Crossing the railway corridor

Trenchless Advisor’s Peter Brown provides a case study to Energy Source & Distribution on how to prepare to re-distribute heavy load demands through an underground high-voltage cable.

Like all major infrastructure, electrical networks need to be upgraded over time or even new networks proposed and designed to improve the overall reliability for its customers. Many of these proposed projects include the installation of cables through busy residential and industrial areas, through parks and bushlands as well as across creeks and rivers, roads and railway corridors.

One such project situated in Sydney’s outer suburbia has proposed a new dedicated underground high-voltage 11kv feeder from a newly constructed zone substation to an existing high voltage overhead mains network. This project will create the ability to re-distribute heavy load demands currently faced in the area on all existing 11kV feeders at the current zone substation and improve overall network reliability to all customers.

As part of the route from the new zone substation to the existing high-voltage overhead, the new 11kV feeder will run beneath the footpath and driveways parallel to a residential road and then swing around to run below an unformed road, creek and marshlands and then continue under a rail corridor and drainage system to exist in close proximity to an existing pole. Due to the nature of the existing surface conditions being a creek along with the existing railway line, it has been proposed to investigate the inclusion of trenchless technology for the installation of the cable through this part of the network’s route.

We undertook an initial site visit so as to review the proposed alignment and design and to also understand the client’s requirements and limitations. Several options discussed with the client and with Dial Before You Dig plans available showing indicative locations for existing underground utilities, a more probable underbore with entry and exit locations was agreed on. This proposed crossing would still have to be confirmed and remain achievable once geotechnical information was available and accurate underground services mapped as well as all information gathered through the rail authority.

It was proposed that the crossing is to include one underbore of Ø450mm to install four Ø140mm conduits and two Ø63mm conduits. One conduit will house the initial installation of a 240mm2, three core copper, 11kV underground cable and the others will remain spares for possible future works and in case of emergency repair. The feasibility and concept design report for this installation shall take into account the geotechnical investigation results, existing utilities, risks and impacts, construction method and technical specifications along with the current rail authority, client and Australian Standards.

Due to the complexity and requirements needed to gain approval to cross the rail corridor using trenchless technology, including the safety aspects, we needed to compile a detailed and suitable feasibility study that complied to all the rail authorities requirements. Several key factors had to be investigated and detailed throughout the report. These included a detailed trenchless feasibility and concept design report; methodology as well as technical drawings and other required documentation. A submission to the local rail authority was also made for approval of the design prior to engaging a contractor to undertake the works. This information was needed to outline the most effective and appropriate trenchless method to be used for this part of the project. Sourcing, pricing and managing appropriate subcontractors to undertake geotechnical investigations, surveys and existing utility location investigation all of which were required for the gathering of vital information need for the feasibility, concept design and rail authority approval.

Before any works were undertaken, a timeline for seeking rail approval and even an approximate start date for construction was determined. As with most projects, there was very little time available to acquire the correct approval from the rail authority before the project was to be tendered and construction begun. From the start of the work until an ‘approval in principle’ was acquired, only seven months was allocated. This timeline had to include completion of all sub-contractors works and their reports supplied to Trenchless Advisor. The feasibility and concept design, technical drawings and track-monitoring plan then had to be compiled, allowing sufficient time within the timeline for the appropriate engineers and personnel from within the rail authority to review and comment on the information and hopefully obtaining the approval in principle. This would then allow the client to move forward with the tender documents and selection of a contractor to undertake the works.

Like any project, a few obstacles were encountered, one being the creek that weaved its way through the rail corridor and the inclusions of several large and small drains guiding the water flow from the surrounding areas. The drains especially were a point of concern, as they limited the exit location for the proposed bore and dictated the depth at which the bore was to be situated. To begin the required works, we organised for the survey and geotechnical investigation to be conducted, including a borehole on either side of the rail corridor to give an indication of the subsurface conditions. The information gathered during the survey and geotechnical investigation were vital for accurately assessing the chosen method and depths required for the underbore and prediction of possible settlement within the rail corridor together with allowing design of a track monitoring plan to be followed during construction. A detailed existing utility search was also undertaken allowing a true mapping of all assets in the area.

