Ocean Engineering 146 (2017) 298–310
Contents lists available at ScienceDirect
Ocean Engineering
journal homepage: www.elsevier.com/locate/oceaneng
Engineering geotechnical investigation for coral reef site of the cross-sea
bridge between Male and Airport Island
C.Q. Zhu a, b, H.F. Liu b, c, *, X. Wang b, c, Q.S. Meng b, R. Wang b
a
b
c
Faculty of Engineering, China University of Geosciences, Wuhan, Hubei 430074, China
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China
University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Coral reefs
Engineering geotechnical investigation
Maldives
Cross-sea bridge
The Male-Airport Island Cross-sea Bridge project is the largest island-linking project in Maldives, the country
known as “the kingdom of coral reefs.” Coral reef is also a special type of rock and soil medium to support significant civil engineering projects. In the Cross-sea Bridge project, the engineering geotechnical investigation of
the coral reef site was divided into two stages: the feasibility study and the construction-drawing design phase.
Engineering geological survey techniques were applied, and together with field exploration methods, such as
geophysical prospecting and drilling exploration, the geological conditions of the site with respect to bridge
engineering demands were comprehensively evaluated. In addition, the design parameters for pile group foundations were proposed based on in-situ tests, such as standard penetration, dynamic sounding, and acoustic wave
testing in borehole as well as laboratory physical and mechanical experiments and bearing-capacity tests for pile
foundations using rock and soil samples drilled from the site. The investigative methods adopted in the
Male–Airport Island Cross-sea Bridge project and the results obtained will provide references for similar engineering projects in the future.
1. Project overview
As a construction project for which China provides a significant
amount of aid, the Mal
e-Airport Island Cross-sea Bridge project in the
Maldives is on the critical path to realizing the 21st Century Maritime
Silk Road envisioned in China's “One Belt, One Road” initiative. The
project is located in North Male Atoll, Maldives, which crosses the
Gaadhoo Koa Strait and connects three adjacent islands, Male Island,
Airport Island, and Hulhumale (Fig. 1) which are both in the atoll. The
project is the most important island linking project in the Maldives.
The project starts in the southeastern corner of Male Island, connects
to a construction plan known as Boduthakurufaanu Magu Road in the
southern side of Male. Then a bridge is designed to cross Gaadhoo Koa
Strait from the end of Boduthakurufaanu Magu Road and make landfall
on the southern side of Airport Island. The endpoint of the bridge connect
to the road from the airport to Hulhumale. The wide of the project is
21.0 m. And total length of the project is approximately 2 km, with the
bridge being 1.39 km long and the total bridge approaches being
610 m long.
The proposed foundation of the bridge is a type of pile group foundation known as “overall steel tube” or conventional large-diameter pile
group foundations known as “separated steel tube”. Both types of pile
group foundation consist of six pieces of 1.5-m-diamter drilled piles.
The bedrock in the proposed project area is composed mainly of coral
reef sediments, a special type of rock and soil medium. Coral reefs are
geological sediments formed by accumulation of skeletons and shells of
dead reef-building corals, which are primarily distributed in the tropical
ocean between the Tropics of Cancer and Capricorn (Wang et al., 1997).
On this type of bedrock, engineering projects are also distributed (King
and Lodge, 1988; Hua, 2015; He et al., 2010; Yuan et al., 2012) (Table 1).
Their special nature is embodied by two aspects of specificity. Initially,
the material composition of coral reef sediments are basically CaCO3.
Additionally, they are formed by organisms in the marine environment.
In relevant specifications of geotechnical engineering projects in China,
coral reefs have not been included for consideration, and previous studies
have shown that this type of rock and soil medium has the following
major characteristics: (1) the size and shape of sedimentary particles
show a relatively large variability, leading to a relatively large spatial
* Corresponding author. State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei
430071, China.
E-mail address: 958800895@qq.com (H.F. Liu).
https://doi.org/10.1016/j.oceaneng.2017.09.039
Received 26 February 2017; Received in revised form 27 July 2017; Accepted 24 September 2017
Available online 7 October 2017
0029-8018/© 2017 Elsevier Ltd. All rights reserved.
C.Q. Zhu et al.
Ocean Engineering 146 (2017) 298–310
Fig. 1. Location diagram showing the proposed project.
Table 1
Engineering projects constructed on coral deposits.
