by the area ratio, which is the ratio of the diameter of the soil–cement column to the diameter of the composite ground, a. The stress on the column was shown to be low for a composite ground with a high value of a, which resulted in less settlement and fast consolidation. For a long soil–cement column, the excess pore pressures in the surrounding clay and the column were essentially the same at a given consolidation time throughout the improvement depth. It is proposed that the soil–cement column and surrounding clay form a compressible ground, and the consolidation occurs in the vertical direction. The composite coefficient of consolidation (c_v(com₎) that was obtained from the physical model test on the composite ground can be used to approximate the rate of consolidation. This approximation was validated via a finite element simulation. The proposed method is highly useful to geotechnical engineers because of its simplicity and reliable prediction.     

Strength characteristics and mechanisms of salt-rich soil–cement

Salt-rich soft soils have not only general characteristics of common soft soils, but also contain high contents of Mg²⁺, Cl^?, and SO₄^2?, which have negative effects on deep mixing method using cement to treat soft soils. Laboratory and field tests were conducted to investigate the effects of changing cement incorporating ratio, water content, cement mixing ratio, and contents of Mg²⁺, Cl^?, and SO₄^2? on the unconfined compressive strength of the salt-rich soil–cement. The microstructure of soil–cement and the mechanism for the strength change of salt-rich soil–cement were investigated using X-ray diffraction, scanning electronic microscopy (SEM), and backscattered diffraction technology. It was found that an increase of cement incorporating ratio enhanced the strength of soil–cement but reduced its strength when water is added. Different amounts of Mg²⁺, Cl^?, and SO₄^2? not only caused the difference in the microstructures of salt-rich soil–cement but also influenced the soil–cement strength.     

Strength and stiffness of compacted chalk putty–cement blends

Chalk breaks easily when subjected to human action such as mechanical handling, earthworks operations or pile installation. These actions break the cemented structure of chalk, which turns into a degraded material known as putty, with lower strength and stiffness than the intact chalk. The addition of Portland cement can improve the behaviour of chalk putties. Yet, there are no studies determining the tensile strength of chalk putty–cement blends, the initial stiffness evolution during the curing time and other design parameters such as friction angle and cohesion of this material. This paper addresses this knowledge gap and provides an interpretation of new experimental results based on the dimensionless index expressed as the ratio between porosity and volumetric content of cement (η/C_iv) or its exponential modification (η/C_iv^a). This index aids the selection of the amount of cement and density for key design parameters of compacted chalk putty–cement blends required in geotechnical engineering projects such as road foundations and pavements, embankments, and also bored concrete pile foundations.

     

Experimental study on the electrical resistivity of soil–cement admixtures

Recently in China, soil–cement is widely used to improve the soft ground in the highway construction engineering. Literature studies are mainly investigating the mechanical properties of the soil–cement, while its properties of the electrical resistivity are not well addressed. In this paper, the properties of the electrical resistivity of the reconstituted soil-cement and the in situ soil–cement columns are investigated. The test results show that the electrical resistivity of the soil–cement increases with the increase in the cement-mixing ratio and curing time, whereas it decreases with the increase in the water content, degree of saturation and water–cement ratio. A simple equation is proposed to predict the electrical resistivity of soil–cement under the condition of the specified curing time and water–cement ratio. It is found that the electrical resistivity has a good relationship with the unconfined compression strength and blow count of SPT. It is expected that the electrical resistivity method can be widely used for checking/controlling the quality of soil–cement in practice.     

Study on frost resistance of rapid-hardening magnesium phosphate-ferro-aluminate composite cement北大核心CSCD

随着冬季抢修抢建混凝土工程的增多,研发适用于严寒地区具有良好抗冻性的快硬水泥意义重大。磷酸镁水泥具有早期强度高、初凝时间短、与混凝土相容性好等特点,被视为混凝土工程的良好快修快建材料。然而,磷酸镁水泥制备的水泥砂浆或混凝土在北方地区的冬季往往会受到冻融循环作用,导致其耐久性和强度出现不同程度的退化。为增强磷酸镁水泥的抗冻性,在磷酸镁水泥制备中用铁铝酸盐水泥代替一定数量的过烧氧化镁,制备出快硬磷酸镁-铁铝酸盐复合水泥。通过冻融循环前后磷酸镁-铁铝酸盐复合水泥砂浆试件的质量损失测试、强度试验、孔隙率测试及SEM-EDS测试,得出：铁铝酸盐水泥代替氧化镁的数量在30%~40%时,制备的磷酸镁-铁铝酸盐复合水泥砂浆试件冻融循环后的质量损失率最小,抗压强度和抗折强度达到峰值,抗压强度和抗折强度剩余率最高,孔隙率最小,因而该配合比的磷酸镁-铁铝酸盐复合水泥具有最好的抗冻性。由SEM-EDS测试可知磷酸镁水泥砂浆试件冻融循环后,基体中的胶凝材料K-鸟粪石部分溶解,试件整体结构疏松,晶体间存在大量间隙;磷酸镁水泥制备中掺入铁铝酸盐水泥后,制得的磷酸镁-铁铝酸盐复合水泥水化生成大量晶体填充于砂浆试件基体内部,无定形水化产物对砂浆试件的强度有一定补偿作用,使得磷酸镁-铁铝酸盐复合水泥砂浆试件在冻融循环后孔隙率大幅减小,密实度得到提高,使得磷酸镁-铁铝酸盐复合水泥的抗冻性能得到了显著增强。磷酸镁-铁铝酸盐复合水泥为北方严寒地区冬季混凝土抢修抢建工程提供了一种新材料。     

