農業部農村發展及水土保持署臺北分署全球資訊網

2023全球卓越建設獎-環境類-金獎
Rehabilition of Province Highway 9 台9甲10.2K大崩塌災害復建工程
設計監造單位：豐華工程顧問股份有限公司
施工廠商：久旺營造股份有限公司、一亨營造股份有限公司
Question 1 General Description of Project：
A.What type of project is it? (i.e. Forest/River rehabilitation) 
This is a major disaster recovery project in northern Taiwan to restore the reservoir area from collapse and debris flows, which involves the restoration of streams, the rehabilitation of mountain forests, the maintenance of water cleanliness, and the restoration of transportation to and from indigenous communities. This project is located in Wulai District of New Taipei City, a famous hot spring resort in northern Taiwan. On August 8, 2015, Typhoon Soudelor brought with it a record-breaking rainfall (with a peak of 70mm and a total accumulated rainfall of 510mm), causing slope erosion, road collapse, debris flows, landslide dams and flooding in many areas of the upper Xindian River basin. Among them, the most serious was a massive landslide of up to 8.8 hectares in the upper reaches on Provincial Highway 9A at the 10.2K mark (Zhongzhi Village, Wulai District). Not only was the upstream road system damaged, such as Jinyanong Road and Zhongzhi Bridge No. 2, upon which the village's livelihood depended, but the large amount of sand and gravel also disrupted the only access road in the downstream area on Provincial Highway 9A at the 10.2K mark, turning Wulai into an isolated area. It is estimated that 860,000 cubic meters of sediment flowed into the reservoir area, affecting the drinking water of approximately 6.5 million people in northern Taiwan.
B.Location of project.
The site of this Rehabilitation of Provincial Highway 9 project was situated in the suburbs of Taipei, the administrative center of Taiwan, towards the Atayal indigenous community "Wulai" at the 10.2K mark.
C.Total area.
On August 8, 2015, Typhoon Soudelor caused the collapse of 8.8 hectares in the upper reaches on Provincial Highway 9A at the 10.2K mark (Zhongzhi Village, Wulai District). Typhoon Dujuan, which came close on the heels of Typhoon Soudelor on September 28, 2015, extended the area of the landslide to 10.6 hectares, endangering the safety of residents in the vicinity and causing renewed outflows of sediment blocking roads at the 10.2K mark on Provincial Highway 9A and destroying precious wildlife habitats.
D.Total number of phases.
This project took about 3.5 years to complete from May 2016 and was then handed over to the competent authority (the Taipei Water Management Office of the Water Resources Agency under the Ministry of Economic Affairs,R.O.C.) on January 14, 2020 for subsequent maintenance, management and monitoring operations. Overall, this rehabilitation project was broken down into three phases, with a total of six works carried out:
1.Gully controls in 2016: The "massive landslide treatment project on Provincial Highway 9A at the 10.2K mark" in the most upstream part of the gully was undertaken, while an investigation and planning exercise was initiated to identify the collapse mechanism and assess the works by years and by phases.
2.Emergency gully controls in 2017: To prevent the collapse in the upper reaches of the rehabilitation area from continuing to grow in size, affecting road access and the safety of inhabitants, three projects were carried out, including the landslide treatment project on the right bank upstream of the Jinyan Bridge, the landslide treatment project on the right bank downstream of the Zhongzhi Bridge No.2, and the potential debris flow torrent New Taipei DF230 control project, in which the dam, which was the key to restoring stability to the gully, was also completed at this stage.
3.Rehabilitation in 2018: To expedite the stabilization of the slopes on both sides of the gully and the rehabilitation of vegetation, the second phase of the potential debris flow torrent New Taipei DF230 control project and the second phase of the landslide treatment project on the right bank downstream of the Zhongzhi Bridge No.2 were undertaken. Given the future use needs of the landowners, a phased arrangement of the on-site fragmented rocks was adopted to effectively disperse the run-off water and reduce erosion.
E.Condition of the environmental area before rehabilitated and restored. (Land, air, water, and ecosystems)
Environmental conditions before rehabilitation and restoration:
1.This project lay in a water resources conservation area, where run-off water flashes down the Nanshi River and then into the Qingtan Weir of the Xindian River in New Taipei City, making it a major source of drinking water for approximately 6.5 million people in northern Taiwan. The exposed collapsed slopes continue to cause the erosion of sediment, increasing the turbidity of the water source, much to the detriment of the drinking water quality of the residents.
2.Wulai is one of the most important sightseeing spots in northern Taiwan. On top of the indigenous attractions, the Wulai Waterfall, the Naitong Waterfall and the Wulai Trolley, the Wulai Hot Springs are also widely known at home and abroad. In the wake of the disaster, the Atayal people in Wulai was isolated, and economic activities and the tourist industry were heavily hit, and the inhabitants lost their livelihood.
3.Previous to the landslide, the area was home to some of the most valuable wildlife, with Formosan macaques, Reeves’s muntjacs, Formosan blue magpies, wild boars and pangolins frequently sighted, as well as many animal trails. However, all the wildlife has disappeared overnight following an avalanche of mud and stones.
