Engineering Character Of Soft Rock And Design Of Pile Embedded In Rock

The man-made pile with large diameter has been widely used in the foundation of tall buildings and large span structures due to its advantages such as high bearing capacity, easy construction, noisless, no vibration, better quality, fast construction, lower expenses and little demand for construction equipments.The foundations of tall building in Sichuan mountain areas are usually built on rocks ,such as sand rock, mud rock, sandy mud rock, mudy sand rock etc. These rocks are classified as soft rocks according to their strength measured by saturated single-axis compressive strength in lab. The bases of man-made piles with large diameter on these types of rocks are usually dug into medium to weak weathered layers in order to get higher end bearing capacity, so these piles are commonly named piles embeded in rock. But in practice of pile design various rocks with different character, such as sandwich of sand and mud rock, mixed layers of different weathed rock, from severe to weak weathed, are often encountered in the same site. Widely scattered test data and design parameters given in the investigation report often bring about difficulty for engineers to make foundation design. It is still under discussion that how to determine correctly the actual bearing capacity of soft rock.There are various calculating methods about bearing capacity for man-made pile with large diameter which base dug into rocks in the national pile design Regulation and Code. But the calculated results by different method are differ from each other even in the same site under the same geology condition. It needs further study that how to get effective test data in pile design and to ensure safty and economy of pile foundation.This pepper takes the actual pile design of liao Jia Gou housing district in China Engineering Physical Research Institute as an example. The auther analyzed and compared over thousand pieces of test data for soft rock provided by in investigation report covering 27 hectares within whole housing district and put emphasis upon studying the two strength parameters of pile foundation design: characteristic value of saturated single-axis compressive strength and ultimate end resistance of pile. Based on the different design codes and other articles ,the bearing capacities of single piles with same diameter and same geology condition are calculated respectively. Finally in design of working drawing some plate-loading test in deep well on soft rock are carried out to determine the characteristic values of rock ultimate strength. In the design both the friction on the pile shaft and the end resistance on enlarged base are taken into account. The settlements of frame pillar are measured according to regulation. All the observation data show that the settlenents of all buildings are allowable.An economic comparison also has been made between the two types of pile foundation design. Averagely the cost of a pile can save about 1400 Yuan RMB using the design method discussed in this peper. Totally about 3,410 thousand Yuan are saved and 15 days are fulfiled the plan ahead of schedule .Owing to single-axis compressive strenth of soft rock is influenced by many factors, the laboratory strength of soft rock is too low to reflect the real bearing capacity of pile base embedded in soft rock. It is not appropriate and economical to use the laboratory strenth of soft rock to calculate vertical bearing capacity of pile base supported by soft rock. Therefore the author recommends that using plate-loading test data on soft rock in deep well calculate the end resistance of pile base as well as taking into account friction on pile shaft in the pile foundation design.