Hard rocks, water seepage: LTA flags unexpected construction challenges along North-South Corridor

SINGAPORE — Construction of the North-South Corridor (NSC) has hit unexpected ground conditions at Novena and Thomson, complicating works as the project enters its most complex engineering phase, the Land Transport Authority said.

In Novena, tunnelling crews encountered a mix of materials, including hard rocks, rather than the more uniform and softer clay material previously anticipated. This caused a change in construction method.

In the Thomson area, the hard rock layer was closer to the surface than expected and large boulders were also found. This slowed progress because more work was needed.

At a media briefing on Sept 8, LTA said that such conditions were fully detected only during excavation. Earlier investigations had been constrained by live traffic, utility networks and existing MRT infrastructure.

Ground conditions are among the challenges adding to the complexity of construction, which is being carried out close to operational rail lines, major roads, utility lines and densely built-up areas.

Asked if these engineering challenges would delay the completion of the corridor, a spokesperson said LTA is monitoring the progress of the works and will give an update on the completion timeline in due course.

LTA had previously announced that the NSC would be ready in phases from 2027, with the northern viaduct from Admiralty West to Lentor Avenue and the road tunnel from Lentor Avenue to East Coast Parkway to be ready in 2029.

During the media briefing, LTA detailed the challenges encountered in Novena, where the road tunnel is being built directly above and below the operational North-South MRT line.

While the southbound tunnel above the MRT tracks has been completed, progress on the northbound tunnel beneath the line has been slowed by mixed rock formations that are harder to dig and underground water seepage that can weaken the structure.

With active train infrastructure running just 1m overhead, LTA pivoted to a different excavation strategy to prevent ground movement and safeguard the tracks.

Instead of excavating the full tunnel width at once, workers are digging it section by section across the width. Work in each section is monitored closely and stabilised before excavation begins in the adjacent section.

Each section of excavation advances the tunnel by 2m in length, less than the 3.3m achieved when engineers built the southbound tunnel using the tunnel box-pushing method.

Also known as box jacking, the construction technique involves a system of powerful hydraulic jacks pushing a rectangular box into the ground material to form the southbound tunnel.

In the Thomson area, unexpectedly harder rock formations were found around 10m closer to the surface than anticipated.

Workers also had to remove large boulders before they could construct reinforced concrete wall panels that outline the tunnel.

Installing these wall panels can take more than four weeks, longer than the typical timeline of one week.

Besides the challenges in Novena and Thomson, complex diversions of gas, water and telecommunication lines have slowed construction of the viaduct in the north, in Gambas Avenue and Woodlands Avenue 8.

In some parts, the rock-level underground was found to be higher and harder than expected. This caused delays in laying the foundations.

This impacted subsequent works, like installing columns and segments, which can only be done after foundation works are completed, LTA said.

The North-South Corridor construction site in Gambas Avenue on Sept 8.

Despite the setbacks, over 90 per cent of foundation works for the viaduct in the north are now completed, said Ang Pui Boon, LTA's deputy group director of the NSC.

The focus is now on works such as the installation of sections over MRT tracks and major road junctions.

The major segment installation works can only be done during four- to five-hour windows overnight to minimise disruption to road users and bus services, said LTA.

As the heavy foundation works are largely complete, it expects construction noise to decrease further as most of the upcoming works are underground. The need for multiple traffic diversions will also be reduced, it said.

Originally planned as an expressway in 2008, the NSC was redesigned as a "transit priority corridor" in 2016.

Spanning 21.5km, it is made up of an 8.8km viaduct, 400m of ground-level roads and a 12.3km underground road tunnel.

It connects Admiralty West to the East Coast Parkway, cutting travel times to the city from towns like Sembawang, Yishun, Ang Mo Kio, Toa Payoh, Novena and Rochor.

Once completed, pedestrian and cycling paths, dedicated bus lanes, and community spaces will be added to the repurposed roads above ground.

Work on the NSC started in 2018 and was originally slated for completion by 2027.

However, Covid-19-pandemic-related disruptions delayed the project by more than a year, pushing the completion to 2029.

This article was first published in The Straits Times . Permission required for reproduction.

