2 Start of Construction
After this research has been done, Construction can start with the site preparation.
- Construction site clearing and excavation
- Construction site leveling and Grading
- Soil Compaction
- Building the Foundations
2.1 Compaction of soil layers
Compaction of soil layers which support loads is a must because it decreases settlement and consequently prevents undesired incidents. Tamping, rolling, and vibration are types of loads employed to compact soil layers. there are several machines used for compaction at construction site such as smooth wheel roller, sheep foot roller, rubber tire, crawler, and tamping plate compactor. Not only does the compaction of soil improve shear strength but also it declines soil permeability and compressibility.
2.2 Building the foundations for Steel structures and Equipment
In engineering, a foundation is the element of a structure which connects it to the ground, transferring loads from the structure to the ground. Foundations are generally considered either shallow or deep.
distribute the weight of the structure over a large area of soil;
avoid unequal settlement;
prevent the lateral movement of the structure; and
increase structural stability.
2.2.1 Shallow foundation
Often called footings, are usually embedded about a meter or so into soil. One common type is the spread footing which consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock.
Another common type of shallow foundation is the slab-on-grade foundation where the weight of the structure is transferred to the soil through a concrete slab placed at the surface. Slab-on-grade foundations can be reinforced mat slabs, which range from 25 cm to several meters thick, depending on the size of the building, or post-tensioned slabs, which are typically at least 20 cm for houses, and thicker for heavier structures.
2.2.2 Deep foundations
Used to transfer the load of a structure down through the upper weak layer of topsoil to the stronger layer of subsoil below. There are different types of deep footings including impact driven piles, drilled shafts, caissons and earth-stabilized columns. The naming conventions for different types of footings vary between different engineers. Historically, piles were wood, later steel, reinforced concrete, and pre-tensioned concrete.
2.2.3 Mono-pile foundation
A type of deep foundation which uses a single, generally large-diameter, structural element embedded into the earth to support all the loads (weight, wind, etc.) of a large above-surface structure.
Many monopile foundations have been used in recent years for economically constructing fixed bottom offshore wind farms in shallow-water subsea locations. For example, one of the projects I worked on, the Greater Gabbard single windfarm of the coast of England went online in 2008 with over 100 turbines, each mounted on a 4.74-meter-diameter monopile footing in ocean depths up to 16 meters of water.
2.3 Laying out underground Piping and (HV) Cables
Although piping acts as a separate discipline, underground piping is mostly done by the Civil Contractor. This mostly because the routing of underground pipes can not be engineered on Isometrics, which is common in piping engineering. Although research has been done upfront about the status of existing underground installations, routing is always somewhat uncertain.
The piping system is taken underground generally for the utility services like cooling water supply to various units and cooling water return to cooling Tower for line sizes normally 18 inch NB and above, other water services with big pipeline sizes, big oil supply lines and various sewer systems in the process units of the chemical, petrochemical and refinery type of plants.
The following are the common underground services in a chemical / petrochemical / refinery plants.
– Cooling water (line size normally ≥18″ NB)
– Fire Water
– Contaminated Rain Water Sewer from process catchment area.(CRWS)
– Oily Water Sewer (OWS)
– Sanitary system
– Storm Water
– Equipment drainage to slop tank
– Electrical cables
– Instrument cables
Some common types of underground piping are:
2.3.1 - Concrete Piping systems (Mostly sewer systems)
Concrete Pipe has stood the test of time as field data confirms service life can exceed well over 100 years. When it comes to structural integrity, hydraulic efficiency, versatility and ease of installation, reinforced concrete pipe out performs all other pipe products. Reinforced Concrete Pipe is manufactured in either a round or elliptical shape. In addition, manholes, inlets and outlets are factory-made to fit most any project. Square and Rectangular shaped Precast Concrete Box Sections are also available.
2.3.2 - GRP (Glassfiber reinforced Piping)
Typical applications are:
Fire fighting mains pipes.
Cooling water lines.
Sewerage and drainage lines.
Process water pipes.
Water purification pipes.
2.3.3 - Underground Steel piping (U/G)
Piping should not be buried or installed underground when it can be reasonably avoided.