Once all the necessary information was collected, a preliminary investigation was begun including the compiling of an electronic bore plan to ensure the bore’s achievability. The information used to compile the electronic bore plan consisted of a wide range of data including expected ground conditions, existing utility location, preferred entry and exit sites specified by the client as well as all specifications and requirements put forward by the rail authority. Other information also important to the bore plan design included appropriate Australian Standards, HDD machine requirements, bend radius, bore depths and product specifications to ensure a safe and practical design was achieved.

The results of the preliminary investigation showed that a horizontal directional drilling (HDD) method was the most effective and appropriate for the requirements achieving a greater depth below the rail. The subsequent investigation and profile indicated a borehole that could be constructed within the silty clay ground conditions from relatively shallow entry and exit pits with minimal chance of settlement. It was also considered the safest method available when crossing the rail corridor and would cause less impact to the surrounding area and flora. The predicted ground settlement based on the ground conditions, depth below rail and method of installation where shown as being limited to approximately one millimetre.

A HDD method was chosen to install the conduits due to the distance of the underbore, existing ground conditions, and depth required under rail as well as ensuring all the client’s requirements were met. An in-depth assessment of the method incorporated with predicted settlement analysis together with planned installation of the bores and the track monitoring planned for construction show that underbores could be achieved with minimal risks. HDD has the ability to both start and finish at shallow depths while maximising the achievable depth under rail limiting the risks during construction.

In a review of the proposed conduit requirements the data shows that due to the depth of the underbore, method of installation, and expected loads, the PN10 Ø140mm and Ø63mm PE conduit was safe for use. A safety factor was also built in the design specifications to ensure a minimum of 100 years’ life is achieved with the product. The product was also preliminarily assessed against the effects of electrolysis and vibration with the expectation that any possible affects would be low.

All information was then collated and forwarded to the client for submission to the rail authority. Contact was kept between all parties during the submission and rail authority design review to ensure any issues, if arising, would be quickly dealt with. Five months into the works undertaken, an ‘approval in principle’ from the rail authority was received by the client. This fell well within the allotted time schedule and provided the client additional time to compile the required tender documentation and source a contractor.

Trenchless Advisor’s Peter Brown provides a case study to

Energy Source & Distribution on how to prepare to re-distribute heavy load demands through an underground high-voltage cable.

Like all major infrastructure, electrical networks need to be upgraded over time or even new networks proposed and designed to improve the overall reliability for its customers. Many of these proposed projects include the installation of cables through busy residential and industrial areas, through parks and bushlands as well as across creeks and rivers, roads and railway corridors.

One such project situated in Sydney’s outer suburbia has proposed a new dedicated underground high-voltage 11kv feeder from a newly constructed zone substation to an existing high voltage overhead mains network. This project will create the ability to re-distribute heavy load demands currently faced in the area on all existing 11kV feeders at the current zone substation and improve overall network reliability to all customers.

As part of the route from the new zone substation to the existing high-voltage overhead, the new 11kV feeder will run beneath the footpath and driveways parallel to a residential road and then swing around to run below an unformed road, creek and marshlands and then continue under a rail corridor and drainage system to exist in close proximity to an existing pole. Due to the nature of the existing surface conditions being a creek along with the existing railway line, it has been proposed to investigate the inclusion of trenchless technology for the installation of the cable through this part of the network’s route.

We undertook an initial site visit so as to review the proposed alignment and design and to also understand the client’s requirements and limitations. Several options discussed with the client and with Dial Before You Dig plans available showing indicative locations for existing underground utilities, a more probable underbore with entry and exit locations was agreed on. This proposed crossing would still have to be confirmed and remain achievable once geotechnical information was available and accurate underground services mapped as well as all information gathered through the rail authority.

It was proposed that the crossing is to include one underbore of Ø450mm to install four Ø140mm conduits and two Ø63mm conduits. One conduit will house the initial installation of a 240mm2, three core copper, 11kV underground cable and the others will remain spares for possible future works and in case of emergency repair. The feasibility and concept design report for this installation shall take into account the geotechnical investigation results, existing utilities, risks and impacts, construction method and technical specifications along with the current rail authority, client and Australian Standards.