Country
Position
Project name
Construction purpose
Foundation form
Construction time
1
Australia
North west shelf of western Australia
Gas extraction
pile foundation in the sea
From 1972/3 to 1987/8
2
Saudi
East bank of Red Sea in Rabigh
Oil-fired power plant
Saudi
Sudan
East bank of Red Sea in Jeddah
West bank of Red Sea in Sudan Port
Natural base foundations
or Partial replacement
Reinforce foundation
Reinforce foundation
From 2009/2 to 2013/1
3
4
North Rankin
“A” Offshore Gas Platform
Saudi RABIGH
2 660 MW Power Plan
Saudi RSGT Port Project
New Container Terminal
Project in Sudan Port
Container terminal
Container terminal
From 2008/1 to 2009/12
From 2006/6 to 2009/11
2. Investigative methods and work assignments
variability in the distribution of porosity in the sediments; (2) the
porosity is far higher than that of terrigenous sediments; (3) the strength
of the particle is lower than that of quartz particles, and due to the existence of intergranular pores, making it fragile; (4) sediments can easily
experience deuteric alteration such as cementation, and after being
cemented, the cementation degree and the type of the sediments structure significantly influence the engineering properties (Given and Wilkinson, 1985).
Since there had never been such a large-scale island-linking project
undertaken in the local area, therefore, the implementation of the project
is undoubtedly a huge challenge for geotechnical engineers.
According to Provision No. 6.11.3 in the “Code for Highway Engineering Geological Investigation” of People's Republic of China Industry
Standard JTG C20-2011 (The ministry of transport of the people's republic of China, 2011), the exploratory points basically focus on the
proposed piers (Fig. 2). The entire investigation process can be divided
into
two
stages:
the
feasibility
study
and
the
construction-drawing design.
The major tasks during the feasibility study which is lasted from May
20, 2015 to June 13, 2015 included engineering geological survey,
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Ocean Engineering 146 (2017) 298–310
Fig. 2. Scheme of distribution of investigative workload.
Fig. 3. Drilling construction on the local site. (a) deep sea area, (b) land area, and (c) coastal area.
2.2. Geological drilling
mapping, and geophysical prospecting and drilling. And the
construction-drawing design stage lasted from June 15, 2015 to July 24,
2015. The major task during the stage is geological drilling.
Six boreholes with the depth of the boreholes varied between 11.7
and 71.7 m were put into effect during the feasibility study, among which
two boreholes (BH01 and BH02) were located in Male Island with the
elevation of orifice ranged from 1.18 to 1.88 m; one drilling hole (BH30)
was located in Airport Island, and the elevation of orifice was1.2 m; the
other three boreholes (BH13–BH15) were located in the water on the side
close to Male Island (namely, Pier 19) within a distance between holes of
approximately 20 m, and the elevation of orifice was between 26.49
2.1. Engineering geological survey and mapping
Before the mapping task, geological data of the bridge site area were
collected. In the task of geological mapping, topographic map was used
as basemap, then comprehensive mapping of engineering geology was
taken out on the base of topographic map in the same scale of 1:1,000.
Fig. 4. (a) Schematic diagram of side-scan sonar imaging, (b) track line distribution diagram, and (c) towfish dragged in the sea.
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Ocean Engineering 146 (2017) 298–310
Fig. 5. (a) electric spark source, (b) electric spark source excitates seismic waves, and (c) marine seismic streamer receives seismic waves.
Fig. 6. (a) shallow seismic reflection in Airport Island, and (b) shallow seismic reflection in Male Island.
and 34.32 m.
During the construction-drawing design stage, 16 boreholes with
depths varying between 40.30 and 76.40 m were created instead of 24
preseted boreholes. Because in the investigation period, it was the
Southwest monsoon season, and the sea surface was exposed to high
winds and waves, with the surge height typically of 1.5–2.0 m, and
locally reaching 3 m and higher. Moreover, since the water depth is
relatively shallow anchor boats and vessels cannot enter the designated
region, and therefore the work of drilling eight holes (BH05–BH08 and
BH26–BH29) located in the breaking-wave regions was postponed to be
carried out during bridge construction with the help of an established
construction platform (see in Fig. 3).
In the task of geological drilling, drilling technologies such as premium slurry for rotary drilling and whole hole-section coring were
adopted; the open-hole caliber was Φ130 mm, and the end-hole caliber
was at least Φ91 mm. Meanwhile, during the drilling process, in-situ tests
such as the standard penetration test (SPT) and the heavy-cone dynamic
penetration test (DPT) were carried out.
