Special ground anchors in geotechnical engineering

From past to present, the deep excavations have been needed in various places around the world. Due to the requirements of the infrastructures, deep excavations and shoring systems which are ones of the important subjects of the geotechnical engineering are provided the construction of the deep structures. Although, the cost of slope method is more lower than the other deep excavations method, it cannot be applicable in crowded areas due to the neighbourhood structures, infrastructures, subway lines, etc. Therefore, the construction of shoring system is inevitable in order to excavate deep foundation levels in crowded areas. The shoring systems can be designed as cantilever or horizontal supported. In Turkey, there are many types of shoring systems which are applicable, for instance, the bored pile shoring system, reinforced shear wall shoring system, shotcrete wall shoring system, diaphragm wall shoring system, sheet pile shoring system, etc. Generally, the shoring systems need horizontal supports like prestressed ground anchor, soil nail, steel strut, reinforced concrete beam and tie-back with respect to adjacent structures, soil/rock conditions and their service life. The most preferred horizontal support structures which are constructed in Turkey are prestressed ground anchors due to the low cost, more experience, and applicable for the almost all soil and rock conditions. The prestressed ground anchor can be defined as structural component placed in soil or rock that is used to transfer an prestressed load into the ground in order to support vertical structures. The basic components of the traditional prestressed ground anchor are anchorage, free (unbonded) length and bonded length. The unbonded length is that part of the prestressing steel that is free to extend elastically and transfer the prestressed load to the bonded length of the anchor. The bond length is defined as prestressing steel that is bonded to the grout and transfer the applied prestressed load to the soil/rock. An important development in ground anchor technology have taken place since 1980s, has been that single bore multiple anchor systems. The construction of single bore multiple anchors which are one type of ground anchors have been started to use in deep excavations and shoring systems. The concept of single bore multiple anchor system have allowed an important increase in anchor ultimate load capacity in soils. Especially, in soils which have poor engineering properties, the single bore multiple anchors provide more effective solutions than traditional prestressed ground anchors and contribute the project costs and schedules. The subjects of the second part of this thesis have been deep excavations and shoring systems. In that part, the types and general principals of shoring system and methods of deep excavation have been mentioned. In third section, the types and component of traditional prestressed ground anchors have been described. Then, the method of construction of ground anchors have been mentioned and acceptance criterias of testing of them have been explained. After that, soil investigation works and design principals of ground anchors have been mentioned. In fourth part of this study, the single bore multiple anchors which are one of the special ground anchors have been explained and the main similarites and differences between traditional prestressed ground anchors and single bore multiple anchors have been presented. Single bore multiple anchors have included several anchor units in same bore, each of them has its own short bond length. The main difference between the single bore multiple anchors and traditional prestressed ground anchors is that the single bore multiple anchors have bonded length at different locations in drilled hole. Located of the unbonded length at different locations has provided that concentrated of shear stress around bonded length at different part of drilled hole. It has provided the division of the prestressing load into the each anchor bond length. Therefore, the soil have been exposed to less shear stress (divided shear stress) due to the transfer of the prestressing load to the different locations. In this way, the excessive prestressing loads can be carried by the anchor system. At fifth part of this thesis, a case history study have been analized and evaluated. Soil investigation works and soil and rock profile of the investigation area, designed shoring sytems, performance test results of single bore multiple anchors and comparison of traditional prestressed ground anchors and single bore multiple anchors have been mentioned, respectively. In order to constructed the deep foundation of the constructions in site, some soil investigation works have been performed. In the scope of the soil investigation works, 29 boreholes have been conducted in investigation area. Standard Penetration Tests have been performed during the borehole works. The disturbed and undisturbed samples which had been taken from the boreholes have been sended to the Soil and Rock Mechanics Laboratory in order to perform some laboratory tests. The result of the laboratory tests have been evaluated in accordance of the Soil Mechanics and Geotechnical Engineering Principals. The components of the shoring system which are horizontal and vertical support structures have been designed in considiration of these laboratory test results. Diaphragm wall type shoring system has been selected as a vertical support structures in order to provide impermeability. These diaphragm walls had been planned to support with two rows of traditional prestressed ground anchors. However, in the investigation area, the traditional prestressed ground anchors would have carried approximately ultimate load of 280kN according to the some basic assumptions. This situation could cause the more project cost and inefficient project schedule. In order to proof the ultimate load capacity of the single bore multiple anchors, at the beginning of the project, 10 performance tests have been performed. Then, the results of these tests have indicated that the ultimate load capacity of single bore multiple anchors have been between load of 600kN and 1050kN with respect to the analysis of elastic movements, permanent movements and creep test criterias. No load loss on test anchors during the creep tests was encountered within a normal time period at maximum test load. This system have provided the avoiding the progressive debonding. According to the results and evaluations of performance tests, instead of traditional prestressed ground anchors, the construction of single bore multiple anchors has been preferred. Taking everything into account, it can be seen that the single bore multiple anchors ultimate load capacity have been more than two times of ultimate load capacity of traditional ground anchors at the same soil conditions. For the traditional ground anchors, anchor bonded length longer than 10.00m do not contribute the ultimate load capacity of anchors. However, using the single bore multiple anchor system, the bonded length of anchor system can be extended. Besides, the single bore multiple anchors have a larger factor of safety against creep than traditional ground anchors. The stress distribution during the prestressing of anchor tendons is non-uniform due to general incompatibility between elastic modulus of the anchor tendon, anchor grout and the ground. In the majority of traditional ground anchors, the bond stress is focused at the proximal end of the bonded length, while the distal end of the fixed length remains stress free. As prestressing load is increased, the bond stress at proximal end of the bonded length along either the tendon and grout interface or grout or ground interface is exceeded. Then, the bond stress reduces to a residual value at that location and movement occurs. Using single bore multiple anchor system have permitted the transfer of prestressed load to the distal end. Finally, it can be easily seen that instead of two rows of traditional prestressed ground anchor systems, one row of single bore multiple anchor system can be constructed as a horizontal support component. So, this situation have been provided much less project cost and more efficient schedule.