Hadley, David

Hart, Jonathan

Koor, Nick

Portsmouth University Research Portal (Pure)

The Foothills Bypass – from design to construction 
 
N.P. Koor, D. Hadley & J.R. Hart 
Scott Wilson (HK) Ltd 
 
 
 
 
 
 
 
 
 
 
 
 
ABSTRACT: The Foothills Bypass is a dual 2-lane highway on the western side of Tuen Mun in the New 
Territories of Hong Kong. The road is constructed on 1.7 km of embankment along the foothills of Tsing 
Shan and on viaduct at its northern end where it links with a grade separated interchange. The stabilisation of 
a large area of existing instability (Area 19) forms a major and integral part of the works. The bypass 
embankment was incorporated into the stabilisation of Area 19 as a toe weight with additional filling and 
extensive sub-soil drainage also being applied to the slope as part of the stabilisation measures. This paper 
briefly describes; the  design of the Area 19 stabilisation works; the construction works and; the results and 
implications of confirmatory site investigation made during construction. 
 
 
1 INTRODUCTION 
 
The Foothills Bypass is a 2.6 km long, dual 2-lane 
road from Tuen Mun West (near Butterfly Beach) 
connecting to Wong Chu Road (Figure 1). The road 
is constructed on 1.7 km of embankment along the 
foothills of Tsing Shan and 0.9 km on viaduct, 
which takes it over Lung Mun Road to connect into 
Wong Chu Road. Associated with the bypass 
construction is the stabilisation of a large area of 
instability (600 m by 300 m) located within Tuen 
Mun Area 19, up-slope of the proposed bypass. 
Total construction cost for the embankment and 
slope stabilisation works is in the order of 400 
million Hong Kong dollars. 
Area 19 has had a long history of instability both 
geological as described by Langford et al. (1986) 
and, more recent due to borrowing activity in 1977 
as described by Hunt (1982) and Taylor & Hadley 
(2000). At the time of designing the Foothills 
Bypass embankment, it was assumed that the 
landscape along the bypass route south of Area 19 
(Figure 1) did not contain similar geological 
instability as Area 19. However, due to uncertainty 
in this assumption and, the lack of available ground 
investigation at the design stage, a detailed 
confirmatory site investigation programme was 
carried out during the construction phase of the 
project.  This paper briefly describes; the design and 
construction of the Area 19 slope stabilisation works 
and, the results and implications of the confirmatory 
site investigation. 
 
 
 
Figure 1. Foothills Bypass. 
 
2 GEOLOGY, GEOMORPHOLOGY AND 
HYDROGEOLOGY 
 
The geology of the Tsing Shan range area is quite 
varied and this is reflected in its geomorphology 
(Figure 2).  The upper very steep slopes of Tsing 
Shan, which consist of megacrystic fine to medium 
grained granites, dominate the surrounding 
landscape.  Downslope, the granite gives way to a 
distinct set of rounded spurs and associated less 
steep slopes underlain by metasedimentary strata of 
the Tuen Mun Formation, which comprise siltstones, 
quartzites, tuffites and conglomerates. 
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TOE BERMS
FIGURE 2
TUEN MUN
AREA 19
LUNG MUN ROAD
TUEN MUN
HIGHWAY
WONG CHU ROAD
TSING SHAN RANGE
BYPASS
FOOTHILLS
Figure 2. Tsing Shan range looking south-west  
(see Figure 1). 
 
Further downhill, and marked by another change 
in gradient, is the subdued and gently sloping terrain 
of the footslopes, which are underlain by volcanics 
also of the Tuen Mun Formation.  The volcanic 
rocks consist primarily of andesitic lava’s. The 
andesite weathers to a firm to stiff becoming very 
stiff greenish grey slightly clayey silt with closely 
spaced relict joints. The joint surfaces, frequently 
polished/slickensided as a result of previous 
movement and often manganese-coated, are 
generally discontinuous. In addition to the joints, 
continuous (c. 10’s of metres), low angle, through-
going, shear surfaces have been observed in the 
uppermost few metres of the weathered andesite in 
Area 19 both during investigations in the early 
1980’s (Hunt 1982) and during the current 
stabilisation works. These shear surfaces are smooth 
undulating and often highly polished and striated 
and are interpreted as being the surfaces of rupture 
of both recent and relict landslides. The presence of 
such discontinuities within the soil mass has a 
significant effect on its overall behaviour, 
particularly the mass strength. Large areas of the 
gentler lower slopes are covered by Quaternary 
colluvium. 
Groundwater monitoring results indicate that 
groundwater levels within the colluvium and the 
decomposed volcanics vary both seasonally and in 
response to individual rainstorms. Generally, 
however, no large rapid rises in transient 
groundwater levels, in response to individual 
rainstorms, are observed within either the colluvium 
or the underlying volcanics. The responses that do 
occur tend to be over a period of three to seven days 
or longer. Perched groundwater levels often develop 
within the colluvium, particularly during the wet 
season. Vertical ground water flow is generally from 
the colluvium into the decomposed volcanics below.  
Artesian conditions, frequently encountered within 
the footslopes of mountain ranges, have not been 
observed. 
The Foothills Bypass is founded generally on 
colluvium overlying completely decomposed 
andesite along most of the route apart from the 
southern end where it is founded directly onto 
partially weathered granite. 
 
