Cheung, A

Koor, Nick

Portsmouth University Research Portal (Pure)

 DOUBLE CORROSION PROTECTION OF LONG SOIL NAILS 
AT DEEP BAY LINK – A CASE HISTORY 
 
 
Nick Koor1 & Anson Cheung2 
 
 
Abstract: Preliminary soil nails up to 30 m in length were installed at the Deep Bay 
Link project to test the effectiveness of grouting long nails. The nails incorporated a 
double corrosion protection system consisting of a corrugated HDPE sheath. Each 
preliminary nail was successfully tested to two times the design load and several were 
exhumed to inspect the grouted nail section. Grout was seen to have completely filled 
the internal and external void around the corrugated sheath with excellent contact 
between the grout and the surrounding saprolite. Permanent soil nails with double 
corrosion protection up to 28 m long are now being successfully installed at the site. 
 
 
INTRODUCTION 
 Deep Bay Link (Figure 1) is a duel-three lane trunk road providing a strategic link 
between the Hong Kong Shenzhen Western Corridor and the existing Yuen Long Highway. 
Due to its strategic nature the HK$4.0B project has been fast tracked with a construction 
period of 28 months. Gammon Construction Ltd. are the Main Contractor for the northern 
section and construction is currently underway and due to be open to traffic by end-2005. The 
link traverses the broad low lying valley separating Yuen Long and Tuen Mun and is primarily 
supported by a reinforced concrete viaduct except where the road passes through the northern 
extremity of the Tsing Shan Range (Figure 1) in a 40 m deep cut. Soil nails support the cut 
which has up to 60° side slopes and nails up to 28 m long. It is the soil nailed slopes in the 
northern part of the project which are the subject of this paper.  
 
THE SITE 
Topography 
 The northern part of the site through which the road is in cut is characterised by a north 
south trending valley surrounded by rounded granitic hills. The valley bottom is at about 
+40mPD and the surrounding hill slopes have elevations ranging from about +60 to +80mPD 
which slope gently towards the valley at about 30. Ephemeral streams drain into the valley 
from the surrounding hills. The natural terrain is generally sparsely vegetated with small trees 
and shrubs with some areas of gulley erosion where the vegetation is sparse. 
 
Geological Setting 
 The Hong Kong Geological Survey Yuen Long Sheet 6 indicates that the site of the 
major cut is located in an area underlain by regionally metamorphosed fine grained granite. A 
major slip strike shear zone which trends north-east separates the fine grained granite from 
Tuffs and Tuff Breccias of the Tuen Mun Formation about 200 m to the south-east. Zones of 
intense shearing along this shear zone have produced mylonites within both the granite and 
volcanic rocks of the area. Quartz veins and minor quartzphyric rhyolite dykes are also 
                                                 
1 Senior Resident Geotechnical Engineer – Ove Arup & Partners Hong Kong Ltd 
2  Resident Geotechnical Engineer – Ove Arup & Partners Hong Kong Ltd 
 
Figure 1 - Deep Bay Link & associated road transportation routes in Western New 
Territories – Hong Kong 
 
present within the granite. Quaternary superficial deposits occur within the valley floors and 
are generally termed debris flow deposits on the geological sheet but they may be better 
named colluvium. Little colluvium is shown to exist on the granitic side-slopes. 
 
 
CUT SLOPE DESIGN 
Ground Conditions 
 Detailed ground investigation consisting of vertical drillholes, trial pits and laboratory 
testing confirmed that the site is underlain by metamorphosed fine grained granite with little 
or no colluvium being present. The granite is variably weathered with continuous Grade III 
rock as encountered in the drillholes at between 5 and 40m below existing ground levels, the 
shallower being in the base of the valley. Mylonite zones were also encountered in the 
drillholes and later observed in the cut slope face. Groundwater levels range between 0 and 
20m below existing ground levels. 
 