Environmental conditions after rehabilitation and restoration:
1.After 3.5 years of work on this project, 37 gravel sand slide control facilities were installed to effectively control the stream centerline and adjust the stream bed gradient, stabilize the slopes on both river banks, and protect the exposed collapsed land with beam-free frames, with native plant seeds used for hydroseeding, leading to excellent vegetation restoration, reducing erosion and restoring the cleanliness of the water source. In addition, horizontal collector pipes with 87 boreholes (6,800m) were installed by exploiting the topographic drop, effectively lowering the water table and recharging the water source with discharged groundwater.
2.Protecting the only access route for indigenous people on Provincial Highway 9A allows for the smooth transport of agricultural products and access for tourists, thereby reviving the local economy.
3.After the stabilization of the gully and the rehabilitation of the vegetation on the slopes, the wildlife habitats gradually came back to life, with Formosan macaques, Formosan blue magpies, wild boars, pangolins and Reeves’s muntjacs populating the area once again.
F.Studies carried out to indicate the condition of the environmental area.
This project conducted a 3-year survey from 2017, with the results of the research described below:
1.Analysis of rainfall frequency: The analysis of extreme precipitation events in the rehabilitation area over the years found that both Typhoon Soudelor and Typhoon Dujuan in 2015 had rainfall intensities exceeding the 100-year return period and were characterized by short-duration heavy downpours, thus wreaking havoc on a large scale.
2.Hydrological analysis: With the rehabilitation area situated in the watershed area of the Nanshi River, the data from the nearby Datongshan rainfall station was employed as the main basis for the subsequent analysis of rainfall frequency, rainfall intensity, concentration time, run-off coefficient and flood peak discharge.
3.Topographic and geological analysis: According to the topographic analysis using the Light Detection And Ranging data of different periods, the bed of the stream was eroded to a depth of 7-20 meters by Typhoon Soudelor in 2015, and five sliding masses (S1-S5) were plotted and 14-boreholes drilling were set up to a total depth of 682 meters. The drill cores indicate that the rock mantle is 30-50m thick and therefore susceptible to landslides due to run-off. In addition, the groundwater and tilt displacement monitoring results showed that the sliding mass S3 on the right bank downstream of the Zhongzhi Bridge No. 2 was most active due to the change in the water table, and the landslide depth was about 18m below the surface.
4.Groundwater survey: Groundwater distribution in the rehabilitation area was investigated by means of ground-resistance testing, with a total length of 1,170m over 4 survey lines. Groundwater flow conditions were also confirmed based on groundwater detecting layer and groundwater flow direction tests, which served as a basis for the laying of the horizontal collector pipes for groundwater drainage. 
5.Geology of the area: The strata exposed in the rehabilitation area were mainly composed of the Oligocene Kankou formation (Kk) and the Oligocene Tsuku formation (Tsk) on the upper slopes. No geological structures pass through the catchment area of the rehabilitation area, while the adjacent geological structures comprise the Chatienshan anticline and the Zhongzhi fault.
6.Environmental geology: According to the 1 in 25,000 geological hazard potential map for 2002 by the Central Geological Survey, Ministry of Economic Affairs, there were three gullies in the vicinity of the rehabilitation area, all with potential debris flow. The middle and lower reaches of the potential debris flow torrent New Taipei DF230 had medium to low potential rock debris avalanche areas, while the upper reaches had medium to high potential rock debris and rock slide areas.
7.Economic sector: Includes a survey of population distribution, current land use, and the current state of transport, livelihoods and tourism development.
8.Ecological survey: According to local aborigines and Zhongzhi Village headman Huang Chung-hsin in Wulai District where Provincial Highway 9A at the 10.2K mark is located, the collapsed area was home to Formosan macaques, Formosan blue magpies, and other animals such as pangolins, Reeves’s muntjacs and wild boars. After the landslide, all the rare animals were nowhere to be found, but the habitat has now been brought back to life.
G.How long did the project take to complete?
This project commenced in May 2016 and was completed in 3.5 years, and then handed over to the responsible agency: the Taipei Water Management Office of the Water Resources Agency under the MOEA on January 14, 2020 for ongoing monitoring and maintenance.
H.Was this a Government or private initiative?
The upper reaches of the Xindian River are a very important reservoir area in northern Taiwan under the jurisdiction of the Taipei Water Management Office of the Water Resources Agency under the MOEA(R.O.C.) Due to the large-scale destruction caused by Typhoon Soudelor in 2015 that affected the drinking water of 6.5 million people in Metropolitan Taipei Area, the Taipei Branch of the Soil and Water Conservation Bureau Taipei Branch,Council of Agriculture, Executive Yuan(R.O.C.) provided assistance in the implementation of the project based on the whole of government principle. The total cost of this project was approximately NT$300 million, which was a government-funded public works project.
Question 2: Planning and Design
A.What is the initial concept of the master plan?