Fig. 7. Schematic diagram of acoustic logging principle.
Fig. 8. Field work of acoustic logging.
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axis of the bridge, and 1 longitudinal measuring line was laid at each side
of the central axis by the spacing of 75 m, the measuring line number was
CS1 to CS3. Simultaneously, 4 longitudinal measuring lines were laid at
120 m spacing, perpendicular to the central axis in the area of main
bridge pier. The measuring line number was CS4 to CS7 (Fig. 4(b)
and (c)).
2.3.2. Seismic imaging
Seismic imaging method is a new engineering geophysical prospecting technology after seismic reflection wave multiple coverage
technology. The method employs geophysical methods based on differences in rock wave impedance (difference between rock density and
velocity). There is no need for NMO correction (when the offset is zero),
so it had the advantage of no waveform stretching distortion caused by
NMO or near surface wide-angle reflection distortion.
In the geophysical prospecting task, 1 longitudinal measuring line
was laid along the central axis of the bridge, 4 longitudinal measuring
lines are arranged on each side of the central axis, the line number was
YX1 to YX9. Meanwhile, 3 longitudinal measuring lines were laid in the
deep seabed trough, perpendicular to the central axis (Fig. 5).
Fig. 9. Vibration exciting wave velocity tester and suspension probe in borehole.
2.3.3. Shallow seismic reflection
The multiple coverage method in high resolution seismic reflection
exploration refers to the superposition of multiple seismic traces at
different excitation points and different receiving points from the same
reflection points at the underground interface.
1 longitudinal measuring line was laid along the central axis of the
bridge in the Male island, and 1 longitudinal measuring line was arranged on each side of the central axis, the line number was DZ1 ~ DZ3.
Moreover, 1 longitudinal measuring line was laid along the central axis of
the bridge, and 1 longitudinal measuring line was arranged on each side
of the central axis, the line number was DZ4 to DZ6 (Fig. 6).
2.3.4. Magnetic surveying
Magnetic surveying was used to investigate underwater topography,
reefs, and sunken vessels.1 longitudinal measuring line was laid along the
central axis of the bridge, 1 line was laid at each side by the spacing of
50–100 m. Moreover, 3 transverse measuring lines were also arranged. In
the obvious change of elevation, as well as within the scope of main pier
and auxiliary pier, the detection interval was encrypted by 10 m spacing.
If the magnetic anomaly occurred, the measuring line shall be encrypted
in the position of magnetic anomaly body, then the specific position and
shape would be further detected.
Fig. 10. Multibeam three-dimensional diagram of the underwater slope near the shore of
Male Island.
2.3.5. Ultrasonic logging
Ultrasonic logging is to excite and accept ultrasonic through short
distance in borehole to determine the change of ultrasonic velocity along
the borehole depth in the rock near the wellbore, and to determine the
integrity of rock formation. In the task, a special transceiver ultrasonic
transducer was placed in drilling, measuring point by point (Fig. 7).
The acoustic logging works were arranged in 4 holes, at the main pier
and auxiliary pier of the bridge. They were used to test the elastic wave
velocity of different rock layers in the hole, to estimate the integrity of
the rock mass and to calculate the elastic mechanical parameters of the
rock mass (Fig. 8).
2.3. Geophysical prospecting
Geophysical prospecting work including side-scan sonar imaging,
seismic imaging, shallow seismic reflection, underwater photography
and magnetic surveying were carried out in the water area near primary
piers. And tests including ultrasonic logging, shear wave velocity testing,
and borehole TV inspection for boreholes were also performed.
2.3.1. Side-scan sonar imaging
The principle of side-scan sonar imaging is using a transducer towfish
to drag out a wide beam of intermittent acoustic pulses over the seabed,
scanning the seabed at both sides of the transducer. The reflected pulses
from various targets on the seabed are received by the same transducer
(Fig. 4(a)). The reflected signals are continuously recorded in the
recording chart. These signals represent reflections from the gravel, rock,
outcrops, and pipes of the seabed. The intensity of the reflected signals
depends on the reflected target. Based on the study of the intensity of
reflected signals and the recording of images, a geological interpretation
of sonar reflected signals can be made, and the size and height of various
targets on the seabed can be estimated.