新加坡——陆路交通管理局表示，南北走廊（NSC）在诺维娜和汤申的建设遇到了意想不到的地质条件，这使得该项目进入最复杂的工程阶段时，施工变得更加复杂。

在诺维纳，隧道施工人员发现隧道内土层成分复杂，包括坚硬的岩石，而非之前预想的较为均匀柔软的黏土。这导致施工方法发生了改变。

在汤姆逊地区，坚硬的岩层比预期更接近地表，而且还发现了巨大的岩石。这导致勘探进度放缓，因为需要进行更多的工作。

陆路交通管理局在9月8日的新闻发布会上表示，这些情况只有在挖掘过程中才被完全发现。此前的调查受到交通、公用设施网络和现有地铁基础设施的限制。

地质条件是增加施工复杂性的挑战之一，施工地点靠近运营中的铁路线、主要道路、公用设施线路和人口稠密的建成区。

当被问及这些工程难题是否会延误走廊的竣工时，一位发言人表示，陆路交通管理局正在密切关注工程进展，并将在适当时候公布竣工时间表。

陆路交通管理局此前宣布，国家安全通道将从 2027 年开始分阶段建成，其中从海军部西到伦多大道的北部高架桥和从伦多大道到东海岸公园大道的公路隧道将于 2029 年建成。

在媒体简报会上，陆路交通管理局详细介绍了诺维娜遇到的挑战，因为那里修建的公路隧道正好位于运营中的南北地铁线路的正上方和下方。

虽然地铁轨道上方的南行隧道已经完工，但线路下方的北行隧道由于混合岩层难以挖掘以及地下水渗漏会削弱结构，进展缓慢。

由于上方仅 1 米处就有正在运行的列车基础设施，陆路交通管理局 (LTA) 采取了不同的挖掘策略，以防止地面移动并保护轨道。

为了避免一次性挖掘整个隧道宽度，工人们采取分段挖掘的方式。每一段的施工都受到严密监控，并在相邻段开始挖掘前确保其稳定。

每次开挖只能推进隧道 2 米，比工程师采用隧道箱顶推法建造南行隧道时推进的 3.3 米要少。

这种施工技术也称为箱形顶推法，它利用强大的液压千斤顶系统将一个矩形箱体推入地下材料中，从而形成南向隧道。

在汤姆逊地区，人们意外地发现，在距离地表约 10 米处，存在比预期更坚硬的岩层。

工人们还必须先移走大块岩石，才能建造勾勒出隧道轮廓的钢筋混凝土墙板。

安装这些墙板可能需要四个多星期，比通常一周的工期要长。

除了诺维娜和汤申的挑战之外，煤气、水和电信线路的复杂改道也减缓了北部甘巴斯大道和兀兰8道高架桥的建设。

在某些区域，地下岩层比预期更高更硬，这导致地基施工延误。

陆路交通管理局表示，这影响了后续工程，例如安装柱子和构件，这些工程只能在基础工程完成后才能进行。

9月8日，甘巴斯大道南北走廊施工现场。

陆路交通管理局国家安全委员会副组长洪培文表示，尽管遭遇挫折，但北部高架桥的基础工程目前已完成 90% 以上。

目前的工作重点是地铁轨道和主要道路交叉口上方安装路段等工程。

陆路交通管理局表示，主要路段安装工程只能在夜间四到五个小时的窗口期内进行，以最大程度地减少对道路使用者和公交服务的影响。

由于大型基础工程已基本完工，预计施工噪音将进一步降低，因为接下来的大部分工程都在地下进行。该公司表示，同时，多次交通改道的需求也将减少。

国家安全走廊最初于 2008 年规划为高速公路，后于 2016 年重新设计为“公交优先走廊”。

该工程全长 21.5 公里，由 8.8 公里的高架桥、400 米的地面道路和 12.3 公里的地下公路隧道组成。

它连接海军部西和东海岸公园大道，缩短了从三巴旺、义顺、宏茂桥、大巴窑、诺维娜和罗卓等城镇前往市区的旅行时间。

建成后，改造后的道路上方将增设人行道、自行车道、专用公交车道和社区空间。

国家安全委员会 (NSC) 的建设工作于 2018 年启动，原计划于 2027 年完成。

然而，新冠疫情相关的干扰使该项目延误了一年多，竣工日期推迟到 2029 年。

本文最初发表于《海峡时报》。转载需获得许可。