Fundamentals of cement-admixed clay in deep mixing and its behavior as foundation support of reinforced embankment on subsiding soft clay ground | |
| Author | Lorenzo, Glen A. |
| Call Number | AIT Diss. no.GE-04-01 |
| Subject(s) | Cement--Additives--Thailand--Bangkok Embankments--Thailand--Bangkok |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Civil Engineering |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. GE-04-01 |
| Abstract | The deep mixing method (DMM) of soft ground improvement using either mechanical mixing or jet mixing with cement slurry would increase the water content in the clay due to the effect of remolding and mixing with the associated addition of water prior to the feeding of cement admixture. The water in the cement slurry would further increase the water content in the clay-cement paste. This present study has investigated, in addition to the previous study which utilized only cement content and curing time as controlled parameters, the effects of higher mixing clay water content on the strength and compressibility behavior of cement-admixed clay. Higher mixing clay water content means the total clay water content (Cw) in the clay-cement mixture, inclusive of the water from the cement slurry, is equal to or greater than the liquid limit (LL) of the base clay. In this study, the mixing clay water contents were varied from LL up to 1.5LL of the base clay. The curing periods were 7, 14 and 28 days for the physical and unconfined compression (UC) tests, and 28 days for oedometer and consolidated-undrained (CU) triaxial tests. New fundamental parameters such as the ratio of after-curing void ratio (eot) and cement content (Aw) have been found sufficient to characterize the strength and compressibility of high water content cementadmixed clay in deep mixing application. From analyses performed on the results of unconfined compression (UC) test as well as consolidated-undrained (CU) test, the ratio eot/Aw has been proven to combine together the influences of clay water content, cement content and curing time as well as curing pressure. The unconfined compression strength (qu) of cement-admixed clay from both laboratory-mix and field-mix specimens have yielded unique relationship with eot/Aw ratio. Moreover, the deviator stress and excess pore water pressure responses as well as the stress paths during consolidated-undrained triaxial tests were also successfully characterized by the eot/Aw ratio. Furthermore, the results of oedometer consolidation tests revealed that Aw governs the position of the post-yield compression line, and the eot determines the magnitude of the onedimensional vertical yield stress, y v ' , at particular Aw. An empirical compressibility model that predicts the one-dimensional compression line of cement admixed clay has been proposed. Moreover, the y v ' of cementadmixed clay has yielded a reasonable correlation with qu. Normalizing the after-curing unit weight, after-curing water content and after-curing specific gravity were incorporated in an empirical relationship to obtain eot. Moreover, from the results of unconfined compression and oedometer compression tests, a new concept of optimum mixing clay water content for economical and efficient mixing of cement and clay for DMM application was discovered. The confirmation supporting the existence of optimum mixing clay water content is presented and a schematic diagram idealizing the clay-water-air-cement interactions is illustrated. The optimum mixing clay water content (Cw,opt) is defined as the total clay water content of the clay-water-cement mixture that would yield the highest possible improvement in strength of cured cement-admixed clay. Based on the test results presented, the Cw,opt would fall near the liquid limit (LL) of the base clay, preferably within LL up to 1.10LL. Significantly, at this optimum mixing clay water content, to yield a certain magnitude of shear strength, only 10% cement content by weight is needed instead of the corresponding 17% in the conventional method of mixing. Therefore, the main advantage of mixing the cement slurry into the clay at optimum clay water content is the reduction of cement content and cost by as much as 40%. In the new analytical method for the design and analysis of deep mixing pile improved ground, both short-term and long-term analyses were considered. In the short-term analysis, the existing method of bearing capacity evaluation for deep mixing pile has been modified to account for the improvement in strength of the adjacent soil in the vicinity of deep mixing pile owing to thixotropic recovery, reconsolidation, cement penetration and cation diffusion during and after DMM pile installation. The adhesion factor of the deep mixing pile has been found to be greater than unity. In the long-term, the effect of local differential settlement between deep mixing pile and the adjacent clay as well as the effect of piezometric drawdown, which causes ground subsidence, on the development of negative skin friction (downdrag) and on the compression of the deep mixing piles were studied. For piezometric drawdown that has been present for several years, the elimination of piezometric drawdown in the analysis will lead to: (1) overestimation of the consolidation settlement and (2) underestimation of the downdrag skin friction and, hence, axial stresses in the deep mixing piles. The additional axial stresses induced by the downdrag skin friction on the deep mixing pile intensifies the final axial stresses in the pile and, hence, reduces the factor of safety against bearing capacity at long-term. Furthermore, a new empirical method of predicting the strength of cement-admixed clay has been formulated for use in the analytical calculation of the strength of deep mixing pile. Consequently, a design procedure of estimating the cement content (Aw) and the amount of water to be added per meter length of deep mixing pile at a given specific gravity and natural water content of the base clay has been established. A scheme of deep mixing pile installation for foundation support of bridge approach embankment on soft ground has been proposed. Two methods of proportioning the lengths of cement deep mixing piles for bridge approach foundation were formulated. The first method is general, while the second is an alternative method for quick calculation. To avoid consequent undulation of the pavement at bridge approach, it is crucial that the deep mixing piles supporting the approach embankment must be installed at varying lengths, with closer spacing and full penetration through the soft clay layer near the abutment and, then, decreasing smoothly towards a determined length of improvement. Significantly, the smooth variation of the lengths of deep mixing piles is as important as the smooth vertical alignment of the crown of the pavement. A full scale improved soft clay foundation by cement jet mixing overlain by 6.0 m high reinforced embankment was constructed in Wangnoi District, Ayuthaya, Thailand and monitored in order to study its consolidation and deformation characteristics. Based on the results of this full scale study, a new analytical method of estimating the rate and magnitude of the consolidation settlement of DMM pile improved ground overlain by reinforced embankment has been formulated. Moreover, the reliable elastic as well as consolidation and compressibility parameters were obtained through back-analyses of the actual surface and deep settlements as follows: modulus of elasticity of the deep mixing pile (Eup) and the adjacent clay at the proximity of the pile (Eus) of 105qu and 280Suv, respectively, where qu is the 28-day unconfined compression strength of the deep mixing pile and Suv is the corrected vane shear strength of the intact base clay; average mobilized coefficient of downdrag skin friction (ave) of 0.101; neutral axis of the downdrag skin friction to be reasonably located at the bottom of the upper-third of the soft clay layer; coefficient of consolidation of the deep mixing pile (cv,p) of 800 m2/yr; coefficient of consolidation of the surrounding clay (cv,c) of 2.0 m2/yr; compressibility ratio (mv,p/mv,c) of 0.10; permeability ratio, kv,p/kv,c, of 40; and weighting factors and corresponding to equal strain and equal stress conditions of 80% and 20%, respectively. |
| Year | 2005 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. GE-04-01 |
| Type | Dissertation |
| School | School of Civil Engineering |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Geotechnical Engineering (GE) |
| Chairperson(s) | Bergado, Dennes T.; |
| Examination Committee(s) | Kasem Petchgate;Noppadol Phienwej;Glawe, Ulrich;Takemura, Jiro;Jian-Hua, Yin; |
| Scholarship Donor(s) | The Government of Japan; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2005 |