 
3 GEOLOGICAL MODEL RELATING TO 
INSTABILITY IN AREA 19 
 
A geological model based on that proposed by 
Langford et al. (1986) explains the origins of the 
instability in Area 19 and is consistent with 
observations made to date. The model postulates that 
during the Quaternary Period, rapid erosion through 
the colluvium and completely decomposed andesite 
by downcutting streams resulted in landsliding with 
the surfaces of rupture being mainly within the 
partially weathered andesite.  The relict surfaces of 
rupture are very weak and are believed to be at or 
close to residual strength which has been measured 
as low as  = 9 (Taylor & Hadley 2000). It is 
believed that it is these very weak relict surfaces of 
rupture that were reactivated during the borrowing 
activity in 1977. It is likely that such failure planes 
are present within the andesite up to its contact with 
the metasediments. 
 
 
4 AREA 19 SLOPE STABILISATION DESIGN 
 
One of the greatest challenges to the design of the 
slope stabilisation works was the determination of 
representative mass shear strength parameters to use 
in the design. This is covered in detail by Taylor & 
Hadley (2000) and is not repeated here. 
Figure 3. The comparative approach. 
 
The “comparative approach”, which made the 
fundamental assumption that the present slopes are 
metastable, was used as the design tool. This method 
comprised assigning unity to the factor of safety and 
FOOTHILLS BYPASS 
AREA 19 
 
TSING SHAN 
(CASTLE PEAK) 
GRANITE 
 
ANDESITE 
META-SEDIMENTS 
 
Figure 4. Sequence of works for 6m deep trench drain installation in Area 19.
 
 
constructing a family of potential slip surfaces 
within the decomposed volcanics in the existing 
ground model in order to determine the 
corresponding shear strength parameters along these 
slip surfaces (Figure 3). The values determined for 
the drained angle of friction () varied from 12 to 
20 and were critically dependent on groundwater 
assumptions.  This range is in general agreement 
with both the back analysed shear strength 
parameters and the results of laboratory testing 
(Taylor & Hadley 2000). The drained cohesion (c) 
was varied between 0 and 2 kPa during this exercise. 
The essence of this approach is that a particular slip 
surface and its corresponding shear strength 
parameters were then inserted into the proposed 
earthworks layout and the resulting factor of safety 
determined (Figure 3). The fundamental benefit 
derived from this approach is the enhancement 
achieved in the design factor of safety. 
The bypass was constructed on a 20 m high fill
 
Figure 5. Construction sequence – Area 19. 
 
Figure 6. Trial trench log with interpretation. 
 
embankment, which forms a toe weight to Area 19 
with filling up-slope at an angle of 11º (Figure 3). 
Sub-surface drainage in the form of 6 m deep 
counter-fort drains at 12 m centres were 
incorporated together with extensive surface 
drainage measures. To monitor the long term 
performance of the slope stabilisation measures, a 
comprehensive network of geotechnical 
instrumentation is being installed and linked to an 
automatic monitoring system which allows real time 
movement and pore pressure to be observed. 
 
 
5 CONSTRUCTION IN AREA 19 
 
Construction of the Foothills Bypass commenced in 
early 1999 and is due for completion in the last 
quarter of 2001. The landslide stabilisation works in 
Area 19 will cost approximately 140 million Hong 
Kong dollars and consist of; removal of about 0.2 
million m3 of unsuitable soil from the slope; 
construction of 17 km of 6 m deep trench drains 
(Figure 4); and, the compaction of 1.8 million m3 of 
fill to form the embankment toe weight and slope 
filling. The construction phase of the project 
required significant geotechnical monitoring and 
control and therefore a  full time Resident 
Geotechnical Engineer was part of the site 
supervisory team. 
Critical to the success of the slope stabilisation 
and to avoid large-scale slope movements was the 
sequencing of the drainage and earthworks in Area 
19 (Figure 5).  Excavation fronts were limited to 
50 m wide bays to allow trench drain construction 
and filling from the toe up.  This ensured that 
unloading at the toe of the slope was minimised.  It 
also meant that support was removed only over a 
relatively short width thus limiting the size of any 
incipient ground movement into the excavation. As 
the deep trench drain construction (Figure 4) was 
generally in advance of the bulk excavation it had a 
stabilising effect by actively reducing pore pressures 
in the slope and reinforcing the ground ahead of bulk 
excavation. This procedure proved very successful 
and only small-scale containable slope movements 
were triggered during the initial excavation and 
trench drain construction stage. 
 
