Helical piles

Geotechnical engineering has many application areas. These areas are deep and shallow foundations design, deep excavation support systems design, soil improvement projects, slope stabilitys analysis, off-shore geotechnics, underpinning, earthworks design, geotechnical earthquake enginnering and etc. In this thesis, helical piles which used for deep foundations design, deep excavation support systems design and underpinning design besides the conventional methods will be introduced. Helical piles were invented by Alexander Mitchell in early 1800's in England. First invented helical piles made of wood and they were used in lighthouses's foundations and piers. Modern helical piles are made of steel. Helical piles have one or more helical bearing plates and there is a distance between helical bearing plates. Helical piles use as single pile or pile group. Helical piles resist tension, compression and lateral loads. Helical piles widely used for foundations of energy lines and multi-story buildings, buoyancy control of pipelines, underpinning systems and as tiebacks for deep excavation support systems. Bored and driven piles are conventional methods of deep foundation systems and they are widely used. Helical piles are different from these methods and used in many developed countries (USA; England), but helical piles are not used in Turkey. Before the bored pile installation, soil is bored. Driven piles are drived into the soil. In order to install the helical pile into the soil, neither boring, nor driving process will not be used. Helical pile shaft is rotated into the soil by using a torque motor. Limit state analyses is used for to determine the ultimate bearing capacity of helical piles. According to the this method, The ultimate bearing capacity of helical pile is calculated with both individual bearing method and cylindrical shear method. The minimum value of these methods has been selected and this minimum value is the ultimate bearing capacity of helical pile. Moreover LCPC method is also used to determine the ultimate bearing capacity of helical piles. Also the average torque value which is obtained during the installation is used for to determine the bearing capacity of helical pile. Helical piles ultimate bearing capacities were calculeted using limit state analysis in many different soil types and different helical sizes within scope of this master thesis. It is observed that calculation methods directly depends on inter helix spacing. These observations and other results are evaluated in this master thesis. Helical pile axial compression and axial tension tests are conducted to determine the helical piles compression and tension capacity. Helical pile compression load tests are conducted in accordance with ASTM D1143. Helical pile tension load tests are conducted in accordance with ASTM D3689. Axial loading test results can be generally evaluated by Davisson method or 5% displacement method. Helical lateral load test is conducted to determine the helical pile lateral load capacity. Helical pile lateral load tests are conducted in accordance with ASTM D3966. Lateral loading test results also can be generally evaluated by Davisson method or 5% displacement method. Helical piles are also used in deep excavation support systems and slope stability systems design. Helical piles used as helical pre-stressed anchor, helical anchor or helical soil nail. Helical piles can use with shotcrete wall, sheet pile wall, bored pile wall and braced wall. Helical piles can resist lateral loads and because of the this reason, it can be used as a tiebacks in deep excation systems. It can be also used for retaining wall foundations and it can be used as strut. Deep excavation support system stability can be determine by the software which is based on finite element method. Helical piles are used for underpinning existing structures. Underpinning is done for repair of damaged foundations, to increase load carrying capacity. Helical piles used with underpinning brackets to underpin the existing foundations. There are two types of underpinning brackets. Plate bracket and angle bracket. Both of them can be used with helical piles. But angle brackest are most common bracket types. There many historical buildings which underpin using helical piles in USA. Helical piles are also used to underpin existing structures located near braced excavations. Helical piles can be used with grouting systems. Grout encloses the helical pile shaft and increases adhesion. If adhesion increases, bearing capacity will increase. Cement is used as a grout material. Helical piles are very effective solution for foundations which locate in expansive soils. The zone of soil which effect by expansive soil is called "active zone". This zone can swell and swelling pressure can damage to the existing foundation. Therefore helical pile must be extended to the stable soil.Swell pressure can be determined by odometer test. Helical pile's slender shaft is not effected by expansive soils. Helical bearing plates can resist pullout forces which occur by expansive soil. If proper equipments and procedures are provided, installation of helical piles is very simple. Helical piles usage growes because of this simplicity. Helical piles can be install variable soil conditions. The helical pile is rotated into the soil by application of torsion using torque motor. Helical pile installation equipments are torque motor, torque indicator, drive tool, drive pin and hydraulic machine. Hand installation equiments does not consist hydraulic machine. Reaction bar and foot control are used instead of hydraulic machine. Torque measurement during the helical pile installation is very important because helical pile bearing capacity can be calculated using the average torque value during the installation. There are two common ways in order to measure installation torque. First method is shear pin indicator. This method's disadvantage is torque measurement during the installation is not continuous. Second method is mechanical dial indicator. This method's disadvantages are it can be expensive and they are very sensitive. Helical piles can be effected by corrosion because they made of steel. Therefore helical pile must be protected against to the corrosion. There are three main methods for helical pile corrosion protection. First method is the zinc galvanizing. Zinc galvanizing helps protect steel by corrosion. Batch hot-dip galvanizing is most common galvanizing method. Second corrosion protection method is powder coating. Powder coating is less durable than zinc galvanizing. Both zinc galvanizing and powder coatings are coating protection methods. The third corrosion protection method is cathodic protection method and sacrifical anodes is most common method is cathodic protections.