Slope Design 
 Limits set on land-take for the project predicted the adoption of either steep soil nailed 
slopes or some form of vertical retaining structure supporting a cut slope. Due to programme 
and cost considerations it was decided to adopt the steep soil nailed solution (Figure 2) as 
 
Deep Bay 
 
 
Tuen Mun Road 
Shekou 
Yuen Long Highway 
Castle Peak Road Deep Bay Link 
Route 3 
N Hong Kong Shenzhen Western Corridor 
Tsing Shan Range 
 
Figure 2 - Layout of cut slopes at northern end of Deep Bay Link 
 
these would be quicker and cheaper to construct. Soil nailed slopes were designed with slope 
angles of between 50° and 60° incorporating over 2000 soil nails. Slopes up to 40 m high with 
up to five berms have been designed using conventional techniques used in Hong Kong 
(Figure 3). Due to the relatively steep side slopes and conservative design groundwater levels 
adopted, the nails supporting the lower two berms are up to 28 m long and have a design load 
of up to 460kN. The slope design life is 120 years. 
 
 
Figure 3 - Typical section through cut slopes at northern end of Deep Bay Link 
 
Ground Aggressivity 
 In order to assess the potential corrosion of any steel elements installed in the slopes, a 
series of tests were made during the ground investigation to determine ground aggressivity. 
The testing and corrosivity assessment followed the recommendations made by Murray 
(1993). The results of the testing are summarised in Table 1. The results indicate a ranking 
mark of -6 which indicates that the soil condition is likely to be aggressive and therefore 
For Section see Figure 3 
Preliminary Soil Nail 
Installation Zone 
corrosion protection is required. This result agrees with the study reported by Shiu & Cheung 
(2003) where they state that a significant portion of the local (Hong Kong) soils has a high 
corrosion potential. 
 
Table 1 - Aggressivity table 
Item Measured Value Marks 
Soil Composition 
Material containing not more than 75% and 10% of particles 
passing the 63 micron and 2 micron BS sieve sizes 
respectively.  The material passing the 425 micron BS sieve, 
when tested in accordance with BS 1377, have a plasticity 
index less than 6 
0 
Groundwater level at buried 
position 
Soil nail reinforcement will be submerged under design 
groundwater table 
-4 
Resistivity (ohm – cm) 10,000 or more 0 
Moisture Content Average 25.5% -1 
pH Value Average 5.2 -2 
Soluble Sulphate (ppm) Average 105 0 
Cinder and Coke or made ground None 0 
Redox potential Average 417 +2 
Presence of sulphate and hydrogen 
sulphide 
0.09% (None) 0 
Presence of carbonate 0.18% (Trace) 0 
Chloride ion (ppm) 200 -1 
 Total -6 
 
Buildabiltiy and Corrosion Protection 
 Several factors have to be taken into account when designing long soil nails in corrosive 
ground conditions. They are; the buildabiltiy of the nail; what corrosion protection system to 
use and; potential grouting difficulties. These three critical elements need to be considered 
holistically rather than in isolation.  
The high nail design loads require the use of standard 50 mm deformed bars (Fy = 
460N/mm2) or smaller diameter (c.36mm) high yield bars such as that manufactured by 
Dywidag or Macalloy (Fy =1080N/mm
2). Other options such as carbon fiber strands were 
considered, but at the time of design such systems had not been used in Hong Kong and 
therefore the time constraints for Government approval ruled out their use. The pros and cons 
of using the two bar types are at Table 2. Due to the obvious practical difficulties in installing 
28 m long 50 mm deformed bars the high strength bar option was investigated further as 
lead-in times for material ordering and shipment were available within the Contract period. 
 
Table 2 - Pros & cons for different bar options 
Attribute Standard Bar (50 mm) High Strength Bar (36 mm) 
Availability Easily available in Hong Kong Limited number of manufacturers 
Weight Very heavy and difficult to man-handle Much easier to man-handle 
Testing Most laboratories in Hong Kong set-up 
to test 
Difficult to test with modifications 
required to test rigs 
Corrosion Protection Standard zinc protection Zinc protection not recommended – use 
other methods such as plastic sheaths 
Quality Control Variable depending on source Very good quality 
Lead-in Time Short Long – three months or more 
   
At the time of designing the nails there was no real guidance in Hong Kong on what 
corrosion protection measures are applicable to soil nails for different ground conditions. 
From the test results it was considered that the steel needed full corrosion protection. 
Concerns regarding the potential for hydrogen embrittlement of high strength steel elements 
which have been zinc coated, Arup (2003), meant that an alternative to standard zinc coating 
was required. Options such as heat shrunk plastic or epoxy coatings to the bars were 
considered, but finally a double corrosion system was adopted consisting of a HDPE 
corrugated full length sheath which surrounds the bar and is filled with grout in lieu of the 
zinc coating. Manufactures of the high strength steel provide such double corrosion systems 
which compliment their range of bar sizes and include couplers, spacers and end cap with 
integral grouting tube. The Main Contractor opted for the Dywidag-Systems International 
double corrosion protection system (Plate 1). This consists of a 100 mm diameter sheath with 
end cap for the 36 mm diameter high strength bar to be installed into a 150 mm diameter hole. 
 
