Strong tower foundations are essential for the stability of towers, especially in adverse weather conditions such as hurricanes and strong winds. Tower foundation designs should be customised to make them suitable for the local terrain and reduce the time requiÂÂrÂed for construction. Over the past 20 yeaÂrs, tower foundations have evolved significantly. In India, some of the comÂmoÂnly used tower foundations are grillage, precast, reinforced cement concrete (RCC) spread, pile type and plain cement concrete (PCC). Their choice depends on the project-wise requirements.
Power Line presents an overview of tower foundation designs and their laying techniques…
Micropiling
One of the key emerging foundation designs is micropiling. Conventionally used for laying the foundation for high-rise buildings, micropiles have gained traction in the installation of transmission towers. Following the success of transmission projects developed on miÂcro piles, the design has been replicated in several transmission projects. MicroÂpiling has gained popularity in congesÂted environments due to its no-noise and low vibration features. These microÂpiles can be used in a wide range of geotechnical conditions and that makes thÂem an ideal option for transmission projects for diverse geograÂphical landscaÂpes, including deserts, mounÂtains and mariÂne environments.
Micropiling was undertaken in Kerala for monopole transmission towers. The transmission line project was built 60 years ago and underwent an upgradÂation from the 66 kV to the 132 kV level.
Grillage foundation
A grillage foundation consists of several layers of beams made of steel, precast concrete, or timber (usually laid at right angles to each other) used to disperse heavy point loads from the superstructure to an acceptable ground-bearing pressure. It is suitable in situations where the foundation of a structure is constrÂucted as a shallow-type foundation over wet or loose soil. The deep foundation method is typically used in loose soil, whereas the grillage foundation is recommended when constructing a deep foundation is not possible. This type of foundation is considered when the foundation depth is restricted to 1-1.5 metres due to geographical barriers. It can also sustain and transfer heavy loads from the structure to large areas.
The grillage foundation can be classified into two categories, depending on the material of construction – steel grillage foundation and timber grillage foundation. The steel grillage foundation consists of two- or more layers of steel beÂaÂÂms, with one or two-tier rolled steel emÂbedded in cement concrete. MeanÂwhile, the timber grillage foundation is best suited for lightweight structures, in areÂas where the soil-bearing capacity is not more than 6 tonnes per square metre. A brick or block masonry wall is constructed over this foundation. It is also quite suitable for areas facing the issue of waÂterÂlogged grounds. It is used to support structures that are designed to traÂnsfer and distribute heavy concentrated loads on soil and that have a low bearing capaÂcity. It is quite useful in firm soil areas where concreting is costly. It is a factory-made foundation that takes up less construction cycle time.
Precast foundation
This type of foundation is built off-site and is then transported to the installation location. The foundation is manufactured in factories in a controlled casting environment. It is also quite advantageÂÂoÂus when it comes to controlling the mix, placement and curing, allowing easier and more stringent quality control.
Moreover, weather changes do not influence or delay the execution of a project. Precast foundations are quite sustainable, efficient and economical. They are especially useful in areas where the perÂiod of construction is limited. The dimeÂnÂsions of the foundations are governed by the uplift capacity of the cranes.
HooÂks are provided for lifting.
Precast foundations offer several advantages. Since materials for multiple projects can be bought in advance by a pre-casting entity, the project developer can take the advantage of quantity discounts and lower costs. When using precast foundations, a small workforce is requiÂrÂed with a minimum skill set. Once on-site, this type of foundation can be insÂtallÂed immediately as there is also no waiting time required for it to gain
strenÂgth. The modularity of precast foundations quickens the installation process. Accelerated curing leads to a significant improvement in the strength gain and reduces the time between casting the part and putting it into service. The process is tightly controlled, from material selection to consolidation to curing, creating extremely durable concrete foundations.
RCC foundationÂ
RCC-spread foundations, made of an RCC base slab or mat, can be designed for a vaÂriÂety of soil conditions and may be used in locations where it is not possible to lay cement concrete footings. Various kinds of RCC-spread footings can be designed for tower foundations – single-step type, multiple-step type, and chamfered-step type. It is structurally the best, even though it can be costly. RCC-spread-type foundations are technically and economically the most favourable, especially when there are heavy loads on the foundation and the soil is of poor quality.
PCC foundation
PCC-type foundations consist of a plain concrete footing pad with a reinforced chimney. Pile-type foundations are used when the soil is weak or for special applications like riÂver crossing towers. The stub of the toÂwer is anchored to the bottom of the pad (either pyramidal in shape or steppÂed) by rods and the chimney. Although stepped footings require less shuttering material, they need more attention during construction in order to avoid cold joints between the steps. Moreover, pyÂraÂmidal footings entail a costlier framework.
Direct embedment foundation
Direct embedment foundations are used for various structures including towers. The process involves digging a hole in the ground, followed by the placement of the base of the pole into the hole. This type of embedment eliminates the need for a costly anchor pole base footing. The installation time required is also less, independent of the weather conditions. It also does not have any exposed base plates or anchor bolts.
Helical foundation
Helical foundations have a low demand for resources and can be executed and tested easily. The equipment used to install helical piles is often smaller and less expensive than that used in other piling techniques. It can be installed at any time of the year and can be reÂmoÂved quÂite easily as well. These piles are made of secured steel with low-fitted steel plates welded to the base. Tension and compression are two things that allow helical piles to stabilise objects in a variety of soil conditions. For insÂtaÂlling helical piles, machine-mounted hydraulic or electrically powered drills are used. Helical piles can be installed in confined spaces that are difficult to access. They also utilise a low-noise, minimal vibration installation method, preventing any disturbance to neighbours and protecting the existing landscape from damage. They can be used for repairing existing foundations or for building entirely new ones.
Conclusion
Designing and laying foundations is a challenging task given the risks posed by geology. Thus, selecting the most suitable foundation for towers is critical for the stability of transmission towers and lines. The lines that carry higher voltages also require bigger and heavier tower foundations. To meet the changing reqÂuiÂrements of utilities, it is essential to focus on building appropriate and roÂbust tower foundations.