GT型土壤固化剂改良土的工程特性研究

GT型土壤固化剂是一种新型的土壤改性加固材料,以高钙灰和脱硫石膏两种工业废料为主要原料,辅以生石灰、水泥、熟石膏、硫酸铝及明矾石等次要成分,采用生石灰消解法除去脱硫石膏中的自由水,按全粉料配料的方法研制而成。按一定掺量向土中掺入石灰和GT型土壤固化剂制成石灰改良土样和固化剂改良土样并进行养护、浸水,对土样进行击实试验、直剪试验、压缩试验和渗透试验。试验结果表明固化剂改良土的击实效果、抗剪强度、压缩性、抗渗透性等工程特性明显优于石灰改良土。分析了GT型土壤固化剂加固土的机理,为进一步的研究和工程应用提供参考。     

强夯动力性能的显式非线性数值分析

充分考虑碰撞中出现的材料非线性、几何非线性、接触非线性、运动非线性以及它们之间相互耦合的特性,利用显式瞬态非线性有限元分析技术分析了强夯对地基土的冲击碰撞过程,得到了夯锤与土体之间的撞击力时间历程、土体位移、应力的时间历程,并得到了落距、夯锤重量、夯锤底面积与强夯加固深度之间的相互关系,分析结果表明：土体位移滞后于土体应力,再现了夯锤与土体碰撞的整个过程,反映了强夯过程中的一般现象和基本规律。     

A new approach for stabilization of lateritic soil with Portland cement and sand: strength and durability

The presence of lateritic soils occurs in tropical and subtropical regions. The improvement of lateritic soils that are not suitable for a particular purpose through techniques that combine modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction is seen as an alternative. In this context, a dosage method to use a local lateritic soil as construction material in a most rational way reducing the economic and environmental impacts related to this activity is still missing. Therefore, the current research aims to evaluate the performance of a lateritic soil via modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction. For this, unconfined compression, and durability (wetting and drying) tests were carried out on specimens of compacted clayey gravel lateritic soil, whose granulometry was modified by the insertion of distinct amounts (from zero to 45%) of weathered sand, treated with distinct Portland cement contents (from 4 to 10%), molded at different dry unit weights (from 16.8 to 20.1 kN/m³) and cured for 7 and 28 days. Results of the mechanical tests have shown the significant influence exerted by cement content and dry unit weight of the blend, followed by curing time and finally sand insertion. Satisfactory correlations between the response variables (q_u and ALM) and the adjusted porosity/cement index (η/C_v) were obtained. Furthermore, an innovative approach to replacing the laborious durability test is proposed.

     

Cement-Stabilization of Sabkha Soils from Al-Auzayba,Sultanate of Oman

Sabkha soils are salt-bearing formations that are formed in arid regions. In their in situ states the sabkha soils have high compressibility and low shear strength. These soils are also heterogeneous and their properties depend on the type and amount of salt present. Thus, these soils are not suitable for support of infrastructures without the risk of high settlement and/or bearing capacity failure. This paper investigates the possibility of using cement to improve the shear strength of sabkha soils for possible use as a foundation-bearing soil. The sabkha soil used in this study is a sandy sabkha obtained from the coastal plains at Al-Auzayba, Sultanate of Oman. Cement was added in percentages of 2.5, 5, 7.5 and 10%, by dry weight of soil. The soil-stabilizer mixers were allowed to cure for 7, 14 and 28 days. Laboratory tests such as compaction, unconfined compression, consolidated undrained triaxial and durability tests were performed to measure the engineering characteristics of the stabilized material. The results showed substantial improvements in the shear strength of the sabkha–cement mixtures and the mixtures are also durable with small weight loss after 12 wetting/drying cycles. Thus, cement can be used to improve the shear strength of sabkha soils. Furthermore, the effective stress path and the tress-strain relation of the sabkha–cement mixtures follow trends similar to those of cemented calcareous soils.     