New Taipei City's Wulai District is renowned as a hot spring tourist destination in northern Taiwan. According to statistics by the New Taipei City Government in 2019, the Atayal indigenous population stood at about 6,433, with most of them engaged in tourism and agriculture. On August 8, 2015, Typhoon Soudelor brought with it a record-breaking rainfall (with a peak of 70mm and a total accumulated rainfall of 510mm), causing slope erosion, road collapse, debris flows, landslide dams and flooding in the upper Xindian River basin, wreaking havoc throughout Wulai District. Among them, the most serious was a massive landslide of up to 8.8 hectares in the upper reaches at the 10.2K mark on Provincial Highway 9A (Zhongzhi Village, Wulai District). Not only were Jinyanong Road and Zhongzhi Bridge No. 2 upstream damaged, upon which the village's livelihood depended, but the large amount of sand and gravel also disrupted the only access road in the downstream area at the 10.2K mark on Provincial Highway 9A, turning Wulai into an isolated area and cutting villagers off, which left life in a state of emergency, while rare animal habitats were extensively damaged.
As the upper reaches of Xindian River is a very important reservoir area in northern Taiwan under the jurisdiction of the Taipei Water Management Office and the widespread devastation caused by the storm could not be dealt with by a single agency, the Executive Yuan instructed the Water Resources Agency based on the whole of government principle to bring in various agencies for meetings. Because the disaster affected the drinking water of 6.5 million people in Metropolitan Taipei Area, the Soil and Water Conservation Bureau the Council of Agriculture, the Executive Yuan(R.O.C.) provided assistance in the implementation of the project for the reconstruction of the affected areas in the shortest possible time.
This project was based on four main guiding principles:
1.Wrapping up the remediation of the potential debris flow torrents as soon as possible to avoid further damage to the main traffic routes and to allow the residents of Wulai to have a safe route home.
2.Safeguarding the reservoir area to supply clean water to the 6.5 million people in Metropolitan Taipei Area.
3.Stabilizing the slopes of both sides of the potential debris flow torrents and restoring the exposed collapsed land with native plants to provide habitats and wildlife corridors for rare animals.
4.The engineering design should factor in the results of the various surveys, the choice of construction methods should be tailored to the local conditions, the most economical engineering volume should be assessed to avoid excessive excavation, in order to achieve energy saving and carbon reduction, and the convenience of subsequent maintenance and management should also be taken into consideration.
B.From the studies carried out of the existing conditions what are the plans to restore or enhance the environment?
A survey showed that the stream bed in the restoration area was eroded to a depth of 7-20 meters, with a steep slope of 25-33%. In addition, the geology on both banks is a thick layer of rock debris, which is susceptible to collapse and exposure due to run-off. Further, the rise and fall of groundwater levels also affect the stability of the sliding masses, so the design guidelines are described below:
1.Control of stream centerline and slope drop: The adjustment of the slope of the river bed was overall considered according to the topographic changes and the stabilized toe of the slope on both banks as a benchmark longitudinal grade. Thirty-seven gravel sand slide control facilities were installed to effectively control the stream centerline and adjust the stream bed gradient to 9%.
2.Stabilization of the slopes on both banks: The use of beam-free frames to protect the exposed collapsed areas and the use of native plant seeds for hydroseeding have resulted in excellent vegetation restoration, reducing erosion and restoring the cleanliness of the water source.
3.Groundwater drainage: Horizontal collector pipes with 87 boreholes (6,800m) were installed at the toe of the slope by exploiting the topographic drop, effectively lowering the water table, based on the results of ground resistance detection and other tests.
4.Safe discharge of surface run-off: Installation of shotcrete drains around the perimeter of the collapsed site has intercepted surface run-off water into the collapsed site and reduced the occurrence of erosion.
5.Restoration of wildlife corridors: Simple wildlife corridors made of timber allow mammals and reptiles to pass through the gravel sand slide control facilities and revetments.
C.What innovation in designed for the planning of the project?
The most important task of this project was to quickly stabilize the slopes on both sides of the potential debris flow torrent to avoid further damage to the main traffic routes and to give the residents of Wulai a safe way to return home. A geological survey and monitoring results showed that the sliding mass S3 on the right bank downstream of Zhongzhi Bridge No. 2 was most active due to the influence of groundwater and has been cracking since 2017, affecting the construction of the lower slope and the safety of the adjacent houses. It is therefore important to be fully informed about the distribution and flow of groundwater. As a result, an extensive surface drainage survey was carried out to eliminate possible sources of groundwater recharge. Groundwater distribution was then confirmed using ground resistance detection and drilling core comparison. Finally, groundwater detecting layer and groundwater flow direction tests were performed to pinpoint groundwater flow conditions, completing an investigation on a wide range of surface, linear and point scales, which served as a reference for the angle and direction in which the horizontal collector pipes were installed, thus effectively lowering the water table and mitigating the slope slippage. In addition, an endoscopic device developed in-house was used to reach into the horizontal collector pipe to record the actual water discharge, representing a major innovation.
Moreover, to enhance the curing effect of the cement mortar after the hitting of soil nails into the slope, a hollow drill rod was used in this project to facilitate future inspection of the driving depth and cement mortar filling.
D.What are the materials to be used?