 
 
 
Figure 7. Aerial photograph re-interpretation. 
 
6 CONFIRMATORY SITE INVESTIGATION 
SOUTH OF AREA 19 
 
Extensive confirmatory ground investigation, not 
possible at the design stage, enabled constant 
refining, and updating of the geological model used 
to develop the geotechnical solution for the bypass 
works. Trial trenching to the south of Area 19 
(Figure 6) combined with geomorphological 
mapping and re-interpretation of aerial photographs 
established that areas of instability and relict 
landslides existed along the embankment route 
(Figure 7). These fossil or relict landslides occur 
within the completely decomposed andesite and 
appear to be of a similar form to those described in 
Area 19. Block samples of completely decomposed 
andesite containing shear surfaces were tested in a 
large shear box and measured field strengths of 
c=0 kPa and = 12 to 19 which are within the 
same range as determined for Area 19 (Taylor & 
Hadley 2000) and close to the mass strength 
parameters used in the “comparative approach”. 
Following this work, the geological model derived 
for the route south of Area 19 was modified to take 
into account the findings of the confirmatory 
investigation and embankment stability was then re-
assessed. The weak shear surfaces within the 
andesite resulted in embankment toe stability having 
less than the required factor of safety and therefore 
embankment redesign was required.  To ensure that 
the progress of the contractor building the bypass 
was not compromised, a “bolt-on” solution of toe 
berms, designed to increase toe stability, were 
adopted along the down-slope side of the 
embankment (Figure 1).  Due to space restrictions 
and the existing topography at one critical section, 
ground improvement in the form of grout columns 
(66 no. of 750 mm diameter 12 m long at 2 m 
centres) constructed using the continuous-flight-
auger technique were required together with a rock 
fill toe berm (Figure 8).  The grout columns enhance 
the shear strength of the completely decomposed 
andesite and disrupt the continuity of any continuous 
weak shear surfaces.  The continuous-flight-auger 
technique was adopted due to; suitable ground 
conditions; speed of construction (a production rate 
of up to 5 per day was achieved); and, the method is 
both quiet and vibration free which was critical due 
to the close proximity of the Tuen Mun Riding 
School. The embankment south of Area 19 is now 
complete including the toe berms and no instability 
has been recorded to date. 
Figure 8. Toe berm with ground improvement. 
 
 
7 CONCLUSIONS 
 
For projects in geologically unstable areas, a sound 
engineering geological model is required for an 
economic and safe design. Adequate ground 
investigation information during design is vital to 
ensure major re-design is not required during 
construction, but ongoing investigation, mapping 
and verification tasks as construction proceeds are 
also essential. An experienced engineering geologist 
or geotechnical engineer, full time on site, thus plays 
an essential role in the success of the construction 
phase of projects in geologically demanding areas. 
 
 
 
 
ACKNOWLEDGEMENTS 
 
This paper is published with the permission of the 
Director of the Territory Development Department, 
Hong Kong SAR Government. 
 
 
REFERENCES 
 
Hunt, T. (1982). Slope failures in colluvium 
overlying weak residual soils in Hong Kong. 
Proceedings of the ASCE Speciallity Conference 
on Engineering and Construction in Tropical and 
Residual Soils, Honolulu, pp 443-462. 
Langford, R.L., Arthurton, R.S. & Lai, K.W. (1986). 
Geological Survey and Ground Investigations in 
Tuen Mun New Town, Hong Kong.  The Role of 
Geology in Urban Development.  Geological 
Society of Hong Kong.  Bulletin No 3, pp 199-
208. 
Taylor, G.R. & Hadley, D. (2000).  Stabilisation of a 
Large Landslide in Tuen Mun, Hong Kong. 
Landslides in research, theory and practice. Ed. 
by Bromhead, E. Dixon, N. and Ibsen, M-L. 
Proceedings of the 8th International Symposium 
on Landslides, Cardiff, vol. 3 pp1454-1460.  


Foothills Bypass, Hong Kong – Design to Construction

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Foothills Bypass, Hong Kong – Design to Construction

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