强夯加固回填土地基的三维数值模拟

针对回填土地基,基于ABAQUS有限元软件,采用大变形几何非线性三维有限元方法对强夯加固效果进行了数值模拟。分析中考虑每一次夯击引起的塑性变形,确定前次夯击后土体塑性区开展的形状,并调整塑性区的土性参数,以此作为下次夯击分析的基础,重复上述步骤来实现对强夯整个过程的模拟。结合工程实例分析,得到了在不同夯击能、不同夯击次数作用下土体塑性区开展的情况,有效地预测了强夯加固区在空间上的分布规律。研究结论对类似的强夯工程有一定的参考价值。     

固化淤泥压缩特性的试验研究

淤泥固化技术在国内已经进入到工程应用阶段,明确固化淤泥的压缩特性对于指导淤泥固化工程的设计具有重要意义。通过单因素试验方案对不同初始含水率、不同水泥添加量、不同养护龄期的固化淤泥的压缩特性进行了研究。结果发现,固化淤泥压缩特性的最大特点是存在固结屈服应力,当荷载小于固结屈服应力时固化淤泥的压缩性非常小,而固化淤泥屈服之后的压缩性是屈服前压缩性的20倍以上,并且远大于未处理淤泥的压缩特性。淤泥本身是高压缩性的土,固化处理以后变为中等压缩性和低压缩性的土体。固化淤泥的固结屈服应力随水泥量增加线性增大,龄期越长、含水率越低固结屈服应力越大。固化淤泥的这种在固结屈服应力处发生突变的压缩特性和天然沉积结构性土类似,可以用双对数压缩模式来表示固化淤泥的这种压缩特性。     

Effect of Pozzolanic Reactivity on Compressibility Characteristics of Stabilised Low Lime Fly Ashes

This paper presents the effects of addition of lime and lime along with gypsum on the compressibility behavior of two class F fly ashes. Since the fly ashes develop strength and exhibit lower compression, consolidation testing with conventional duration of load increment may not be appropriate. Hence, an attempt has been made to assess the minimum duration of load increment necessary to study the compressibility characteristics of such materials. Thus compressibility behavior of fly ashes with additives has been studied using conventional consolidation test with different durations of load increments varying from 30 min to 48 h. The results obtained indicated that 30 min of duration of load increment can be used to assess the compressibility behavior of such materials. The effect of lime which reduces the compression is seen to be maximum from the results obtained with the duration of load increment of 30 min but gradually reduces with higher durations of load increment. It has also been observed that the rate of decrease in the compressibility is maximum up to 2.5% lime and gradual thereafter. The compressibility of lime treated fly ashes further reduces when gypsum is incorporated, the optimum gypsum percentage being 2.5. This reduction in the compressibility of fly ashes enhanced by incorporating lime and gypsum makes them versatile in the construction of embankments and for structural fills, particularly reducing the time required in between laying of each lift.     

Mechanical behavior and microstructural variation of loess under dynamic compaction

The laboratory dynamic compaction test was used to study the effects of dynamic impact loads on the shear strength characteristics of loess. Loess specimens were compacted by different numbers of blows and their shear strengths measured. The experimental results revealed that the shear strength varies in stages with the number of blows. During the initial stage, the shear strength increases with the number of blows. It reaches a peak value and then decreases with the number of blows. To reveal the relationship between this alternation in shear strength and microstructural variation, quantitative microstructural analyses were performed on some tested specimens at different stages of strength evolution. This microstructural study demonstrated that the variation of shear strength is directly related to the alteration of microstructure. It is concluded that the shear strength of loess is basically controlled by its microstructural state. Growing particle size heterogeneity, varying particle orientations and increasing microstructural damage are the main causes of the alternation of shear strength.     