Calcium-aluminum-silicate-hydrate “cement” phases and rare Ca-zeolite association at Colle Fabbri, Central Italy

Very high temperature, Ca-rich alkaline magma intruded an argillite formation at Colle Fabbri, Central Italy, producing cordierite-tridymite metamorphism in the country rocks. An intense Ba-rich sulphate-carbonate-alkaline hydrothermal plume produced a zone of mineralization several meters thick around the igneous body. Reaction of hydrothermal fluids with country rocks formed calcium-silicate-hydrate (CSH), i.e., tobermorite-afwillite-jennite; calcium-aluminum-silicate-hydrate (CASH) — “cement” phases - i.e., thaumasite, strätlingite and an ettringite-like phase and several different species of zeolites: chabazite-Ca, willhendersonite, gismon-dine, three phases bearing Ca with the same or perhaps lower symmetry of phillipsite-Ca, levyne-Ca and the Ca-rich analogue of merlinoite. In addition, apophyllite-(KF) and/or apophyllite-(KOH), Ca?Ba-carbonates, portlandite and sulphates were present. A new polymorph from the pyrrhotite group, containing three layers of sphalerite-type structure in the unit cell, is reported for the first time. Such a complex association is unique. Most of these minerals are specifically related to hydration processes of: (1) pyrometamorphic metacarbonate/metapelitic rocks (natural analogues of cement clinkers); (2) mineralization between intrusive stocks and slates; and (3) high-calcium, alkaline igneous rocks such as melilitites and foidites as well as carbonatites. The Colle Fabbri outcrop offers an opportunity to study in situ complex crystalline overgrowth and specific crystal chemistry in mineral phases formed in igneous to hydrothermal conditions.     

Assessment of cement’s dust impact on the soil using principal component analysis and GIS

To evaluate the impact of the cement’s dust emitted by the eastern Moroccan cement factory (Oujda Holcim) and deposited on the soil of the Ain Lahjar commune, 58 samples of soil were collected around the factory on a radius of 3 km approximately. The physicochemical analyses of the upper 3 cm of the soil surface samples correspond to the pH, electrical conductivity, the chlorides and the oxides of Ca, Al, Fe, K, Mn, S and Si which were analyzed by the fluorescence of X-rays. The preliminary results demonstrate that this dust are especially basic and contain a high free lime (43% CaO). The principal component analysis (applied on the 58 samples of soil) allows deducting that the free lime and the sulfur oxide are the tracer elements of this form of pollution. Furthermore, the spatial projection of the factor scores of the principal component analysis using the geographical information system permits to determine the spatial distribution of more polluted areas of soils as well as to estimate their surface (63.3% of the survey zone).     

Stress–dilatancy behavior of cemented sand: comparison between bonding provided by cement and biocement

Acta Geotechnica - Biocement as an alternative to cement for soil improvement has been studied for the past decade. A comparative study on the cementation effect on the mechanical behavior of sand...     

Shear strength characteristics of Irbid clayey soil mixed with iron filling and iron filling–cement mixture

Iron filling and iron filling–cement mixture were used to improve the shear strength characteristics of Irbid clayey soil. For this purpose, five types of Irbid clay soils were obtained and mixed with iron filling and iron filling–cement mixture at different percentages. Two sets of prepared samples were mixed with the admixture. The first set was prepared by mixing the soil samples with iron filling alone at 2.5, 5.0, 7.5, and 10% by dry weight of the soil. The second set was prepared by mixing with iron filling–cement mixture at equal ratio of the same percentages of the first set. An unconfined compression test was performed in this study to measure the shear strength properties of the soils. The test results showed that the increase in the percentages of the iron filling and iron filling–cement mixture up to 10% will result in increasing the maximum dry density of the soil and increase the unconfined compressive strength and the secant of modulus of elasticity of the clayey soil. Also, the addition of iron filling–cement mixture increased the unconfined compressive strength and secant modulus of elasticity of the clayey soil higher than the addition of iron filling alone.     

