A STUDY ON GEOPOLYMER STABILIZED AND FIBER REINFORCED SOFT CLAY COLUMNS

Abstract

The soft clay deposits (cu < 25 kPa) occur along the coastlines and estuaries of several world nations including India. In view of the enormous economic activity along the coastlines, it is imperative to take up huge infrastructure building (such as transportation routes, ports and harbour structures, multi-storeyed structures, residential and industrial utilities, etc.) over these unsuitable deposits inevitably. The sustained research by various investigators across the globe enabled the engineering community to develop remedial techniques such as soil replacement, stone columns, preloading with vertical drains, electro osmosis and soil-lime or soil-cement piles by deep soil mixing in order to make these deposits viable for construction activity. Among these techniques, only deep mixing method could modify the ground within short time and the remaining techniques require considerable time periods before the expected level of improvement could be achieved. In view of this, for time-bound projects, deep mixing method becomes an inevitable choice. This technique essentially consists of installing soil-binder columns by mixing binder (dry or wet) in the existing soft soil with the help of augers below ground surface. The soil-binder columns thus formed act as reinforcement for the soft ground improving its overall performance (increased bearing capacity and reduced settlements) to support low to medium load structures. The lime and cement have been traditionally used as binders for this purpose and currently, it is felt that the use of these traditional cementing materials with high carbon footprint are to be discouraged. Also, lower durability of these materials is reported. As an alternative to these conventional binding materials, geopolymer technology has been introduced and continuously being investigated for concrete making as well as for soil stabilization. The major difference in concrete making and soil stabilization using geopolymerisation arises from the fact that the entire geopolymerisation mechanism in soil stabilization, which is susceptible to aspects like silica and alumina supply, alkaline concentration, water content, etc., can be altered by the existence of soil. In view of the high degree of variability of soft clay deposits, especially the natural water content, several uncertainties arise, for which systematic investigations are inevitable in order to build confidence in the construction industry. This technology basically involves the preparation of an inorganic alumina silicate material formed by combining reference materials called ‘precursors’ possessing high amorphous silica (Si) and alumina (Al) with alkaline solutions called alkali activators or reactants of required concentration to get the target strength and durability requirements. Out of the various industrial by-products and wastes that are used as vi precursors, fly ash, ground granulated blast furnace slag (GGBS) and metakaolin are most widely used. The general inference from these studies is that these precursors in the presence of NaOH+Na2SiO3 impart high strength and durability to the treated soil. Also, among the several hydroxide and silicate combinations rich in soluble metals like sodium (Na) and potassium (K) as potential alkaline medium, Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3) combination (NaOH+Na2SiO3) proved to be the most effective one and was broadly accepted by the cement and concrete industry researchers. However, contradictory results are also reported by previous researchers where geopolymers made with GGBS impart high early strength at ambient temperature curing, unlike geopolymers made of fly ash which require vigorous working environment and high temperatures of curing (60 °C– 200 °C) to initiate the reactions. Further, it is indicated that the use of 𝑁𝑎𝑆𝑖𝑂 in the geopolymerisation process is suggested to be discouraged in view of its higher carbon footprint during its manufacture and transportation. As per this, some researchers have attempted to use lower alkali concentrations in their studies. However, the use of geopolymers at higher binder contents with high alkali concentrations becomes inevitable to satisfy the target UCS ranging from 1034 kPa to 4137 kPa of soil-binder columns required for DSM treatment of soft soils for wide range of applications. A detailed laboratory testing is taken up to understand the strength behaviour of soil geopolymers using 𝑁𝑎𝑂𝐻 alone as alkali activator in the present work along with GGBS as precursor in view of its higher strength gain at ambient temperature. As the soil-geopolymer mixes become brittle at high alkali concentrations and binder contents, polypropylene fibers are incorporated into the mixes to improve their ductility as suggested by the researchers. Also, considering the case of intrusion or extrusion of water (moisture fluctuations) from the soil surrounding the soil-geopolymer columns, the durability (against wetting and drying) of the soil-geopolymer mix specimens needs to be studied. Although fibers do not influence the changes in the hardened soil-geopolymer matrix due to wetting and drying, their role is still crucial in arresting the propagation of micro cracks formed during wetting and drying cycles, thus reducing the mass loss and volume change. The scope of the present study is to synthesize an appropriate geopolymer (GP) binder with GGBS and 𝑁𝑎𝑂𝐻 reinforced with polypropylene (PP) fibers to stabilize a highly plastic soft clay at high water contents (around liquid limit) and testing its efficacy with respect to strength and durability characteristics of the stabilized soft clay in deep mixing applications. The objectives of the present research work are kept as follows: vii 1. To study the strength aspects of geopolymer stabilized soft clay with 𝑁𝑎𝑂𝐻 as a sole alkali at higher concentrations and GGBS as binder at its higher contents by performing laboratory tests by varying the mix proportions. 2. To assess the effect of polypropylene fiber inclusion on the strength and stiffness of soil-geopolymer mix specimens by varying fiber dosages. 3. To perform durability studies on the soil-geopolymer mix specimens of suitable proportions with and without fiber reinforcement and assess their mass loss, volume change and residual strength when subjected to wetting and drying cycles. 4. To estimate the load capacity of model soft clay bed reinforced with end bearing and floating soil-geopolymer columns with and without polypropylene fiber inclusion. The experimental investigations are planned in line with the above objectives to determine the strength in terms of unconfined compressive strength (UCS) and flexural strength for different soil - geopolymer mix specimens with and without fiber reinfocement cured at ambient temperature for 3, 7, 14 and 28 days. The results from these tests were compared with that of the soil-cement specimens. The microstructure of selected treated soil samples after the strength tests was studied with the help of Scanning Electron Microscopy (SEM) and EDAX to understand the mechanism of strength improvement. The selected soil-geopolymer mix specimens with and without fiber reinforcement were subjected to durability test against wetting and drying cycles after 28 days of curing. The load tests on model soft clay bed with end bearing and floating soil – geopolymer columns, without and with fiber reinforcement were carried out to understand the load carrying capacity under axial loading. From the present study it is understood that the specimens treated with GP showed higher UCS values compared to cement-treated specimens for the same dosage, and this may be due to the combined effect of pozzolanic and geopolymeric reactions of GP. To meet the target strength requirement for DSM applications, a binder dosage of greater than or equal to 20% and A/B ratio of greater than or equal to 0.75 are required. With increase in initial soil moisture content (higher than liquid limit), the strength of the treated specimens under unconfined compression and flexure is reduced thus requiring higher binder dosage to meet the DSM requirements at higher water contents. Out of the various combinations of the mixes tried, the geopolymer treated soil mixes with binder content of 30% and A/B ratio of 0.75 reinforced with 1% PP fibers by dry weight of soil could satisfy the strength and durability requirements viii and hence found to be the optimum mix combination for deep soil mixing applications for soils with liquid limit in the range about 68%. For end bearing columns condition with any area ratio, the fiber reinforcement has shown improved load-deformation behavior as compared to the unreinforced system. For floating columns condition, the soil-geopolymer column reinforced soil bed has shown a block failure pattern and hence, the effect of high column strength and fiber reinforcement has insignificant effect on its load capacity. To attain a particular UCS, the Geopolymer stabilisation with GGBS and 𝑁𝑎𝑂𝐻 is found to be economical compared to the Cement stabilisation as per the prevailing market rates.