The methods for the underground installation of cables can be categorized as “Open-cut’’ and ‘’No-dig’’:
- Direct burial: This is the simplest, cheapest, quickest approach and involves digging up the sidewalk (or the first traffic lane, if the sidewalk is too narrow), laying the cables at a soil depth as recommended by standards and restoring the surface. This method involves the least derating of cables due to good heat dissipation. However, it offers the least protection from physical damage, and maintenance or expansion would require digging up each time.
- Semi-direct burial: The difference with direct burial is that the cables run through flexible HDPE conduits which enhance protection from physical damage and make it easier to replace damaged cables or add new ones through simple pulling through hand-hole or manhole covers at either end. However, both construction and cable costs go up (heat dissipation is lower due to the conduits).
- Concrete encased duct bank: The difference with semi-direct burial is that a concrete encasement is built on all sides of the duct bank to provide the maximum mechanical protection, and prevent damage to nearby cables during a short-circuit. However, both construction and cable costs go up (heat dissipation is further reduced due to the concrete encasement).
- Horizontal Directional Drilling: This uses a drilling machine placed at one end of the stretch to be covered ( e.g. across a busy street) which bores an underground hole through to the other end. Thereafter a HDPE conduit is pulled through the hole, and finally cables are pulled through the conduit. This offers similar protection from damage and requires the same cable derating as Semi-direct burial but has higher construction costs than all open-cut methods. However it disrupts public transportation the least.
- Pipe Jacking: This uses a hydraulic pipe jacking machine placed at one end of the stretch to be covered (e.g. across a busy street or culvert) which pushes a large steel pipe through to the other end. Thereafter, the excavated soil is removed from within the pipe. Then conduits are pulled through the pipe and the empty space between the conduits and the pipe is packed with mortar. Finally cables are pulled through the conduits. This method offers the least disruption and the highest physical protection and ease of maintenance but is also the most expensive in terms of construction costs and cable derating.
In summary, the selection of installation method involves tradeoffs between safety, disruptions to public transportation, space availability, duration and complexity of construction, ease of maintenance on the one side and costs on the other.
Reference: Power Cables Handbook, Volumes 2 & 3, Conversion of Overhead Lines to Underground Cables, Copyright@2014, International Copper Association South East Asia Ltd. And Provincial Electricity Authority, Thailand