採用水平導向鑽挖法—敷設四條132千伏的電纜管道
Installation of 4 nos. Cable Ducts by Horizontal Directional Drilling

Background
In October 2002, Hong Kong Electric Co., Ltd. awarded a contract to Wo Hing Construction Co., Ltd. (Wo Hing) for the installation of 4 nos. 132 kV cable ducts from Wong Chuk Hang to Deep Water Bay and also from South Bay to Chung Hom Kok. Wo Hing, in turn, engaged Utilities Construction Ltd. (Utilities) to be the specialist contractor to install four cable ducts using the horizontal directional drilling (HDD) method.

Construction
Being the specialist contractor, Utilities employed the HDD method to install the cable ducts from Chung Hom Kok Road to Headland Road. Each cable duct was 270 m in length with 300 mm diameter.
The sequence of the works is as follows:

1. Construction of launching and receiving pits
2. Pilot hole drilling - 150 mm diameter
3. Reaming the pilot hole from the bore size of 150 mm to 300 mm and then to 400 mm
4. Pulling back the HDPE pipes inside the bore hole
5. Grouting the gap between the HDPE pipe and the bore hole.

The HDD method was adopted by Utilities due to a number of reasons. First of all, there was a reservoir in the middle of the site that restricted the cable ducts to run across the boundary of the reservoir. There were busy roads adjacent to the site, such as Stanley Gap Road and Repulse Bay Road, and the traditional cut-and-cover method will have severe impact on the traffic and disturbance to the public. Hence, the HDD method, together with an advanced survey system, Paratrack, was proposed to solve the above problems. This method not only reduces the construction time but also minimises the traffic impact and disturbance to the public. Utilities used the PE 100, SDR 11 HDPE pipes as the cable ducts. As each pipe was 12 m long, they were joined together to form the 270m long cable duct by butt fusion equipment.

Utilities commenced the works in June 2003 to set up equipment on site. After four months of preparation, the drilling commenced in October. Each pilot hole took about 2 to 3 weeks to complete, and finally all were completed on schedule in April 2004.

Machinery
Paratrack 2, an advanced survey system, was adopted by Utilities to assist in drilling the four parallel and curve drill path at a depth of around 100 m below ground surface. This survey system is a magnetic tracking system employing state-of-the-art electronics with advanced magnetic tracking capabilities. It was deployed from entry to exit along the centerline to enable tracking of the bore accurately along 100% of its length.

Butt fusion equipment, including a control unit with a hydraulic system and a microcomputer, an automatic trimmer with a position sensor, a heating mirror and a chassis, was used to connect the HDPE pipes.

Challenges
The topography of the site posed a great challenge to Utilities. As the alignment of the cable ducts was designed to run from Chung Hum Kok Road to Headland Road, crossing underneath the hill with a reservoir on top, it was quite difficult for Utilities to drill the four parallel cable ducts in a curved alignment in an unknown underground condition. The advanced survey system was therefore employed to locate the position of the drill bit during the course of the pilot drilling.

Limited working space was another problem that Utilities came across. The hill within the site, together with the heavy-traffic road and a slope stabilization work nearby, resulted in a congested working space. Utilities consumed four months to set up the equipment before the commencement of the excavation.

背景
香港電燈公司於二零零二年十月批出一份合約予和興建築有限公司，分別在黃竹坑至深灣，以及南灣至舂坎角敷設四條一百三十二千伏的電纜管道。和興遂委托公建工程有限公司（下稱公建工程）為其中一項鑽孔工程的專業承造商，採用水平導向鑽挖方法敷設四條電纜管道。

建造方法
公建工程採用了水平導向鑽挖方法於舂磡角道至赫蘭道之間敷設四電纜管道。每條電纜管道長二百七十米，直徑為三百毫米。整個敷設電纜管道的程序概括如下：

1. 建造開挖豎井及接收豎井
   
2. 鑽挖直徑一百五十毫米的試驗導管
   
3. 把試驗導管的直徑由一百五十毫米鑽大至三百毫米，再擴大至四百毫米
   
4. 把聚乙炳烯喉管置於鑽挖完成的管道內
   
5. 在聚乙炳烯喉管和鑽挖完成管道之間的縫隙灌上混凝土
   

公建工程之所以採用水平導向鑽挖方法出於多個原因。首先，工地的中央有一個水庫，電纜管道不能敷設在水庫的範圍內。此外，由於工地附近有多條繁忙道路，如赤柱峽道和淺水灣道，故此不宜採用傳統的明挖隨填方法，否則工程會嚴重阻礙交通及對公眾構成滋擾。為此，公建工程採用水平導向鑽挖技術及先進的探測儀器－Paratrack克服以上困難。這技術不但可縮短建造時間，而且能減低工程對附近交通及公眾造成的影響。此外，公建工程還選用了PE 100 SDR 11的聚乙炳烯喉管來敷設電纜管道。由於每條喉管只得十二米長，故此需使用對接設備把喉管接駁至二百七十米長。

公建工程於二零零三年六月開始在工地裝置設備。經過四個月的準備工夫，鑽挖工程於同年十月展開。每條導管約需鑽挖二至三個星期，所有電纜管道最終如期於二零零四年四月鑽挖完成。

機械
Paratrack 2是一套先進的探測系統，協助公建工程在地面下一百米深的位置鑽出四條平行但呈曲線的管道。這套磁力追蹤系統備有最先進的電子器件，可透過磁力追蹤探測物件，公建工程在整個鑽挖過程都須依賴這套系統輔助。

公建工程還使用了對接設備接駁聚乙炳烯喉管，當中包括控制系統、液壓系統、微型電腦、自動修剪機、定位感應器、加熱鏡及底盤。

挑戰
是項工程的最大難題是工地地形對工程的阻礙。四條管道的定線介乎舂磡角道至赫蘭道之間，途中需穿越一座建有水庫的山下。由於地下狀況不明，敷設四條平行但呈曲線的電纜管道確有一定難度，公建工程遂採用先進的探測系統，確定鑽頭在鑽挖過程中的位置。

狹窄的工作空間亦是公建工程須克服的挑戰。工地內有一座山，鄰近又有繁忙的道路，還有正在施工的斜坡穩固工程。以上種種因由使工作空間狹窄，公建工程需要四個月的時間才能在工地設置所需的機械設備。