Analysis of concrete linings of irrigation canal's behavior constructed over improved bases with lime and cement

Since basis contain swelling soil leads to volume changing of soil under concrete linings cause cracking of panels consequently, the operation of irrigation canals would disturb. Chen (1975) and Justo et al (1985) reported that the main causes of irrigation canal failure are volume changing of canal base material in the form of swelling and shrinking. Different methods have been presented to overcome or reduce the risk of such problematic soils to light structures. The magnitude of acting force and bending moments on the canal linings depends upon the canal size and dimension, as well as operating conditions namely full operation or rapid drawdown. Ahmadi et al (2009) recommended that in order to minimize the hazard of concrete cracking the optimum proportion of bed width to canal bottom (B/H) must be followed. Regardless of sidewall slope of the canal, the optimum value for B/H is around 1 to 1.5. However, one of the most popular methods that could be employed to reduce swelling potential of expansive soil is a chemical modification of this kind of soil using by admixtures such as lime and cement. Sahoo and Pradhan (2010) reported the effectiveness of chemical improvement of expansive soils to control and reduce canal failures. Adding admixtures such as lime, cement and fly ash improve soil texture by generating new pozzolan actions (Sahrma et al, 2012). Based on research of Chen (1975) the optimum value for lime content at clayey soil is about 2% to 8% in order to stabilize and reduce expansive potential. Brown and Robinson (1986) conducted a study to determine needed resistance of soil-lime-cement mixture to prevention of seepage. Brown and Sarker (1987) experimentally compared the bending resistance of soil samples improved by lime, cement and lime-cement. The confirmed that the samples contain both admixtures of lime and cement had high strength.
In this study, the behavior of canal linings constructed on the modified basis via lime-cement chemical admixture has been studied and compared with whom which was constructed on the unmodified bases. In order to study the effect of improved based material on the behavior of canal linings, different content of lime and lime cement were added to soil and experimentally the uniaxial compression test conducted on the samples. The test results show that specimens contain 2% and 4% of lime-cement faced with maximum strength. Therefore, this material selected to involve in analysis of behavior of canal linings, in other words, soil material properties improved via 2% and 4% complex of lime and cement considered as base characteristics. Furthermore, in order to study the acting bending moment and shear forces on the concrete linings numerical simulation of canal behavior was executed using by Plaxis as a finite element base geotechnical model. In order to achieve realistic behavior of canal deformation the complete section of geometry of canal created in the model, nevertheless, the trapezoidal canal section is symmetric and might be considered half. To calibrate the model, uniaxial compression test results were imposed. Different sizes of lined canal with different side slopes equal to z=1, z=1.5 and z= 2 were analyzed and the acting bending moments on the concrete linings under three different conditions, include end of construction, full capacity and drawdown were comprised. Also, the soil behavior considered as elasto-plastic and canal linings as elastic, for both of them Mohr-Coulomb failure envelope were used.
The results illustrated adding 2% lime and 4%lime mixed with 4% cement lead to significant reduction in the acting bending moments on the concrete linings. Furthermore, the state of 4% of cement and lime in reduction of bending moments on canal lining is more efficient than the specimens with only 2% lime. Also, the compared results showed, the acting bending moments rises with increasing of canal dimension. The comparison between acting bending moment on the canal linings informs that the maximum bending moment acts on the linings in the full operation condition. This state of operation could be considered as critical state for canals constructed where base material be improved via lime-cement complex. Results related to rapid drawdown condition, also, revealed that at the proportion of width of canal to the depth (B/H), between 0.5 to 1 the minimum bending moments would act to linings and this is in agreement with findings of Ahmadi et al (2009).