The engineering materials primarily used in this project comprised reinforcing bar, formwork, concrete, self-drilling soil nails with hollow drill rods, chain link fence and HDPE water collecting pipes, while all other materials were sourced on-site, such as rubble, soil and primary seeds for hydroseeding.
E.What changes in design after implementation?
This project was subject to change from time to time after implementation to suit on-site conditions, which is described below:
1.The foundation of the gravel sand slide control facilities: The foundation of the facilities was designed according to the original slope of the river bed, and the relevant geological information was recorded in detail during the construction process, which was fed back to the survey company to update the local geological data on the one hand, and to reduce the depth of the foundation excavation in order to achieve the effect of energy saving and carbon reduction if it had already penetrated deep into the rock bed.
2.The direction in which the horizontal collector pipes were laid: The horizontal collector pipes installed for groundwater drainage were based on comprehensive consideration of ground resistance detection and drilling core data. Changes in water discharge were recorded in detail throughout the construction process, which were fed back to the survey company to update the local geological data on the one hand, and served as a reference for the subsequent adjustment of the laying direction on the other.
Question 3: Implementation
A.How many phases were carried out to complete the project?
This project took about 3.5 years to complete from May 2016 and was then handed over to the competent authority (the Taipei Water Management Office of the Water Resources Agency under the MOEA) on January 14, 2020 for subsequent maintenance, management and monitoring operations. Overall, this rehabilitation project was broken down into three phases, with a total of six works carried out:
1.Gully controls in 2016: The "massive landslide remediation project on Provincial Highway 9A at the 10.2K mark" in the most upstream part of the gully was undertaken, while an investigation and planning exercise was initiated to identify the collapse mechanism and assess the works by years and by phases.
2.Emergency gully controls in 2017: To prevent the collapse in the upper reaches of the rehabilitation area from continuing to grow in size, affecting road access and the safety of inhabitants, three projects were carried out, including the landslide remediation project on the right bank upstream of the Jinyan Bridge, the landslide remediation project on the right bank downstream of the Zhongzhi Bridge No.2, and the potential debris flow torrent New Taipei DF230 remediation project, in which the dam, which was the key to restoring stability to the gully, was also completed at this stage.
3.Rehabilitation in 2018: To expedite the stabilization of the slopes on both sides of the gully and the rehabilitation of vegetation, the second phase of the potential debris flow torrent New Taipei DF230 control project and the second phase of the landslide treatment project on the right bank downstream of the Zhongzhi Bridge No.2 were undertaken. Given the future use needs of the landowners, a phased arrangement of the on-site fragmented rocks was adopted to effectively disperse the run-off and reduce erosion.
B.What types of machineries was used to rehabitate/ restores the area? 
Two types of machinery were utilized in this project to restore the environment:
1.Heavy-duty machinery: Subsequent to the disaster, the entire slope and river bed were scattered with fallen trees and boulders, which necessitated heavy-duty machinery such as excavators and bulldozers to excavate and clear the access road for entry, and heavy-duty machinery such as concrete trucks to carry construction materials and drilling machines to lay horizontal collector pipes were particularly warranted during construction.
2.Survey and monitoring machinery: To gain knowledge of the disaster mechanism and to investigate the distribution of groundwater in this project, a number of technological machines were applied, such as camera drones, geological drilling machines, ground resistance detectors, groundwater flow testers, rain gauges, ground surface extensometers, site inclinometers, automatic transmission water pressure meters and self-metered water pressure meters. In addition, we used CCD and time-lapse cameras to supervise and record important images during the construction period.
C.Any physical structures build to complement the project and what are the functions of these structures?
1.Continuous gravel sand slide control facilities and bed consolidation works were adopted to adjust the longitudinal slope of the stream bed and control the stream centerline, and in-situ mortared rubble was laid underneath the spillway to reduce the hollowing out of the foundation caused by the erosion of the river bed due to flood peaks.
2.The toes of the slopes on both banks were protected by concrete revetments, while above the outlet height, in-situ rubble was laid dry without the mortar for bank protection to reduce foot erosion and soil loss.
3.Dry-laid retaining walls were erected to prepare the slope in stages to reduce slope erosion, and intercepting ditches with earthbags were installed at slope seepage points to safely discharge run-off water.
4.The exposed slope was covered with chain link fence, and self-drilling soil nails with hollow drill rods were driven at appropriate intervals, followed by reinforcing bar strapping to hold the slope in place, and finally mortar was sprayed to form a revetment with beam-free frames and create a vegetation rehabilitation base to minimize soil loss from the slope.
5.Shotcrete drains were laid around the collapse site to safely divert runoff water from the slope top.
6.The HDPE horizontal collector pipe was installed to drain the groundwater, lowering the water table and easing the slippage of the slope.
D.Please indicate the technical details involved.