Major applications for buried piping are generally cross-country pipelines where security and safety justify burial. Here security implies the difficulty of sabotage and intentional damage to a buried pipeline vis-a-vis an aboveground pipeline. Safety implies the safety of the surroundings (environment / population centers) due to unintentional or accidental damage of the buried pipeline vis-a-vis an aboveground pipeline.
Other major applications for buried piping are firewater piping, critical piping running across the entire plant installation with certain sections aboveground and certain sections underground. Underground sections would generally be routine movement areas including road and rail crossings.
Plastic pipes although having higher corrosion resistance to untreated water have much lower mechanical strength compared to carbon steel pipes when subjected to forces such as impact and vibration.
Problems Associated with Buried Piping:
-Buried Steel pipes are subjected to external corrosion despite mitigating measures such as external coating and cathodic protection
-Draining, cleaning of buried pipes is difficult compared to aboveground pipe.
-Leak detection and repair of of buried pipe is a difficult and expensive exercise. Although modern buried pipeline leak detection systems are available now, they are very expensive to install.
2.3.4 - Underground (HV) Cables
Although Electrical acts as a separate discipline, underground cable installation is mostly done by the Civil Contractor, in close co-operation with the electrical contractor.
Mostly, three types off cable laying can be identified:
184.108.40.206 Direct Laying Of Underground Cables
This method is the most popular as it is simple and cheap. The cables to be laid using this method must have the serving of bituminised paper and hessian tape so as to provide protection against corrosion and electrolysis. Rarely used in Refinaries or oil and Gas, specially NOT for HV Cables
A trench of about 1.5 meters deep and 45 cm wide is dug.
Then the trench is covered with a 10 cm thick layer of fine sand.
The cable is laid over the sand bed. The sand bed protects the cable from the moisture from the ground.
Then the laid cable is again covered with a layer of sand of about 10 cm thick.
When multiple cables are to be laid in the same trench, a horizontal or verticle spacing of about 30 cm is provided to reduce the effect of mutual heating. Spacing between the cables also ensures a fault occurring on one cable does not damage the adjacent cable.
The trench is then covered with bricks and soil to protect the cable from mechanical injury.
Simpler and cheaper than the other two methods
Heat generated in cables is easily dissipated in the ground.
To install new cables for fulfilling an increased load demand, completely new excavation has to be done which costs as much as the new installation.
Alterations in the cable network are not easy.
Maintenance cost is higher.
Identifying the location of a fault is difficult.
This method can not be used in congested areas with a lot of daily traffic.
220.127.116.11 Draw-In System
In this method, cast iron or concrete pipes or ducts are laid underground with manholes at suitable positions along the cable route. The cables are then pulled into the pipes from the manholes. Usually, an additional pipe/duct is also provided along with the three cable ducts for carrying relay protection connections and pilot wires. Distance between the manholes should be such that pulling in the cables is easier. At corners or while changing the direction of route, radius of the corners must be longer. The cables that are to be laid in this way need not be armoured but must be provided with the serving of hessian and jute in order to protect them when being pulled.
Repairs, additions or alterations to the cable network can be easily made from manholes without re-excavation.
In this method, as the cables need not be armoured, the cable jointing procedure becomes simpler.
Maintenance cost is quite lower.
Fewer chances of fault occurrence due to the strong mechanical protection provided by the system.
The initial cost is very high.
Due to unfavourable conditions for dissipation of heat, current carrying capacity of the cables is reduced.
18.104.22.168 Solid System
In this method, the cable is laid into troughing of cast iron, stoneware, asphalt or treated wood. When the cable is laid into the position, the troughing is filled with a bituminous of asphaltic compound and then covered over. Cables to be laid in this manner could be just lead covered as the troughing provides a good mechanical protection.
This method is very rarely used nowadays as it is more expensive and requires skilled labour and favourable weather conditions.
Example : Cable played under Asphalt
2.4 Erection and outfitting buildings
Because of the complexity of this topic, more information will follow in a later, separate Blog.
2.5 Layout access roads and railways
Already during the Construction phase preparation of Roads and Railways is started and Access roads are prepared.
However, almost always, finalisation off the Roads and walkways is left until the near end of the project. Roads can get damaged by heavy construction machinery, access to the underground soil might still be needed for last minute engineering changes, and walkways and lampposts might get damaged by lifting activities.