MRT Blue Line Extension Project Field Trip

PMI Thailand Event : Field Trip to the MRT Blue Line Extension Project: Hua Lamphong – Bang Khae and Bangsue – Tha Phra Sections (Contract No.2)

Date and Time : Tuesday 25th August 2015 from 1.00 pm – 4.30 pm

Host : CH Karnchang PCL., Tha Phra Field Office

Speaker : Dr. Sopat Voravivatana, Assistant Project Manager

Compiler & Editor: Prasith Ongthawekiat

Introduction

This project is a 27 KMs with 19 stations (i.e. 4 underground plus 15 elevated stations) extension of the existing 21.7 KMs with 18 stations blue line subway of the MRTA (Mass Rapid Transit Authority of Thailand). The extension, crossing Chao Phraya River, mostly covers the western area of Bangkok in addition to the existing eastern coverage and will complete the blue line loop.

There are 5 contracts awarded for this project. Contract no.1 and 2 extend the underground line from existing Hua Lamphong terminal subway station whilst contract no.3 and 4 are the aboveground lines ramped up from existing Bang sue terminal subway station to the elevated levels including depot and park and ride buildings. Contract no.5 and 6 are the last separated contracts for all track works and M&E works respectively. This field trip visits contract no.2 site which is carried out by CH. Karnchang PCL. This contract no.2 could be the most difficult design and construction part of the entire project. East side of the site situated adjacent museums near Rattanakosin Island which is the area of very high historical significance of the Bangkok’s Phra Nakhon District where construction of elevated structures is not allowed and high underground construction technology is required to build the tunnel penetrating 10m below the bottom of the Chao Phraya River.

Cut and Cover Tunneling (C&C)

The 2.6 KM distance of contract no.2 works started from the open cut excavation at the area called “cut and cover” part, to construct base slab (wall) and erect the Tunnel Boring Machines (TBM) to start launching of the machine for both western and eastern bound from this point by pushing itself from base slab (wall). This area was also designed to construct the transition tunnel that ramps up from underground level to join the elevated level towards the Tha Phra station of another contract.

Tunnel Boring Machine (TBM) Tunneling

Boring of the tunnel starting from the cut and cover launching area has been done by a Japanese technology shield machine using earth pressure balance technique. The machine is designed for operation in soft soil condition with underground water. Front earth pressure plus water pressure will be balanced with the excavated earth pressure accumulated in the cutter head chamber at all times during drilling ahead by control gate opening to properly release the excavated soil to the back of the machine via the spiral axis escalating the soil to a belt conveyor hauling system towards the pit at the end for soil disposal.

Tunnel Segments production and installation

A tunnel cylindrical unit was designed to be 0.3 meter thick and 1.2 meter long and made of pre-cast reinforced concrete. Each machine’s penetration will allow the installation of one tunnel unit at a time. The machine will move ahead by pushing at the last installed tunnel unit. Balance pushing will allow the machine to go straight while unbalance pushing will allow the machine to turn left or right as required.

One unit of tunnel cylinder includes 6 arc segments to be installed to form a ring with inner diameter of 5.70 m or outer diameter of 6.0 m. The manually operated robotic arm equipped inside the TBM will grab each typical segment to sequentially erect the piece no.4,3,5,2,6 and finally fill in the last shortest piece no.1 to complete the ring shape. Curve bolts (with its nuts) will fix each sectional joint to tighten the ring and another curve bolts will fasten each longitudinal joint of a 1.2m cylindrical unit to the one installed before.

Erection of all other associated facilities, e.g. earth disposal belt conveyor, material supply trolley, gangway pipe works, lighting, cabling and others inside the concrete units will then follow any extension of the tunnel.

Tunnel alignment and grouting

Some tunnel unit may not be a straight segment used for straight tunnel line. Tapered segments will be used to form a curved tunnel line where it is necessary for a train to turn, with some radius, right or left from the straight tunnel line. Combination of both segment types can be used to get designated radius where applicable.

By the necessity of boring a bit larger diameter than the installed tunnel cylinders, the TBM always leaves a gap between earth tube and the segment. The machine is then equipped with tail grouting system to provide concrete grouting to fulfill the gap in order to avoid damage caused by any subside of the surrounding earth and stabilize the tunnel to the equilibrium of surrounding pressure.

Construction of two underground stations and one intervention shaft

As mentioned before regarding very high historical significance of Rattanakosin Island, the excavation to accommodate the construction of Sanam Chai underground station (coded as BS12) cannot be done with open cut method occupying site wide area. CH Karnchang Pcl., hence, introduces the pipe roofing technique by installing a raft of large pipes to act like a roof structure to cover the site and support the roads with heavy traffic above whilst below excavation is underway. This method minimizes occupancy of aboveground site area and prevents worse traffic flow.

Issaraphap station (coded as BS13) is slightly smaller and shallower than Sanam Chai station and could be less complicated than Sanam Chai station in terms of construction techniques.

Intervention shaft (coded as IVS3) located in between the two aforementioned underground stations has two main functions as follows:-

Engineering function: The shaft acts as a pressure relief valve that ventilates the air pressure arising from the running of the train inside the tunnel like a piston in a cylinder.
Safety function: The shaft acts as an emergency exit for passengers when evacuation from the train is required.

This trip did not visit Issaraphap station and intervention shaft so this article skips describing further details.

Construction scheduling and tracking

The design and construction of this contract no.2 commenced since 4th April 2011 and will end on 5^th June 2016. Time chainage Diagram has generally been used for scheduling and tracking of a project that activities must progress in relation to direction and distance like the construction of roads, railways, pipelines, tunnels, transmission Lines etc. plus activities that must progress in each location along the way to match the scheduled arrival of said linear works.

This diagram technique has also been used by the contractor to plan and control this project. Vertical axis represents the interval of times and horizontal axis represents the interval of chainages. The bars represented activities for each work packages (i.e.C&C, BS13, IVS3, BS12) and lines represented activities for western bound and eastern bound tunnel lines) are plotted into the grids within the two axis. The contract no.2‘s diagram below reports approximately 76% progress as of June 2015.