Abstract
In the present study the foundation of digester tank, main part of sewage treatment plant, is reanalyzed analytically and numerically to check the adequacy of such foundation to support superstructure loading. The foundation of digester tank consists of raft foundation and bored piles. The diameter of raft is 33 m and thickness of 1 m, while the piles are bored type of diameter 0.6 m and length 15 m. After testing eleven working piles, it is found that three piles cannot support a load of 1.5 times the working load (1305 kN) safely or in other words the factor of safety of these failed piles is less than 1.5. The results of filed pile tests are reanalyzed using two well-known methods, Davisson’s method and Brinch-Hansen method to check the ultimate carrying capacity of tested piles. Also, this paper includes analysis of previous soil investigation report and conducting additional soil investigation by drilling three boreholes to secure the soil parameters used in the analytical and numerical analysis of digester tank foundation. SAFE 12 software is used to analysis the foundation of structure as piled-raft instead of pile group to interest from the interaction between soil and raft foundation. The results of analysis showed that the piles failed in the tests can support its share of the superstructure load by a factor of safety 1.8 and the piles success in the field tests can support its share of the superstructure load by a factor of safety not less than 2.86. Also, the settlement under structure will be less than 100 mm, where using piled-raft analysis reduces the settlement to be within allowable limits.
Highlights
The digester tank is considered important structures in comparison with other structures that are consist the sewage treatment plant
The upper part is of a conical shape with outer radius of 12.5 m and height of 6.5 m, the middle part has a cylindrical shape with outer radius of 12.5 m at its upper part (7.3 m height) and 12.8 m at its lower part (6.45 m height) and the lower part of the digester is of an inverted conical shape with outer radius of 12.8 m
Two of boreholes were drilled to a depth of 20 m and the third borehole was drilled to a depth of 10 m [13].The data obtained from the additional soil investigation suggested that the allowable bearing capacity of shallow foundation at depth 1.5 m is ranging from 55-80 kPa and at depth 4.5 m is ranging from 57.5-110 kPa.the bored pile of 600 mm diameter and 15 m depth has an allowable carrying capacity of 710 kN
Summary
The digester tank is considered important structures in comparison with other structures that are consist the sewage treatment plant. Dezfouli [4] studied the effects of reinforcement elements such as geogrid in the cushion layer of the non-connected pile-raft foundation in sandy soil on the mechanism of load transfer and the shares of piles and raft from the total applied load. The present study is a case study focused on reanalyzed a constructed raft and piles to calculate the allowable carrying capacity of piles and apply SAFE 12 software to analyze the system as piled-raft instead of analysis the system as pile group This problem raised after testing eleven working piles in the field, it is found that three piles cannot support a load of 1.5 times the design load of 1305 kN safely or in other words the factor of safety of these failed piles is less than 1.5 [10,11]. This project can be considered is trial to analyze the foundation as piled-raft instead of pile group which is considered of of the sustainability development aspects by changing the design criteria
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