COLLAPSING SLOPES S3 GEOGRAPHY Cheung Chuk Shan College
LANDSLIDES CASE STUDIES 2
Landslide case study 1: SHUM WAN ROAD 1995 Date Area Description of the hillslope
Exact time of slope collapse Slide scar Impact brought by the landslide
Weather conditions that day Slope materials
Drainage conditions
13 August 1995 Shum Wan Road, Wong Chuk Hang, Southern District • original hillside between Shum Wan Road and Nam Long Shan Road • 70 m above • steepness around 27° • covered with dense vegetation • 4:00 a.m. • • • •
140 m in length and varies from about 60 m in width Severely damaged three shipyards on the sea front A fire subsequently occurred in the collapsed structures 2 killed and 5 injured
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rainfall in August 1995 is the highest ever recorded for the month of August
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relatively permeable mainly clay materials seeping of rainwater into the slope was common drains were observed to be blocked after the landslides the drain carried 470 litres of rainwater per second
LANDSLIDES CASE STUDIES 3
S3 GEOGRAPHY LANDSLIDE INVESTIGATION REPORT 1 Date of landslide
13 August 1995
Location of landslide
Shum Wan Road, Wong Chuk Hang, Southern District
Which factor(s) is/are responsible for increasing the stress of the slope?
Heavy and prolonged rainfall - The landslide took place during the rainy season - 13 August had the highest rainfall recorded for the month - the heavy and prolonged rainfall added weight to the soil Poor drainage system - drains were observed to be blocked - rainwater could not be drained away Road construction - a secondary road, Nam Long Shan Road, was found on the upper part of the landslide site - the slope was cut and it was over-steepened, steepness around 27°, increasing the risk of landslides - traffic might produce vibration though not quite significant Weak slope materials - the slope materials were found relatively permeable - with clay materials which were quite unstable - seeping of rainwater into the slope was common Dense vegetation cover - the plant roots might loosen the soil due to biological weathering - this made the slope unstable
Which factor(s) is/are responsible for reducing the strength of the slope?
What is the key factor leading to the landslide? Suggest some precautionary measures to be taken to prevent the same hazard.
Investigation team members
Human activities caused the landslides: - drains were blocked around the landslide site. - The permeable soil layer allowed rainwater to seep through. - to construct a good drainage system (both surface and sub-surface ones) - to repair and inspect the drainage system more frequently by Drainage Service Department - to strengthen the slope by retaining walls or cement - to restrict development on slope 3B 2,3,6,11,12,17,25,27,28,30 3C 2,5,10,15,16,17,19,26,28,29,40,41 3D 1,6,8,10,12,15,17,19,20,29,39 3E 1,2,3,4,9,15,16,18,19,20,33,36
LANDSLIDES CASE STUDIES 4
Landslide case study 2: PO SHAN ROAD 1972 Date Area Description of the hillslope
Exact time of slope collapse Slide scar Impact brought by the landslide Weather conditions that day Slope materials Drainage conditions
15 June 1972 Po Shan Road, Mid-levels, Central • The slope gradient above Po Shan Road was about 35o. • Downslope between Po Shan Road and Conduit Road the slope gradient was more gentle. • A construction site above Conduit Road, where the major part of the landslide was located, was being redeveloped at the time of the landslide. • In late 1971 two landslides occurred on the construction site. • Most of the failed area is underlain by weathered rock of volcanic origin blanketed with 5 m - 10 m thick soil and rocks • unknown • • • •
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140 m in length and varies from about 60 m in width 67 killed and 20 injured 2 buildings destroyed and 1 building severely damaged About 1400 mm of rainfall was recorded between May and June 1972 and in particular more than 650 mm of rainfall was recorded from 16 to 18 June 1972. urban development along the slope cracks were found in drainage channels
LANDSLIDES CASE STUDIES 5
LANDSLIDES CASE STUDIES 6
S3 GEOGRAPHY LANDSLIDE INVESTIGATION REPORT 2 Date of landslide Location of landslide Which factor(s) is/are responsible for increasing the stress of the slope?
Which factor(s) is/are responsible for reducing the strength of the slope?
What is the key factor leading to the landslide?