Study on soil–cement grout interface shear strength of soil nailing by direct shear box testing method

An important design parameter in cement-grouted soil nailed structures is the shear strength at the interface between the grouted nail and the surrounding soil. Both field and laboratory pull-out tests are normally used to investigate this interface shear strength. However, these tests have some limitations. In this study, direct shear box tests are adopted to investigate the interface shear strength behaviour between a completely decomposed granite (CDG) soil and a cement grout plate. Tests were carried out in a large direct shear test apparatus over a range of constant normal stress, soil moisture content, and soil–cement grout interface surface waviness. The laboratory test procedures are briefly described and the main test results are presented, followed by a discussion of the shear behaviour of the soil–cement grout interface. The interface shear behaviour is compared with the shear strength behaviour of the same soil tested under comparable conditions. It is shown that the shear stress–displacement behaviour of the soil–cement grout interface is similar to that of the soil alone. The test results indicate that the interface shear strength of the CDG and cement grout material depends on the normal stress level, the soil moisture content, and the interface surface waviness.     

Experimental investigations on ultimate bearing capacity of peat stabilized by a group of soil–cement column: a comparative study

The aim of this paper was to determine the ultimate vertical bearing capacity of rectangular rigid footings resting on homogeneous peat stabilized by a group of cement deep mixing (CDM) columns. For this purpose, a series of physical modeling tests involving end-bearing and floating CDM columns were performed. Three length/depth ratios of 0.25, 0.5, and 0.75 and three area improvement ratios of 13.1, 19.6, and 26.2 % were considered. Bearing capacity of the footings was studied using different analytical procedures. The results indicated that compared to unimproved peat, the average ultimate bearing capacity (UBC) improvement of floating and end-bearing CDM columns were 60 and 223 %, respectively. The current study found that simple Brom’s method predicted the UBC of the peat stabilized with floating CDM columns with reasonable accuracy, but underestimated the UBC by up to 25 % in the case of end-bearing CDM columns. Published laboratory experiences of stabilizing soft soils using soil–cement columns were also collated in this paper.     

Vertical bearing capacity behaviour of single T-shaped soil–cement column in soft ground: laboratory modelling,field test,and calculation

The T-shaped soil–cement column is a variable-diameter column, which has an enlarged column cap at the shallow depth, resulting in the column shape being analogous to the letter “T”. In this study, 1-g laboratory and full-scale field loading tests were employed to investigate the vertical bearing capacity behaviour of a single T-shaped column in soft ground. Pressure cells were set in a T-shaped column in the field to measure the vertical column stress above and below the column cap during the loading test. After the loading test, several columns were excavated to investigate their failure modes. The results indicated that, since the section area of the column cap was remarkably higher than that of the deep-depth column, the stress concentration occurred in the deep-depth column just under the cap, leading to column failure. Based on this failure mode, a simplified method was proposed to estimate the ultimate bearing capacity of a single T-shaped column; the comparison of estimated and measured results indicated the applicability of the proposed method.     

Armouring of well cement in H2S–CO2 saturated brine by calcite coating – Experiments and numerical modelling

The active acid gas (H₂S–CO₂ mixture) injection operations in North America provide practical experience for the operators in charge of industrial scale CO₂ geological storage sites. Potential leakage via wells and their environmental impacts make well construction durability an issue for efficiency/safety of gas geological storage. In such operations, the well cement is in contact with reservoir brines and the injected gas, meaning that gas–water–solid chemical reactions may change the physical properties of the cement and its ability to confine the gas downhole. The cement-forming Calcium silicate hydrates carbonation (by CO₂) and ferrite sulfidation (by H₂S) reactions are expected. The main objective of this study is to determine their consequences on cement mineralogy and transfer ability. Fifteen and 60 days duration batch experiments were performed in which well cement bars were immersed in brine itself caped by a H₂S–CO₂ phase at 500 bar–120 °C. Scanning electron microscopy including observations/analyses and elemental mapping, mineralogical mapping by micro-Raman spectroscopy, X-ray diffraction and water porosimetry were used to characterize the aged cement. Speciation by micro-Raman spectroscopy of brine trapped within synthetic fluid inclusions were also performed. The expected calcium silicate hydrates carbonation and ferrite sulfidation reactions were evidenced. Furthermore, armouring of the cement through the fast creation of a non-porous calcite coating, global porosity decrease of the cement (clogging) and mineral assemblage conservation were demonstrated. The low W/R ratio of the experimental system (allowing the cement to buffer the interstitial and external solution pH at basic values) and mixed species diffusion and chemical reactions are proposed to explain these features. This interpretation is confirmed by reactive transport modelling performed with the HYTEC code. The observed cement armouring, clogging and mineral assemblage conservation suggest that the tested cement has improved transfer properties in the experimental conditions. This work suggests that in both acid gas and CO₂ geological storage, clogging of cement or at least mineral assemblage conservation and slowing of carbonation progress could occur in near-well zones where slight water flow occurs e.g. in the vicinity of caprock shales.     

