Civil Insight: A Technical Magazine Volume 2 | Page 24

24 TEACHERS’ SECTION CIVIL INSIGHT 2018 tapered piles by introducing stress-dilatancy relation postulated by Bolton (1986, 1987). In the proposed stress- dilatancy relationship, the increase in confi ning pressure will increase the relative density along with increase in angle of internal friction and dilatancy and makes it possible to compute at each segment of pile during pile penetration (Manandhar, 2010; Manandhar and Yasufuku, 2011, 2013). This drawback is improved and both properties are considered to evaluate the skin friction at each depth iteratively. The parametric studies on different types of model tests, prototype tests and real type pile tests are assessed to predict the proposed model. In the proposed model, this drawback has been improved and the skin friction has been calculated successfully. The results are verifi ed using parametric study on different types of model tests, prototype tests, and real type pile tests. EFFECTS OF STRESS DILATANCY AND CYLINDRICAL CAVITY EXPANSION THEORY The soil is presumed to be dilated plastically at a constant rate (Davis, 1968). Generally, zero dilatants angle was considered to compute large strain analyses. In stress-dilatancy relation, secant angle of internal friction, rate of dilatancy towards critical states and both effective stress and soil density parameters are interdependent in their strength parameters. In reality, the angle of internal friction and the rate of dilation towards the critical state is the function of both density and effective stresswhich cannot be avoided in the computational procedure, however it is more complex during calculation. In addition, density and confi ning pressure undergo change when tapered piles penetrate with settlement ratios. Confi ning pressure increases with increasing relative density and angle of internal friction with decreasing dilatancy together with pile penetration (Manandhar 2010, Manandhar and Yasufuku 2012). For the necessary computations, pile and soil materials are incorporated from Manandhar and Yasufuku (2012) as shownin Tables 1 and 2. Therefore, it is important to give closed-form solution to evaluate the skin friction of tapered piles introducing stress-dilatancy relationship in cylindrical cavity expansion theory. The stress-dilatancy relation established by Bolton (1986, 1987) has been introduced in Yu and Houlsby’s (1991) cavity expansion theory to check the bearing behavior of different types of piles. For a plane strain, the stress-dilatancy relation can be expressed in the following term: I ᇱ ୫ୟ୶ െ I ᇱ ୡ୴ ൌ ͲǤͺ\ ୫ୟ୶ ൌ ͷ ୖ୭  (1a) ୖ ൌ ୈ ሺͳͲ െ Ž ’ ᇱ ሻ െ  ͳ   (1b) Where, I ᇱ ୫ୟ୶  , I ᇱ ୡ୴ , \ ୫ୟ୶ , and an I ୭ are maximum angle of friction, angle of friction at critical states, maximum ୖ dilation angle and relative dilatancy index at plane strain respectively. The relative dilatancy index IR is a function of relative density ID and mean effective stress p as shown in Eq. (1b). The mean effective stress can be simply defi ned as the mean radial and hoop stresses explained in the cavity expansion theory as follows: ’ ᇱ ൌ ᇱ ஢ ౨ ା஢ ಐ ଶ ଵ ’ ൌ ቈ (1c)  ଢ଼ ଶ ஑ ᇲ ିଵ ൅ ” ൫ಉᇲ షభ൯ ಉᇲ ି ൅ ଢ଼ ஑ ᇲ ିଵ ൅ ୅ ஑ ᇲ ” ൫ಉᇲ షభ൯ ಉᇲ ି ቉  (2a) Replacing the constant of integration, A, the above equation is simplifi ed into the following form as: ᇱ ’ ൌ െ’ ଴ „ ൫ಉᇲ షభ൯ ൫ಉᇲ షభ൯ ି ಉᇲ ಉᇲ ”  (2b) The general Eq. (2b) can be simplifi ed in the elastic-plastic region (Manandhar 2010, Manandhar and Yasufuku 2013). At the boundary of plastic region, the effective mean stress can be modifi ed into the following structure as: ’ ᇱ ൌ െ’ ଴  (3)    Where pc, p 0 , b, r, D and R are described in detail by Manandhar and Yasufuku (2012).