The technology for this project can be classified into three main categories:
First, to gain knowledge of the disaster mechanism and to investigate the distribution of groundwater in this project, a number of technological machines were applied, such as camera drones, geological drilling machines, ground resistance detectors, groundwater flow testers, rain gauges, ground surface extensometers, site inclinometers, automatic transmission water pressure meters and self-metered water pressure meters. In addition, we used CCD and time-lapse cameras to supervise and record important images during the construction period. The main purpose of ground resistance testing was to obtain the isopotential resistivity profile of the stratigraphic layer by placing current and potential electrodes on the survey line and based on the relationship between discharge current and potential difference. Further, the results of surface geological surveys, geological drilling, groundwater monitoring or other exploration results together with the results of ground resistance measurements helped to determine the distribution of strata, fault locations, stratigraphic aquifers or lithology, etc. The groundwater flow test was carried out using the highly sensitive thermal groundwater flow meter (GFD-3A) developed by Geo5 Japan. This meter is highly sensitive and has a 360-degree horizontal measurement capability. 
Second, engineering design related calculations, such as rainfall frequency analysis, hydraulic calculations, stream bed siltation slope analysis, slope stability analysis, structure or stress calculations, etc., were employed as the basis for the design and configuration of structures such as gravel sand slide control facilities. 
Thirdly, ecological restoration techniques, such as ecological surveys and plant surveys, can serve as a basis for the design of wildlife corridors or vegetation hydroseeding.
E.What is the technical innovation employed in the project?
The most important task of this project was to quickly stabilize the slopes on both sides of the potential debris flow torrent to avoid further damage to the main traffic routes and to give the residents of Wulai a safe way to return home. A geological survey and monitoring results showed that the sliding mass S3 on the right bank downstream of Zhongzhi Bridge No. 2 was most active due to the influence of groundwater and has been cracking since 2017, affecting the construction of the lower slope and the safety of the adjacent houses. It is therefore important to be fully informed about the distribution and flow of groundwater. As a result, an extensive surface drainage survey was carried out to eliminate possible sources of groundwater recharge. Groundwater distribution was then confirmed using ground resistance detection and drilling core comparison. Finally, groundwater detecting layer and groundwater flow direction tests were performed to pinpoint groundwater flow conditions, completing an investigation on a wide range of surface, linear and point scales, which served as a reference for the angle and direction in which the horizontal collector pipes were installed, thus effectively lowering the water table and mitigating the slope slippage. In addition, an endoscopic device developed in-house was used to reach into the horizontal collector pipe to record the actual water discharge, representing a major innovation.
Moreover, to enhance the curing effect of the cement mortar after the hitting of soil nails into the slope, a hollow drill rod was used in this project to facilitate future inspection of the driving depth and cement mortar filling.
Question 4: Environmental Impact
A.What's the extra-ordinary step taken to conserve the nature of the existing environment?
All the measures in this project were aimed at protecting the existing natural environment, with the four planning principles outlined below:
1. Wrapping up the remediation of the potential debris flow torrents as soon as possible to avoid further damage to the main traffic routes and to allow the residents of Wulai to have a safe route home.
2. Safeguarding the reservoir area to supply clean water to the 6.5 million people in Metropolitan Taipei Area.
3. Stabilizing the slopes of both sides of the potential debris flow torrents and restoring the exposed collapsed land with native plants to provide habitats and wildlife corridors for rare animals.
4. The engineering design should factor in the results of the various surveys, the choice of construction methods should be tailored to the local conditions, the most economical engineering volume should be assessed to avoid excessive excavation, in order to achieve energy saving and carbon reduction, and the convenience of subsequent maintenance and management should also be taken into consideration.

A survey showed that the stream bed in the restoration area was eroded to a depth of 7-20 meters, with a steep slope of 25-33%. In addition, the geology on both banks is a thick layer of rock debris, which is susceptible to collapse and exposure due to run-off water. Further, the rise and fall of groundwater levels also affect the stability of the sliding masses, so the design guidelines are described below:
1.Control of stream centerline and slope drop: The adjustment of the stream bed gradient was overall considered according to the topographic changes and the stabilized toe of the slope on both banks as a benchmark longitudinal gradient. Thirty-seven gravel sand slide control facilities were set up to effectively control the stream centerline and adjust the stream bed gradient to 9%.
2.Stabilization of the slopes on both banks: The use of beam-free frames to protect the exposed collapsed areas and the use of native plant seeds for hydroseeding have resulted in excellent vegetation restoration, reducing erosion and restoring the cleanliness of the water source.
3.Groundwater drainage: Horizontal collector pipes with 87 boreholes (6,800m) were installed at the toe of the slope by exploiting the topographic drop, effectively lowering the water table, based on the results of ground resistance detection and other tests.
4.Safe discharge of surface run-off water: Installation of shotcrete drains around the perimeter of the collapsed site has intercepted surface run-off water into the collapsed site and reduced the occurrence of erosion.
5.Wildlife corridors: Simple wildlife corridors made of timber allow mammals and reptiles to pass through the gravel sand slide control facilities and revetments.