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones,Central North Sea: an example of cement growth during bioturbation?

Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non‐ferroan, generally unzoned, subhedral to highly anhedral ‘jigsaw piece’ crystals. These are of a similar size to the detrital silicate grains and typically account for ≈5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near‐marine δ¹⁸O values and low, positive δ¹³C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near‐surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent δ¹⁸O. Labile bioclastic debris (e.g. aragonite, Mg‐calcite) supplied isotopically positive carbon to the pore fluids during shallow‐burial dissolution. Such dissolution took place in response to the ambient ‘calcite sea’ conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg²⁺ and Ca²⁺ with the sea‐water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep‐burial diagenesis.     

Chemical study of limestone and clay for cement manufacturing in Darukhula,Nizampur District,Nowshera, North West Frontier Province （N.W.F.P.）, Pakistan

Limestone and clay samples were collected from Darukhula and adjoining areas of the Nizampur District Nowshera, N.W.EE, Pakistan, and analyzed for different parameters in order to search for new reserves of suitable material for the manufacture of different types of cements in N.W.F.P. It was found that the area under study contains three types of limestones, including high grade limestone, Darukhula limestone and siliceous limestone, which contain 53%, 49.03% and 45.19% CaO, respectively, and three types of clay, including maroon color, yellow to yel- lowish-green color and green color clay containing 57.76%, 65.47% and 61.24% SiO2, respectively. Chemical analysis of the limestone and clay samples collected from the deposits in the area under study showed that all the elements found in these samples are within the range of permissible limits for the production of high-strength Portland cement, sulphate resisting cement and white cement. This paper covers the detailed version of the potential raw material deposits at Darukhula and adjoining areas of the Nizampur District.     

The origin and alteration of calcite cement in tight sandstones of the Jurassic Shishugou Group,Fukang Sag,Junggar Basin,NW China: implications for fluid–rock interactions and porosity evolution

The Shishugou Group, which consists of Middle Jurassic Toutunhe Formation and Upper Jurassic Qigu Formation, is currently an important hydrocarbon exploration target in the Fukang Sag of Junggar Basin, China. The Shishugou Group sandstones experienced a complex diagenetic history with deep burial (3600–5800 m) to develop low–ultralow porosity and permeability reservoir with some high-quality reservoirs found in the tight sandstones owing to the reservoir heterogeneity. This integrated petrographic and geochemical study aims to unravel the origin and alteration of calcite cement in the Shishugou Group sandstones and predict fluid–rock interaction and porosity evolution. The Shishugou Group sandstones (Q_43.8F_7.4R_48.8) have a dominant calcite cement with strong heterogeneity forming in two generations: poikilotopic, pore-filling masses that formed at an early diagenetic stage and isolated rhombs or partial grain replacements that formed at a late stage. The Shishugou Group, which are lacustrine sediments formed in low–medium salinity lake water in a semiarid–arid climatic environment, provided the alkaline diagenetic environment needed for precipitation of chlorite and early calcite cements in early diagenesis. The Ca²⁺ of the pore-filling calcite cements was sourced from weathering or dissolution of volcanic clasts in the sediment source or during transport in under oxidising conditions. The δ¹⁸O_V-PDB and δ¹³C_V-PDB values of calcite were significantly controlled by distance from the top unconformity and underlying coal-bearing stratum with carbon sourced from atmospheric CO₂, and organic matter. The early carbonate cement inhibited burial compaction producing intergranular pore spaces with enhanced reservoir properties by late dissolution under acidic conditions. Anhydrite cement reflects reaction of organic acid and hydrocarbon with the sandstones and is associated with fluid migration pathways. The fluid–rock interactions and porosity evolution of the tight deep sandstones produced secondary pores that filled with hydrocarbon charge that forms this deep high-quality reservoir.     

Raw mix designing, clinkerization and manufacturing of high-strength Portland cement from the limestone and clay of Darukhula Nizampur, Nowshera District, North-West Frontier Province （N.W.F.P.）, Pakistan

This paper covers the detailed version of the potential raw material deposits at Darukhula and the adjacent areas of Nizampur, the manufacturing of high-strength Portland cement samples from the same material and comparison of the physical and chemical parameters for resulting cement with British and Pakistan standard specifications, which include compressive strength, setting time, consistency, lechatelier expansion, Blaine and insoluble residue. It was found that the raw material available in the study area meets the standard specifications and the area is feasible for the cement plant installation. The area can provide raw material which is quite sufficient for the running of a cement plant.