The Shear Strength Behavior of Two Peaty Soils

This paper presents experimental research concerning the shear behavior of two types of Italian peat, one normally, the other over consolidated. Organic soils are characterized by very high compressibility and high fiber content; two features that give rise to several problems during laboratory tests. Under consideration here are the effects of fibers and over consolidation on friction angle and stress–strain behavior. These are evaluated by means of undrained compression triaxial tests, with isotropic and anisotropic (K_o) consolidation being performed on natural and remolded samples. The experimental results are also analyzed by means of a bilinear failure criterion based on soil-reinforcement interaction mechanisms.     

INVESTIGATION OF IMPACT STRESSES INDUCED IN LABORATORY DYNAMIC COMPACTION OF SOFT SOILS

The majority of currently available analytical tools to predict ground stresses due to impact are based on linear spring-dashpot dynamic models. Although these simple models adequately represent stiff ground possessing linear visco-elastic behaviour, they suffer from two striking limitations when applied to relatively softer ground; (1) the inability to account for the permanent deformation resulting from impact, (2) failure to incorporate stiffness changes of softer soil within the impact duration. In this paper, the authors present an improved analytical approach formulated on the basis of a series of laboratory impact tests, to address the shortcomings of the current dynamic models in relation to soft soils. In this procedure, the impact zone is modelled as three distinct zones; (1) a zone beneath the falling weight undergoing non-linear axial deformation while being in vertical motion, (2) an inner zone immediately surrounding zone 1 with non-linear shear deformation, and (3) an outer zone undergoing a relatively lower degree of (linear) shear deformation. The soil constitutive parameters pertinent to the model are obtained from a modified dynamic compression test that simulates the impact conditions. It is shown that analytical predictions of the impact stress history and penetration are in agreement with test results. The findings are useful in the exploration of dynamic compaction techniques that will be effective in soft soil improvement.     

红黏土路基填筑压实度控制指标探讨

红黏土属于一类典型的特殊土,路基施工规范指出,特殊填料进行填筑路基时可根据具体情况适当降低压实度要求,并且规定有些高液限、高塑指黏土不能直接作为路基填料填筑。利用某高速公路处的红黏土进行了重型击实和承载比试验,试验结果表明：红黏土在最优含水率附近具有很强的水敏性。最优含水率点对应的CBR值并非最大值,其最大CBR值对应的含水率大于最优含水率3%左右,在此基础上结合土体的强度、压缩性、胀缩性、渗透性等指标随压实度变化的规律,确定该桩号红黏土作为下路堤填料其压实度可降低2.5%,填筑含水率控制在35%左右。     

非饱和填土侧限压缩变形特性试验研究及应用初探

为研究吸力及压实度对非饱和压实填土压缩变形特性的影响,并建立脱湿（吸力增长）状态下的填方土体工后沉降变形修正计算应用模型,开展了控制吸力和压实度的一维非饱和土侧限压缩试验。结果表明：压缩曲线平缓段随压实度和吸力的提高而增长,表明土样的结构屈服强度同步得到提高;在比容v变化差值序列上,饱和土压缩土样最大、常规压缩土样次之、脱湿土样最小,并且吸力越大,比容v变化差值越小。经历脱湿（吸力增长）后的压实土压缩性降低,定义和建立了吸力压缩系数 及其经验模型,用以表征和度量吸力和压实度对压缩特性的影响规律及程度,发现 随竖向应力增加呈现指数型衰减。对模型参数与压实土相关参量间的关联性进行了分析探讨,同一压实度土样不同吸力所对应的参数 均值随压实水平提高而线性减小,参数λ总体上随吸力的增加而增大,但随压实度的提高吸力对于土体抗压性增强的贡献水平降低且参数 试验数值点靠拢线性趋近线,土样的压缩性随压实度和吸力大小变化而动态调整。基于分层总和法的基本原理初步构造建立了高填方非饱和填土压缩变形修正计算模型（MS）,其应用途径应建立在进一步对非饱和压实填土受荷状态下脱湿土-水特征关系的研究基础之上。     