B.What are the efforts made by the developer to minimize the impact to the environment based on:
1.Project Concept
Due to the huge scale of the disaster, this project was divided into three phases to alleviate the impact on the environment. The first phase was aimed at stopping the collapse site from being extended, so that the gully was remediated urgently to prevent further damage caused by erosion as a result of heavy rainfall. In the second and third stages, emergency gully remediation was carried out to address the sources of potential recurrence of the collapse, while restoring the original landscape and recreating the living environment of the indigenous people and the habitat of wildlife, as well as solving the problem of turbidity in drinking water sources. The planning concept was based on the following four points:
(1)Wrapping up the remediation of the potential debris flow torrents as soon as possible to avoid further damage to the main traffic routes and to allow the residents of Wulai to have a safe route home.
(2)Safeguarding the reservoir area to supply clean water to the 6.5 million people in Metropolitan Taipei Area.
(3)Stabilizing the slopes of both sides of the potential debris flow torrents and restoring the exposed collapsed land with native plants to provide habitats and wildlife corridors for rare animals.
(4)The engineering design should factor in the results of the various surveys, the choice of construction methods should be tailored to the local conditions, the most economical engineering volume should be assessed to avoid excessive excavation, in order to achieve energy saving and carbon reduction, and the convenience of subsequent maintenance and management should also be taken into consideration.
2.Planning
This project conducted a 3-year survey from 2017, with the results of the research described below to mitigate the impact on the environment:
(1)Hydrological analysis: With the rehabilitation area situated in the watershed area of the Nanshi River, the data from the nearby Datongshan rainfall station was employed as the main basis for the subsequent analysis of rainfall frequency, rainfall intensity, concentration time, run-off coefficient and flood peak discharge, while the most economical engineering volume was chosen to avoid excessive excavation and additional environmental damage.
(2)Topographic and geological analysis: The topographic analysis using the LiDAR data of different periods was perform to gain an understanding of terrain variation, and geological conditions were determined by drill cores. The groundwater and tilt displacement monitoring results showed the mechanism of the sliding masses and the landslide depth to assess the level of risk and the method of work.
(3) Groundwater survey: Groundwater distribution and flows in the rehabilitation area was investigated by means of ground-resistance detection, groundwater detecting layer and groundwater flow direction tests, which served as a basis for the laying of the horizontal collector pipes for groundwater drainage.
(4)Ecological survey: A preliminary understanding of the ecology of the restoration area was obtained according to local aborigines and the village headman, and monitoring was carried out at all times during the construction period to guard against any impact on the ecological environment.
3.Designing
A survey showed that the stream bed in the restoration area was eroded to a depth of 7-20 meters, with a steep slope of 25-33%. In addition, the geology on both banks is a thick layer of rock debris, which is susceptible to collapse and exposure due to run-off water. Further, the rise and fall of groundwater levels also affect the stability of the sliding masses, so the design guidelines are described below to alleviate the impact on the environment:
(1)Control of stream centerline and slope drop: The adjustment of the slope of the river bed was overall considered according to the topographic changes and the stabilized toe of the slope on both banks as a benchmark longitudinal grade. Thirty-seven gravel sand slide control facilities were installed to effectively control the stream centerline and adjust the stream bed gradient to 9%.
(2)Stabilization of the slopes on both banks: The use of beam-free frames to protect the exposed collapsed areas and the use of native plant seeds for hydroseeding have resulted in excellent vegetation restoration, reducing erosion and restoring the cleanliness of the water source.
(3)Groundwater drainage: Horizontal collector pipes with 87 boreholes (6,800m) were installed at the toe of the slope by exploiting the topographic drop, effectively lowering the water table, based on the results of ground resistance detection and other tests.
(4)Safe discharge of surface run-off water: Installation of shotcrete drains around the perimeter of the collapsed site has intercepted surface run-off water into the collapsed site and reduced the occurrence of erosion.
(5)Restoration of wildlife corridors: Simple wildlife corridors made of timber allow mammals and reptiles to pass through the gravel sand slide control facilities and revetments.
4.Construction & Operation
Since May 2016, the Taipei Branch of the Soil and Water Conservation Bureau had spent 3.5 years to finish the "massive landslide rehabilitation project on Provincial Highway 9A at the 10.2K mark,” with a total project cost of approximately NT$300 million, which has effectively mitigated the sliding masses on the right bank downstream of the Zhongzhi Bridge No. 2 and the cracking of the consequent rock bed on the left bank upstream of the said highway at the 10.2K mark, producing good results of vegetation restoration. According to a commissioning agreement, this project was handed over to the Taipei Water Management Office on January 14, 2020 for follow-up maintenance, management and monitoring operations, to provide a safe living environment for the local people, maintain quality water resources and protect the original habitat of rare wildlife.
5.Implementation: Control of Earthworks/ Control of Erosion/ Control of Water Quality/ Disposal of Solid Waste/ Control of Air Pollution/ Control of Noise projects
This project is one of the most critical geo-engineering projects in northern Taiwan, primarily because it was located at one of the sources of water for approximately 6.5 million people in Metropolitan Taipei Area. Rainwash caused by the landslide would have resulted in turbid raw water flowing directly into the water intake at the Jhihtan Dam and the water purification basin below. If left untreated by the Taipei Water Department, it could lead to major water restrictions across the cities, and the ongoing soil loss caused by the landslide could endanger the safety of residents and damage the environment for wildlife. In addition, this project also kept erosion, water quality and soil loss in check, working towards the United Nations Sustainable Development Goals (SDGs) 6, “Clean Water and Sanitation,” and 15, “Life on Land,” which are described as follows:
(1)Continuous gravel sand slide control facilities and bed consolidation works were adopted to adjust the longitudinal slope of the stream bed and control the stream centerline, and in-situ mortared rubble was laid underneath the spillway to reduce the hollowing out of the foundation caused by the erosion of the river bed due to flood peaks.