 
Plate 1 - Dywidag-Systems International soil nail double corrosion protection system 
 
 In order to address the constructability and grouting issues relevant to such long nails, 
eight preliminary soil nails up to 30 m long were installed at the site, load tested to two times 
the design load and exhumed for inspection. It is believed that at the time of writing that these 
soil nails are the longest installed with a double corrosion protection system in Hong Kong. 
 
PRELIMINARY SOIL NAILS 
Installation 
 The eight nails were installed at a location (Figure 2) which would later be within the 
bulk excavation for the cut slope formation so that there was; an opportunity to expose the 
nails after testing; inspect the grouting and the grout/soil contact; and take samples to be saw 
cut so that the full nail cross-section could be visually inspected. The preliminary nails also 
gave the Contractor an opportunity to test different installation sequences and systems. 
Preliminary nail details are summarised in Table 3. Initially the soil nail hole was designed to 
be 150 mm in diameter. However, due to difficulties in keeping the alignment of the hole 
straight for such long nails the sheath and steel bar could not be fully installed. The hole 
diameter was increased to 165 mm and installation become easy taking less than half and hour. 
Internal and external packers were used to isolate the test length and the nails were grouted 
using standard grouting equipment with the annulus between sheath and soil being grouted 
prior to the internal void. The sequence for the construction of the soil nails with double 
corrosion is at Figure 4. 
 
Pull out Test Results 
Each preliminary nail was tested to two times the design load using standard testing 
equipment. All the preliminary nails passed the pull-out tests, the results are at Table 3.  
 
 
 
 
 
 
 
 
100mm Diameter 
Corrugated 
Sheath 
 
36mm Bar 
Grout Pipes 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4 - Installation sequence of soil nail with double corrosion protection system 
 
Table 3 - Preliminary nail details and results of pull-out tests 
Soil 
Nail 
No. 
Dywidag 
Bar Size 
(mm) 
Nail 
Diameter 
(mm) 
Bar 
Length 
(m) 
Bond 
Length 
(m) 
Effective 
in situ 
Vertical 
Stress 
(kN/m3) 
Test 
Load 
(kN) 
Max 
Shear – 
Soil/Grout 
(kN/m2) 
Measured 
Residual 
Movement 
(mm) 
Measured 
Maximum  
Movement 
(mm) 
Allowable 
Residual 
Movement 
(mm) 
PT1 36 165 20 2.0 189.7 95 92 1.42 8.0 6.0 
PT2 36 165 25 2.0 217.1 100 96 2.56 11.9 6.0 
PT3 36 165 20 4.0 175.2 205 99 4.80 15.3 12.0 
PT4 36 165 25 4.0 181.8 240 116 4.43 21.8 12.0 
PT5 36 165 20 6.0 100.8 210 68 4.77 13.3 18.0 
PT6 36 165 25 6.0 114.4 235 76 2.41 12.2 18.0 
PT7 36 165 30 6.0 134.5 225 72 2.32 21.1 18.0 
PT8 36 165 30 8.0 109.8 280 68 1.61 27.8 24.0 
 
Nail Inspection 
 Once tested, three of the longest nails were carefully excavated (Plate 2) to allow close 
inspection of the nail and to facilitate sampling. Samples were also taken from the exposed 
grouted sections and saw cut (Plate 3). Several observations were made: 
(1) The grout was continuous over the bond length (Plate 4) with no signs of voids 
apart from close to the packer. 
(2) Some grout escaped past the external packer (up to 2 m) which generally formed a 
coating around the drillhole surface with a void between the grout and the sheath. 
(3) Nail PT6 showed signs of crown collapse in the drillhole. The grout within the 
over-break contained inclusions of soil. 
(4) The grouted diameter measured on nail PT6 ranged from 165 to 180 mm. 
(5) The grout over the bonded length had excellent contact with the ground (Plate 4). 
(6) The nails deviated from the design inclination of 10° to the horizontal by up to 10° 
for the 30 m long nail PT8. 
 
