Ground Engineering for Ratho Station - Essay Example

Ground Engineering Introduction The foundation design for the proposed structure for Ratho station consists of following tasks: (i)Assessment of soilcharacteristics (iii) Selection of the type foundation (iii) Computation of foundation load and (iv) Design of the foundation type Soil characteristics at site The bore hole studies conducted at the Ratho station site shows that the soil is predominantly granular. The hard rock stratum is obtained at 10 m below the ground surface with water table observed below 2 m below the ground level. The top 20 cm of the soil layer is organic which is underlain by compacted fill of sandy soil extending to a depth of 4 metres. Below this layer is sandy silt and clay mixed with coarse particles of rounded or angular shape which continues up to 10 m where the hard bed rock is observed. The strength parameters of the soil at the site is as shown in table 1. Table 1 Soil strength parameters Depth Cohesion (C, KN/mm2) Angle of internal friction (') Density (KN/mm3) Bearing capacity, KN/mm2 2.5 metre 0 35 1.95 300 6.5 metre 10 15 2.0 500 Selection of foundation type Based on the soil type observed and the nature of structure to be constructed the most feasible type of foundation for the Ratho station structure would be pile foundation. Pile foundations are chosen for the situations where structures transmit huge loads and suitable soil layer capable of supporting these loads are available only at greater depths. The piles are long and slender members capable of transmitting the loads of the structure into the hard soil layer or the rocks successfully even if very poor or soft soils that are considered to be inferior to meet this purpose is present in the upper layers. Based on the load carrying function piles are classified as end bearing pile, friction pile or combination pile that undertakes both the functions appropriately. The friction piles are suitable for the locations where granular soil is present and frictional resistance between the pile and the soil could be utilised in the resistance. While the end bearing piles helps to transmit the entire load a hard stratum through the tip resistance. A sketch of the load action on the end bearing pile and friction pile are shown in the figure 1. The basic structural components of any pile are pile cap and the body of the pile. Figure 1(a) : End bearing pile Figure 1 (b) : Friction pile (Abebe and Smith, 2005) (Abebe and Smith, 2005) Pile load computation Following assumptions are incorporated in the load estimation on piles (i)All piles are considered as rigid (ii) The pile is pinned at the top and bottom And (iii) All piles carry only vertical force. The different loads that the pile need to carry would be direct loads from the columns and any other imposed loads on the structure. The direct loads from the columns to the pile are already given in the problem. The other loads are the load from the glass facades and the imposed loads. The glass faade load is the load along the edges of the floor and it could be computed by multiplying the unit load of the faade by the distance between the columns. The details of computation is as follows Glass faade load between the columns 1 & 8 Distance between the columns 1 and 8 = 20 metre Unit load of facde between the column 1 and 8 = 35 KN/m The load from the faade is assumed to be transferred equally to both the columns . So the loads are computed as 35 20 / 2 = 350 KN So the columns 1 and 8 would carry load of 350 KN each from the glass facades. Glass faade load between the columns 4 & 5 Distance between the columns 4 and 5 = 20 metre Unit load of facde between the column 4 and 5 = 35 KN/m The load from the faade is assumed to be transferred equally to both the columns . So the loads are computed as 35 20 / 2 = 350 KN So the columns 4 and 5 would carry load of 350 KN each from the glass facades placed between the columns 4 and 5. Glass faade load between the columns 1 and 4 The columns in this span are 1,2,3 and 4. The distance between column 1 and 2 = 10 metre The distance between column 2 and 3 = 14 metre The distance between column 3 and 4 = 10 metre The total distance between the columns 1 and 4 = 24 metre Unit load of facade between the column 1 and 4 = 23 KN/m The load from the faade is assumed to be transferred equally to both the columns . The load transmitted to column 1 = 10 23 / 2 = 115 KN The load transmitted to column 2 = 10 23 / 2 + (14 23 / 2 ) = 276 KN The load transmitted to column 3 = 10 23 / 2 + (14 23 / 2 ) = 276 KN The load transmitted to column 4 = 10 23 / 2 = 115 KN Glass faade load between the columns 8 and 5 The columns in this span are 8,7,6 and 5. The distance between column 8 and 7 = 10 metre The distance between column 7 and 6 = 14 metre The distance between column 6 and 5 = 20 metre The total distance between the columns 8 and 5 = 24 metre Unit load of facade between the column 8 and 5 = 28 KN/m The load from the faade is assumed to be transferred equally to both the columns . The load transmitted to column 8 = 10 28 / 2 = 140 KN The load transmitted to column 7 = 10 28 / 2 + (14 28 / 2 ) = 336 KN The load transmitted to column 6 = 10 28 / 2 + (14 28 / 2 ) = 336 KN The load transmitted to column 5 = 10 28 / 2 = 140 KN Distance between the columns 5 and 8 = 24 metre Thus the loads contributed to each of the columns from the glass faade are added up and final load reaching the pile beneath each column is presented in the Table 1. Also the imposed load for each column is also computed by distributing the load to columns as per the code provisions. Pile reference (Column number) Direct load (KN) Load from facades (KN) Other imposed load (KN) Total load (KN) 1 330 465 250 1045 2 560 276 600 1436 3 560 276 600 1436 4 330 465 250 1045 5 340 490 250 1080 6 550 336 600 1486 7 550 336 600 1486 8 340 490 250 1080 Design of pile foundation As the maximum depth to the hard rock bed is only 10 m and the loads computed for the piles are between 1000 KN to 1500 KN, it is proposed to provide precast - prestressed concrete piles under all columns. The design details of the piles are as follows. Based on the Eurocode provision the design of piles are undertaken using limit state method. The partial factors of safety are incorporated as m , n and Rd . m is the partial safety factor for characteristic values of materials , n is the partial coefficient to ensure strength and stability of structure and is classified into cases A, B and C, Rd is the partial factor to incorporate the test results and future construction. The design details of pile Cross sectional shape of pile : square Cross section dimension of pile : 45.7 45.7 cm2 Minimum effective prestress force : 1010 KN Details of pre-stressing strands : 10 strands of 12.7 mm diameter. Section modulus : 15.93 10-3 Design bearing capacity : 1803 KN Concrete strength : 34.5 MN/m2 Length of pile : 9 m The typical sketch of the design is given below Figure 2a : Cross section of pre-stressed concrete pile Figure 2b : Longitudinal section of pre-stressed concrete pile References Bowles, J E (1997), Foundation analysis and design, 5th Edition, McGraw-Hill Companies Inc. Abebe, A and Smith, G.N. (2005), Pile foundation design: A student guide [Online] available at [22 April 2008] Read More