Dynamic stiffness of laterally loaded pile foundation

Abstract

The study mainly covers the analysis of dynamic properties of Dhaka soil followed by evaluation of dynamic pile head stiffness, using a soil structure interaction analysis tool based on Thin Layered Element Method, TLEM. Frequency dependent pile head stiffness is evaluated by TLEM software for two selected sites of Dhaka city, namely; Mirpur Defense Officers‟ Housing Scheme (DOHS) and Uttara sites. Five borings were conducted at five selected sites and twenty borehole data were collected for all the sites. Shear wave velocity was measured by Small Scale Microtremor Measurement (SSMM) technique for all the selected locations. At the same locations SPTs were also conducted. Using the soil mass density,() and shear wave velocity,(Vs) low-amplitude shear modulus,(Gmax) was estimated by the relation, Gmax= vs2. It is found that clay has got minimum and dense sand has got maximum value of Gmax. In the process of study, correlations between SPT N-value and Vs as well as Gmax were also developed for sand (fine contents, Fc : 12 ~ 37%) and clay (Fc : 53~100%). These equations were compared with other equations developed for similar types of soil including local soils by various authors. It was found that proposed correlations agree with them well. Apart from determining Vs and Gmax, few other dynamic properties like, soil mass density, penetration resistance, Poissoin‟s ratio etc. of selected sites were determined including basic static properties like, specific gravity, grain size analysis, index properties etc. Assessing the static properties out of five sites a reclaimed site, Mirpur DOHS and an original site Uttara were preferred for evaluating dynamic pile head stiffness under lateral loading conditions. Dynamic pile head stiffness was determined using TLEM. Soil dynamic properties obtained through field and laboratory test together with required pile properties were used as basic inputs of TLEM program. Few widely used RCC pile dimensions ranging from 457 to 610 mm with average practical lengths varying from 9 to 25 m were considered for the analysis. The real and imaginary part of the program output provided stiffness and damping respectively. In Mirpur DOHS dominant soil layers were loose silty fine sand, medium to stiff silty clay and dense silty fine sand. A layer of soft organic clay of about 1 to 2 m was found below the upper loose silty sand layer. In this site it was found that stiffness increases with the increment of pile diameter. For a 457 mm pile initial stiffness ranges between 7.17x104 to 6.99x104 tf/m for the variation of pile length between 6 to 25 m. For a 610 mm pile this range was from 9.13x104 to 8.83x104 tf/m. The pile head damping on the other hand showed lower magnitudes (5.48x103 to 7.05x103) compared to the stiffness. Again, keeping the pile length constant with the increment of pile diameter almost similar range of stiffness was observed. It was found that stiffness decreases with the increment of frequency. Here, it was also found that pile head stiffness become almost independent of pile diameter at certain range of frequency (23 to 28 Hz) for a constant length. Higher diameter pile had higher damping in comparison to piles of lower diameters. Damping decrement was more conspicuous within the range of active pile lengths. The stiffness determined using correlated shear wave velocity showed higher stiffness than that of field shear wave velocity. It was noticed that both stiffness and damping increase with the increase of Poisson‟s ratio from 0.30 to 0.49. Homogeneous soil layer, however, provided greater stiffness than that of inhomogeneous layer. Upper 3 m of Uttara site had a soft to medium stiff clay layer followed by a stiff clayey silt with fine sand up to 6 m depth and from then onward up to 15 m depth the layer was medium dense to dense silty fine sand. This site being original land showed greater pile head stiffness in comparison to reclaimed land, Mirpur DOHS where loose fills dominates. It was found that for a 20 m pile having diameter of 457 and 610 mm, pile head stiffness vary from 2.776x105 to 3.49x105tf/m, respectively. Dense sand layer of Uttara site had almost four times greater stiffness than that of Mirpur DOHS site, where loose fills were predominant. In this site it was noticed that increment of pile diameter has greater influence on damping than that of length. Keywords: Dynamic soil properties, soil pile interaction, pile head stiffness and damping.