(2)The toes of the slopes on both banks were protected by concrete revetments, while above the outlet height, in-situ rubble was laid dry without the mortar for bank protection to reduce foot erosion and soil loss.
(3)Dry-laid retaining walls were erected to prepare the slope in stages to reduce slope erosion, and intercepting ditches with earthbags were installed at slope seepage points to safely discharge run-off water.
(4)The exposed slope was covered with chain link fence, and self-drilling soil nails with hollow drill rods were driven at appropriate intervals, followed by reinforcing bar strapping to hold the slope in place, and finally mortar was sprayed to form a revetment with beam-free frames and create a vegetation rehabilitation base to minimize soil loss from the slope.
(5)Shotcrete drains were laid around the collapse site to safely divert runoff water from the slope top and reduce soil erosion caused by the pooling of run-off.
(6)The HDPE horizontal collector pipe was installed to drain the groundwater, lowering the water table and easing the slippage of the slope, and the clean groundwater diverted could recharge the raw water.
C.Is there any energy minimise by efforts? What is done to encourage the reduction of the consumption of energy?
This project was designed to achieve energy and emissions savings by avoiding excessive excavation and additional environmental damage in the most economical way. In addition to the reinforced concrete required to maintain the strength of engineering materials, in-situ materials such as gravel, soil and native plant seeds were used as far as possible. Moreover, the foundation of the gravel sand slide control facilities was designed according to the original slope of the river bed, and the relevant geological information was recorded in detail during the construction process, which was fed back to the survey company to update the local geological data on the one hand, and to reduce the depth of the foundation excavation in order to achieve the effect of energy saving and carbon reduction if it had already penetrated deep into the rock bed.
D.Was recycling and harvesting of rain water run-off incorporated into the water articulation system?
One of the main objectives of this project was to protect the slopes on both banks of the potential debris flow torrent from continuous crumbling and affecting the water quality. As a result, continuous gravel sand slide control facilities and bed consolidation works were introduced to adjust the longitudinal gradient of the stream bed and to control the stream centerline, so that the flow velocity was slowed down and the water became cleaner. The HDPE horizontal collector pipe was installed to drain the groundwater, lowering the water table and easing the slippage of the slope, and the clean groundwater diverted could recharge the raw water and become the drinking water for 6.5 million residents in Metropolitan Taipei Area.
E.Is there any step taken to recycle waste, i.e. make into fertilizer or to provide heat for use?
The waste materials involved in this project were fallen trees, crumbled earth and stone and retaining rail piles. The fallen trees were turned into organic fertilizer by the environmental protection unit, the tumbledown sand and gravel was used for phased backfilling or sand and gravel gabion filling for slopes, and the retaining rail piles were utilized as reinforcement material for the foundation of gravel sand slide control facilities.
F.What are the challenges faced and solution provided?
The biggest challenges facing this project were the race against time and recovery from a major collapse of up to 8 hectares, the constant slippage of the ground, and the communication between the many units working simultaneously, with the countermeasures described below:
1.Time:
The rehabilitation of the access road was prioritized, followed by the remediation of the gully slopes. In order to save time, the work was carried out in parallel with the survey and planning and the project was concluded in 3.5 years. In particular, the initial post-disaster rehabilitation works started on the 25th of July 2016 and were finished on the 17th of June 2017, lasting a total of 318 calendar days. However, due to the rainy weather in northern Taiwan, the number of days when work could not be carried out was as high as 132 calendar days, which demonstrates the difficulty of construction. In addition, the construction period overlapped with the New Taipei City Government’s rehabilitation project for Zhongzhi Bridge No. 2 in Zhongzhi Village, Wulai District, which required the coordination of the movement of construction materials and machinery, thus adding to the difficulties of the construction, and the smooth completion of the works was achieved only through mutual communication.
2.Sliding of strata:
According to the results of the survey, the scale of the landslide continued to escalate in three places, including the right bank upstream of the Jinyan Bridge, the right bank downstream of the Zhongzhi Bridge No. 2, and the left bank upstream of Provincial Highway 9A at the 10.2K mark. As the sediment continued to move down and endangered the safety of neighboring residents, slope protection works such as beam-free frame revetments and slope shotcrete intercepting and drainage ditches were carried out above the exposed rock beds, while below the rock beds, slope preparation and vegetation rehabilitation, slope drainage system and drainage facilities such as drain gutters were undertaken. The results of the survey and monitoring in 2017 revealed the groundwater level and the movement of the masses.  In 2018, the rehabilitation project focused on the protection of the unremediated section of the stream bed and the accelerated removal of groundwater from the right bank to achieve the effect of slowing down the slippery of the masses, as well as the laying of horizontal water collecting pipes to effectively lower the water table and increase the stability of the slopes.