Due to the complexity and requirements needed to gain approval to cross the rail corridor using trenchless technology, including the safety aspects, we needed to compile a detailed and suitable feasibility study that complied to all the rail authorities requirements. Several key factors had to be investigated and detailed throughout the report. These included a detailed trenchless feasibility and concept design report; methodology as well as technical drawings and other required documentation. A submission to the local rail authority was also made for approval of the design prior to engaging a contractor to undertake the works. This information was needed to outline the most effective and appropriate trenchless method to be used for this part of the project. Sourcing, pricing and managing appropriate subcontractors to undertake geotechnical investigations, surveys and existing utility location investigation all of which were required for the gathering of vital information need for the feasibility, concept design and rail authority approval.

Before any works were undertaken, a timeline for seeking rail approval and even an approximate start date for construction was determined. As with most projects, there was very little time available to acquire the correct approval from the rail authority before the project was to be tendered and construction begun. From the start of the work until an ‘approval in principle’ was acquired, only seven months was allocated. This timeline had to include completion of all sub-contractors works and their reports supplied to Trenchless Advisor. The feasibility and concept design, technical drawings and track-monitoring plan then had to be compiled, allowing sufficient time within the timeline for the appropriate engineers and personnel from within the rail authority to review and comment on the information and hopefully obtaining the approval in principle. This would then allow the client to move forward with the tender documents and selection of a contractor to undertake the works.

Like any project, a few obstacles were encountered, one being the creek that weaved its way through the rail corridor and the inclusions of several large and small drains guiding the water flow from the surrounding areas. The drains especially were a point of concern, as they limited the exit location for the proposed bore and dictated the depth at which the bore was to be situated. To begin the required works, we organised for the survey and geotechnical investigation to be conducted, including a borehole on either side of the rail corridor to give an indication of the subsurface conditions. The information gathered during the survey and geotechnical investigation were vital for accurately assessing the chosen method and depths required for the underbore and prediction of possible settlement within the rail corridor together with allowing design of a track monitoring plan to be followed during construction. A detailed existing utility search was also undertaken allowing a true mapping of all assets in the area.

Once all the necessary information was collected, a preliminary investigation was begun including the compiling of an electronic bore plan to ensure the bore’s achievability. The information used to compile the electronic bore plan consisted of a wide range of data including expected ground conditions, existing utility location, preferred entry and exit sites specified by the client as well as all specifications and requirements put forward by the rail authority. Other information also important to the bore plan design included appropriate Australian Standards, HDD machine requirements, bend radius, bore depths and product specifications to ensure a safe and practical design was achieved.

The results of the preliminary investigation showed that a horizontal directional drilling (HDD) method was the most effective and appropriate for the requirements achieving a greater depth below the rail. The subsequent investigation and profile indicated a borehole that could be constructed within the silty clay ground conditions from relatively shallow entry and exit pits with minimal chance of settlement. It was also considered the safest method available when crossing the rail corridor and would cause less impact to the surrounding area and flora. The predicted ground settlement based on the ground conditions, depth below rail and method of installation where shown as being limited to approximately one millimetre.

A HDD method was chosen to install the conduits due to the distance of the underbore, existing ground conditions, and depth required under rail as well as ensuring all the client’s requirements were met. An in-depth assessment of the method incorporated with predicted settlement analysis together with planned installation of the bores and the track monitoring planned for construction show that underbores could be achieved with minimal risks. HDD has the ability to both start and finish at shallow depths while maximising the achievable depth under rail limiting the risks during construction.

In a review of the proposed conduit requirements the data shows that due to the depth of the underbore, method of installation, and expected loads, the PN10 Ø140mm and Ø63mm PE conduit was safe for use. A safety factor was also built in the design specifications to ensure a minimum of 100 years’ life is achieved with the product. The product was also preliminarily assessed against the effects of electrolysis and vibration with the expectation that any possible affects would be low.

All information was then collated and forwarded to the client for submission to the rail authority. Contact was kept between all parties during the submission and rail authority design review to ensure any issues, if arising, would be quickly dealt with. Five months into the works undertaken, an ‘approval in principle’ from the rail authority was received by the client. This fell well within the allotted time schedule and provided the client additional time to compile the required tender documentation and source a contractor.