Suggest some precautionary measures to prevent the same hazard. Investigation team members
18 June 1972 Po Shan Road, Mid-levels, Central High building density - buildings created stress on the steep slope, average gradient about 35o - heavy burden for the slope Transport routes - Roads were built on steep slopes, like Po Shan Road, Kotewall Road, Conduit Road - Heavy traffic on the roads produced vibrations Construction work - the vibration of sheet piling work would loosen the rocks and soil - the construction site was found above Conduit Road Heavy and prolonged rainfall - High rainfall was recorded in May and June 1972 (about 1400 mm) - More than 650 mm of rainfall was recorded from 16 to 18 June 1972 - Rainwater added weight to the soil layer Deep weathering - serious weathered soil was found along the slope, about 5-10 m thick - mainly composed of decomposed volcanic rocks - which were exposed by a previous slide near the landslide site Rain action - heavy rain seeped into the soil layer - friction reduced - soil got saturated very quickly - water acted as lubricant - soil particles became loosened Human activities caused the landslides - over-development on the hill slopes (tall buildings and dense road system) - construction works on the redevelopment site might be the immediate cause of the landslide - regular inspection and maintenance of the slope - to arouse public attention to the importance of slope safety 3B 8,19,23,35,37,40 3C 3,7,11,12,18,23,25,30,35 3D 10,12,13,14,19,20,27,31,35,39 3E 5,6,7,12,21,22,26,27,32,35
LANDSLIDES CASE STUDIES 7
Landslide case study 3: KWUN LUNG LAU 1994 Date Area Description of the hillslope
Exact time of slope collapse Slide scar Impact brought by the landslide Weather conditions that day Slope materials Drainage conditions
23 July 1994 Kwun Lung Lau, Kennedy Town, HK Island • A scar measured 28 m wide and 14 m high • About 1000 m3 of debris were released in the landslide, consisting of mainly soil, fragments of cement, blocks of masonry and trees • 8:53 p.m. • • • • •
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450 m in length The incident resulted in five fatalities and three other people were injured. The landslide occurred following a period of heavy rain. Mainly soil About 2½ hours before the landslide, a hole of approximately 1 m in diameter appeared on the cement surface, and the muddy water came out from it. The soil contained lots of water.
LANDSLIDES CASE STUDIES 8
LANDSLIDES CASE STUDIES 9
S3 GEOGRAPHY LANDSLIDE INVESTIGATION REPORT 3 Date of landslide
23 July 1994
Location of landslide
Kwun Lung Lau, Kennedy Town
Which factor(s) is/are responsible for increasing the stress of the slope?
Building on slopes - Kwun Lung Lau was built on a steep slope, adding weight to the soil layer below Over-steepening of the slope - the slope was cut and covered with cement Heavy and prolonged rainfall - large amount of rainfall (a total of 547 mm recorded 48 hours before the landslide) - it added weight to the slope
Which factor(s) is/are responsible for reducing the strength of the slope?
Leakage of water pipes - leakage of underground water pipes led to rapid saturation of soil - friction of soil particles reduced - soil particles were unable to stick together - a hole of about 1 m in diameter appeared on the cement surface about 2½ hours before the landslide - muddy water came out from the hole Weak retaining wall - the retaining wall failed with soil saturation due to the seeping of rainwater - probably due to improper maintenance
What is the key factor leading to the landslide?
- Lack of maintenance of underground water pipes
Suggest some precautionary measures to be taken to prevent the same hazard.
Investigation team members
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to improve surface and sub-surface drainage system to check the slope regularly to restrict further development on the steep slope to build stronger retaining walls and cover with cement on slopes
3B 4,5,10,21,22,31,38,41 3C 1,4,8,20,27,32,33,38,39 3D 3,5,16,18,20,22,26,28,34,41 3E 11,13,14,23,24,25,28,29,34,37
LANDSLIDES CASE STUDIES 10
Landslide case study 4: FEI TSUI ROAD 1995 Date Area Description of the hillslope
Exact time of slope collapse Slide scar Impact brought by the landslide
13 August 1995 Fei Tsui Road, Chai Wan • Before the landslide, the cut slope inclined towards the north at an average angle of 60° to the horizontal, with a maximum height of about 27 m. • The depth of the rupture surface was about 15 m and the landslide had an estimated failure volume of some 14,000 m3. • 1:15 a.m. • •
Weather conditions that day
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Slope materials
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Drainage conditions
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450 m in length Debris from the landslide covered the lower part of the slope and extended across Fei Tsui Road, situated at the toe of the slope, to the Chai Wan Baptist Church and a playground adjacent to the church. One person was killed and another was slightly injured. A total of 370 mm of daily rainfall was recorded at a rain gauge 220 m north of the landslide location on 12 August 1997, a day prior to the landslide incident. The landslide occurred mainly in weathered volcanic rock No obvious drainage system was found.
LANDSLIDES CASE STUDIES 11
S3 GEOGRAPHY LANDSLIDE INVESTIGATION REPORT 4 Date of landslide
13 August 1995
Location of landslide
Fei Tsui Road, Chai Wan
Which factor(s) is/are responsible for increasing the stress of the slope?
Heavy and prolonged rainfall - a total of 370mm of daily rain - rainwater added weight to the soil Steep slope - landslide occurred on a cut slope with an average angle of 60° - the slope was over-steepened due to road construction, Fei Tsui Road - the pull of gravity was strong on steep slope and led to landslide Presence of building structure on the steep slope - the Chai Wan Salt Water Serviced Reservoir might have added weight / stress to the slope Thick soil layer - weathering of volcanic rocks was serious - weathering formed thick soil layer which was unstable Seepage of rainwater - heavy and prolonged rain acted as lubricant to reduce friction between soil and rocks - soil became saturated and unstable
Which factor(s) is/are responsible for reducing the strength of the slope?
What is the key factor leading to the landslide? Suggest some precautionary measures to be taken to prevent the same hazard.
Weathering of volcanic rocks and over-steepening of slope -
Investigation team members
To build proper drainage system, e.g. surface drainage channels To build stronger retaining wall To restrict further development on the steep slope To plant some trees on the steep slope
3B 7,14,24,32,36,39 3C 6,9,13,14,21,22,24,31,34,36,37 3D 4,9,11,21,25,33,36,37,38,40 3E 3,8,17,30,31,38,39,40