3.Potential debris flow torrent New Taipei DF230 control project: 
This project was the most important project among the disaster recovery works on Provincial Highway 9A at the 10.2K mark, providing about 50,000 cubic meters of sediment siltation space upstream and adjusting the slope of siltation upstream to reduce the erosion of the stream bed. Due to its location at the mouth of the valley, the narrow width and steep gradient made construction extremely arduous, so it was handled slowly with personnel carrying equipment. Moreover, as the lower reaches were adjacent to the old Provincial Highway 9A, traffic control had to be stepped up over the course of construction, with construction routes coordinated with the Zhonghe Branch, First Maintenance Office, Directorate General of Highways under the Ministry of Transportation and Communications.
G.With global warming, what steps are taken to reduce the impact of the project to the environment?
All measures in this project factored in energy saving and carbon reduction to minimize the impact of global warming. This project also kept erosion, water quality and soil loss in check, working towards the United Nations Sustainable Development Goals (SDGs) 6, “Clean Water and Sanitation,” and 15, “Life on Land,” which are described as follows:
1.Continuous gravel sand slide control facilities and bed consolidation works were adopted to adjust the longitudinal slope of the stream bed and control the stream centerline, and in-situ mortared rubble was laid underneath the spillway to reduce the hollowing out of the foundation caused by the erosion of the river bed due to flood peaks.
2.The toes of the slopes on both banks were protected by concrete revetments, while above the outlet height, in-situ rubble was laid dry without the mortar for bank protection to reduce foot erosion and soil loss.
3.Dry-laid retaining walls were erected to prepare the slope in stages to reduce slope erosion, and intercepting ditches with earthbags were installed at slope seepage points to safely discharge run-off water.
4.The exposed slope was covered with chain link fence, and self-drilling soil nails with hollow drill rods were driven at appropriate intervals, followed by reinforcing bar strapping to hold the slope in place, and finally mortar was sprayed to form a revetment with beam-free frames and create a vegetation rehabilitation base to minimize soil loss from the slope.
5.Shotcrete drains were laid around the collapse site to safely divert runoff water from the slope top and reduce soil erosion caused by the pooling of run-off.
6.The HDPE horizontal collector pipe was installed to drain the groundwater, lowering the water table and easing the slippage of the slope, and the clean groundwater diverted could recharge the raw water.
Question 5: Result On Environmental Impact
A.Any study done to find the end result of the project?
The most important task of this project was to quickly stabilize the slopes on both sides of the potential debris flow torrent to avoid further damage to the main traffic routes and to give the residents of Wulai a safe way to return home. Therefore, the main objective of this project was to protect the slopes on both sides of the potential debris flow torrent from continuous collapse and affecting water quality by adopting a series of measures such as continuous gravel sand slide control facilities and bed consolidation works to adjust the longitudinal slope of the stream bed and control the stream centerline, concrete revetment protection at the toes of the slopes on both banks, phased slope preparation to reduce slope erosion, beam-free frame revetment installation to create a vegetation rehabilitation base and HDPE horizontal collector pipes installation to drain groundwater. The results and benefits of this project continue to be analyzed through the monitoring of the water table and slope tilt displacements. These data can be viewed at any time via the website or mobile app, and are often quickly accessed through wide area interpretation by unmanned aerial vehicles (UAVs).
B.How was the project area enhanced, restored or rehabilitated after the completion of the project?
After the completion of this project, the location of the massive landslide on Provincial Highway 9A at the 10.2K mark was restored to its original state. In addition to filling the subsidence area near local residents' homes and making it safer, the responsible authority has been carrying out monitoring work since May 2017 to determine the link between slope activity and geology and groundwater,  and has introduced automated monitoring equipment since 2018, including CCD observation, rain gauge, tilt and water level observation hole, water level observation hole, in-hole extensometer and wireless surface extensometer, and various types of observation devices, which can be used to view the current slope displacements or restoration area through the website and the real-time observation platform, thus completing the installation of "prototype disaster prevention" automated monitoring system.
Firstly, the results of the groundwater level and tilt displacement monitoring revealed that the permanent water table has dropped considerably, from 2.5m to 0.5m due to rainfall, indicating effective groundwater drainage and a reduction in tilt displacement from 4cm to 0.2-0.6cm per month. The recent monitoring results have ruled out any visible displacements and no surface displacements were observed. This project's effectiveness was highlighted by the comparison of aerial photographs from different periods.
With the recovery of vegetation in the disaster area and the stabilization of the crumbling site, over 6,000 Atayal indigenous people in the Wulai region affected had seen an immediate improvement in their transportation and livelihoods, with the only access road to the city reopened and 1,701,000 annual tourist arrivals registered. According to Zhongzhi Village headman Huang Chung-hsin, the local iconic wildlife Formosan macaques and Formosan blue magpies had returned to their habitats and the entire affected area had been revitalized.