Stabilization of lateritic soils with phosphoric acid

Summary This paper describes a laboratory study on the stabilization of lateritic soils with phosphoric acid-H₃PO₄. This method is most promising for road and airport pavement construction in tropical regions where fine textured lateritic soils (red clays and silts) occur over large areas. The iron and aluminum phosphates formed are hard and insoluble. The main source of iron is free iron oxide, and the aluminum sources are free aluminum oxide, exchangeable cations and clay minerals (hydrated aluminum silicates). Four different soil samples were studied, but the most comprehensive study was carried out with a lateritic soil evolved from weathered basaltic bedrock. The variables of the test specimens were: percentage of acid, moulding water content, compaction energy, and curing time. Strength tests performed were the axial or unconfined compression test and the indirect tensile or diametrical compression test. With 5% of phosphoric acid to dry weight of soil, values of compressive strength around 4.0 MPa were obtained after 28 days curing.     

The aging of gypsum in underground mines

The aging properties of gypsum extracted from two underground mines in France (Livry–Gargan and Grozon) have been highlighted by means of observation performed using scanning electron microscopy on samples taken along horizontal boreholes drilled through to the middle of several pillars. The aging process is exhibited by the presence of traces of dissolution (edges of the dissolved crystals, corroded crystalline surfaces, important intra- and intercrystalline space). These dissolution figures decrease in number and in intensity from the wall heading towards the middle of the pillar. The “older” pillars display a greater number of dissolution traces than the “more recent” pillars. Then, the role of the aging has been assessed by determining various physical and mechanical parameters on cylindrical samples of 38 mm in diameter and 76 mm in height, machined from the set of drilled boreholes. The following parameters have been quantified: density, grains density, total porosity, porosity accessible to water, intrinsic permeability to nitrogen, velocity of ultrasonic compression and shear waves, dynamic Young's modulus, dynamic Poisson's ratio, uniaxial compressive strength and static Young's modulus. Significant variations in parameter values between the wall and the middle of the pillar were recorded and recognized as being strongly correlated with the intensity of dissolution traces, and hence with gypsum aging.     

Compressibility of cement-stabilized zinc-contaminated high plasticity clay

The presence of heavy metals at high concentrations (percent levels) in soils has been a growing concern to human health and the environment, and the cement stabilization is considered to be an effective and practical approach to remediate such soils. The compressibility of such stabilized soils is an important consideration for redevelopment of the remediated sites for building and/or roadway construction. This paper investigates the effects of high levels of zinc concentration on the compressibility of natural clay stabilized by cement additive. Several series of laboratory compression (oedometer) tests were conducted on the soil specimens prepared with the zinc concentrations of 0, 0.1, 0.2, 0.5, 1, and 2 %, cement contents of 12 and 15 %, and curing time of 28 days. The results show that the yield stress and compression index at the post-yield state decrease with an increase in the zinc concentration regardless of the cement content. The observed results are attributed to the decrease in the cement hydration of the soil. Overall, this study demonstrates that the cementation structure of the soils is weakened, and the compressibility increases with the elevated zinc concentration, particularly at relatively high levels of zinc concentration.     

Laboratory study of static and dynamic compaction grouting in triaxial condition

This research paper is focused on the fundamental behaviour of applying static and dynamic compaction grouting techniques on completely decomposed granite (CDG) soils in Hong Kong. Using the modified triaxial apparatus and a novel pulse wave generator, laboratory tests were performed to identify the critical controllable factors of static and dynamic compaction grouting techniques in optimizing compaction effectiveness. The distinguishing feature of this laboratory apparatus is that it can simulate triaxial condition of static and dynamic compaction grouting. The effective confining pressure, the lateral pressure coefficient, excess pore water pressure, back pressure and void ratio change of the specimen were measured in this study. At the same time, the dynamic compaction grouting pressure, dynamic compaction frequency, and dynamic compaction duration were controlled. Moreover, the effects of effective confining pressure and injection rate on the compaction efficiency in static tests were studied. The study focused on the effect of dynamic compaction frequency, dynamic compaction duration, lateral pressure coefficient and initial dry density on the compaction efficiency of Hong Kong CDG soils.     