Stage 1 Stage 2 Stage 3 
(7) The saw cut sections revealed continuous sound grout in both the internal and 
external annuli (Plate 5). 
(8) The corrugated sheath tended to float during the grouting of the outside annulus 
however the steel bar remained central (Plate 5). 
 
The installation, testing and exhumation of the preliminary piles demonstrated that the 
corrugated sheath and steel bar could successfully be installed to up to 30 m and that the nails 
could be fully grouted to their distal end. This gave the designer and Contractor confidence to 
proceed with the permanent works installation. 
 
Plate 2 - Preliminary soil nail Plate 3 - Preliminary soil nail 
 being exposed   being saw cut 
 
Plate 4 - Exposed preliminary soil nail Plate 5 - Section through grouted 
     preliminary soil nail 
 
 
PERMANENT NAIL CONSTRUCTION 
 At the time of writing approximately 430 nails with double corrosion protection between 
24 and 28 m long have been successfully installed at Deep Bay Link (Plate 6). Initial 
difficulties were experienced using standard Hong Kong soil nail grouting equipment. The 
Contractor was not able to fully grout these long nails and there was leakage from the outside 
annulus into the central annulus. The Contractor has modified his grouting procedure by using 
slightly larger diameter grout pipes (increased ID from 13 to 17mm), utilizing a pressure grout 
pump (HD Engineering Ltd GP60) and simultaneously grouting both annuli. On average each 
28 m long nail takes about 3 hours to drill in saprolite, 15 to 20 minutes to install the 
Bar 
Spacer 
Sound 
Grout 
Sheath 
Spacer 
36mm 
Dywidag 
Bar 
100mm 
diameter 
Corrugated 
Sheath 
Grout 
Pipes 
8m 
Packer 
1m 
Excellent soil / grout 
contact 
Solid Grout 
Grooves from drilling 
165mm 
corrugated sheath and steel bar and 15 minutes to grout. 
 
 
Plate 6 - General view of 40m high cut slope at northern end of Deep Bay Link during 
soil nail installation (March 2005) 
 
 
DISCUSSION & RECOMMENDATIONS 
 The preliminary soil nail installation programme at Deep Bay Link demonstrates that 
nails up to 30 m long can be successfully installed and grouted using off-the-shelf double 
corrosion protection systems. As successful installation is very much contractor and ground 
condition dependent it is recommended that similar trials as these are made on projects where 
long nails are required. Zinc coating high strength steel is not recommended due to the 
possibility of hydrogen embrittlement. Grouting the annuli should be carried out 
simultaneously to avoid sheath floatation and possible grout leakage from the outer to inner 
annulus or vice-versa. For such long nails it is more efficient to use pressure grouting pumps 
with larger smooth bore grouting pipes than normal to ensure a continuous grouting process. 
Care should be taken when specifying nail diameters for long nails with double corrosion 
systems and designers should allow at least 30 to 40mm clearance between the sheath and the 
drillhole wall to allow for installation difficulties. The maximum drillhole deviation of 10° 
measured for the 30 m nail affects the horizontal nail capacity by less than 5% and is therefore 
considered not to be a concern bearing in mind the conservative design methodologies 
adopted in Hong Kong. Shiu and Cheung (2003) observation that many soils in Hong Kong 
are aggressive is supported and the authors recommend that the use of carbon fiber soil nail 
strands be further developed for Hong Kong conditions. 
 
 
REFERENCES 
Arup (2003). Zinc coating of high strength pre-stressing steel – again! Arup MATERIAL 
notes 2003NM_5 (internal Arup publication). 
Murry, R.T. (1993). “The Development of Specifications for Soil Nailing Research Report 
380” Transport Research Laboratory, Department of Transport, UK. 
Shiu, Y.K. and Cheung, W.M. (2003). "Long-term Durability of Steel Soil Nails" GEO Report 
No. 135. Geotechnical Engineering Office, Civil Engineering Department, The 
Government of the Hong Kong SAR. 
 
 
ACKNOWLEDGEMENTS 
 The Authors wish to thank the Director of Highways Department of The Government of 
the Hong Kong SAR for permission to publish this paper. 


Double corrosion protection of long soil nails at Deep Bay Link – a case history

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Double corrosion protection of long soil nails at Deep Bay Link – a case history

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