Compressibility and collapse characteristics of arid saline sabkha soils

Arid saline soils are well-distributed over the globe, with a variety of nomenclature. Along the seaboard of the Arabian Gulf, these soils exist widely and are known as “sabkhas”. Despite the cemented and saline characteristics of the sabkha matrix, a recent investigation indicated that flooding the saline sabkha with distilled water in the conventional oedometer apparatus was incapable of producing a sudden reduction in volume and/or a significant collapse. This study proposes a modification to the conventional oedometer on undisturbed sabkha specimens to consolidating specimens under a constant head. Tests were, therefore, conducted on undisturbed sabkha specimens to assess their compressibility and collapse potential whereby percolation of water was commenced under two pressures to evaluate the role of sustained pressure on the collapse mechanisms.

Despite the low compressibility of sabkhas, results of these tests indicated that these arid, saline soils possess a high collapse potential attributable primarily to dissolution of sodium chlorides, leaching of calcium ions and soil grain adjustment. The collapse potential increases with an increase in the acting pressure at which percolation of water takes place. In contrast to other typical soils, the collapse of arid, saline soils is not instantaneous but requires sufficient volume of water to percolate in order to enhance the dissolution of the cementing agents.     

疏浚淤泥固化土的压缩特性与结构屈服应力

河道疏浚拓宽以及港口新建扩建等不可避免地产生大量疏浚淤泥,如何处置大量废弃淤泥是工程界关注的一个重要研究课题。固化方法是目前处理疏浚淤泥的常用方法,其目的是将废弃淤泥处理成土工材料加以利用。处理后的淤泥固化土作为工程建设用土必须考虑其工程力学性能。为了研究疏浚淤泥固化土的压缩性状与结构屈服应力,进行了多组不同配比下淤泥固化土的压缩试验。通过室内一维压缩试验,研究了高含水率疏浚淤泥固化土的压缩性状,探讨了固化材料掺量、初始含水率、龄期等对固化土压缩变形特性和结构屈服应力的影响。压缩试验结果表明:与许多天然沉积结构性土及水泥土类似,疏浚淤泥固化土的压缩曲线均存在一个明显的结构屈服点。当上部荷载小于结构屈服应力之前,固化土的压缩性很小; 一旦上部荷载超过结构屈服应力之后,固化土的压缩性急剧增大。分析了淤泥固化土结构屈服应力的影响因素,并将由一维压缩试验得到的结构屈服应力(Py)与由无侧限抗压试验确定的抗压强度(qu)进行了比较,给出了两者之间的定量关系。疏浚淤泥固化土在屈服前后的压缩性状差异很大,这一研究成果对工程应用很有指导意义,当上部荷载较低时,可以充分利用其屈服前的低压缩性,但应确保上部荷载不能超出其结构屈服应力,以避免固化土在缺乏预兆的情况下发生突然破坏。     

强夯法加固岷江防洪堤粉土地基的效果检验

对宜宾市岷江防洪堤工程粉土地基进行了强夯法加固现场试验,介绍了试验区强夯法的施工设计和现场检测与室内试验结果。强夯法加固地基现场检测结果表明,采用1 600 kN?m的夯击能使该粉土地基有效加固深度达到8 m。 当夯点间距为5 m,强夯2遍,满夯1遍后,现场检测和取土实验结果表明：由于强夯作用,粉土层的干密度明显增加,压缩性和渗透性降低。在天然地基中,32.2 %的标准贯入击数小于5,强夯以后,标准贯入击数全部大于7。粉土地基经过强夯处理后,满足了防洪堤地基对承载力和渗透性的要求,消除了Ⅶ度地震液化势。采用正三角形夯点布置区的加固效果明显优于正方形夯点布置区的加固效果。