In the task, 1 longitudinal measuring line was laid along the central
2.3.6. Shear wave velocity testing
Suspension type of probe in borehole is mainly composed of fully
sealed (waterproof) electromagnetic vibration source, two separate hermetically sealed detectors and high strength connecting hose. In the
shear wave velocity testing, shear wave velocity of rock soil layer can be
provided to classify the earthquake resistance of the site (Fig. 9).
Its working principle is when the source is acting an impact force on
the borehole wall, the shear wave propagates downward along the
borehole wall. Two detectors are suspended at the bottom of the source,
when the S wave propagates to the detector position, the fluid coupling
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Table 2
Comprehensive stratigraphic column.
Layer No.
Burial conditions of the rock and soil layer (m)
roof depth
top elevation
Lithologic descriptions and distributions
top thickness
minimum-maximum average
(1)
0.00
0.00
1.18–1.88
1.42
1.30–11.10
4.93
(2)
0.00
0.00
0.00
0.00
0.07–0.05
0.01
29.23 to 46.18
39.25
0.40–0.50
0.45
3.00–18.20
10.48
(4-1)
0.00–11.10
3.36
12.87 to 0.12
6.63
2.40–17.20
7.74
(4-2)
0.00–5.90
1.657
0.00
0.00
34.32 to 0.35
13.70
8.87 to 7.16
8.02
2.30–6.30
4.27
4.00–9.80
6.90
(5-1)a
9.50–19.80
12.98
18.60 to 8.62
12.13
0.90–13.00
9.82
(5-1)b
9.40–28.40
20.33
28.35 to 9.35
20.02
3.60–18.90
8.58
(5-1)c
27.30–32.00
29.65
31.95 to 27.37
29.66
9.60–13.00
11.30
(5-2)a
9.80–17.20
12.30
28.10 to 16.96
21.68
3.20–14.80
10.10
(5-2)b
24.4
24.4
31.56
31.56
16
16
(5-3)
3.00–25.00
10.83
35.52 to 30.37
32.77
13.40–17.50
15.13
(6-1)
10.40–42.70
22.67
69.25 to 46.19
57.87
1.00–17.70
5.53
(6-2)
8.10–46.20
24.85
72.75 to 40.12
56.43
2.00–24.10
7.50
(3)
(4-3)
Fill soil (Qml
4 ): gray or off-white, soils are primarily in slightly or medium densea dense state; they are mainly
distributed in shallow artificial layers of island-surrounding roads and dikes, primarily consisting of coarse
sand (calcareous sands); particles are inhomogeneous, and poorly sorted and graded. mixed with 10–30
percent coral gravels and coral fragments, and locally there may be coral gravels mixing with coral sands; the
grain size of coral gravels is within 2–10 cm, some can be more than 10 cm; types of coral debris that can be
distinguished include staghorn coral and bamboo-like coral; while the diameters of staghorn coral debris are
typically 1–3 cm and the lengths are 2–4 cm, the diameters of bamboo-like coral debris are typically 0.5–2 cm
and the lengths are 3–5 cm; in addition, filling materials comprising household garbage such as plastic bags
and woven bags were found in the lower portion of the fill soil layer in borehole BH02, accounting for around
20% of the total amount. The revealed thickness of the Male coastline is 1.3–2.4 m, and the revealed
thickness of Airport Island's is 11.1 m.
Reef rock block: milk white; primarily consisting of reef limestones that are swept to the seaside under the
action of ocean storms, they are sparsely distributed and mainly located near the seawall of Airport Island.
Gravelly sand mixed with gravel: pale yellow, gray, off-white, saturated; gravel sands are mainly composed
of calcareous bioclastic detritus; grain size of gravel is typically 1–5 cm; some large pieces can be more than
10 cm; with rubbly edges and corners, and show local wear;
few debris that are bamboo-like coral or staghorn coral are mixed, whose typical diameters are 1–2 cm and
lengths are 2–4 cm.
Reef limestone: milk white, off-white; cores are partially semicircular or circular short columns with a length
of 10–20 cm, and partial chunks that are 1–5 cm long; the framework grains are mostly coral gravels between
0.5 and 1.0 cm in diameter, with a few coral gravels measuring 2.0–4.0 cm in diameter; crystalline calcite
cementation; intergranular pore develops, the surface of rock cores is rough, and the rock mass is relatively
fragmentized; this stratum is primarily discovered near the starting point of the bridge and land area sections
in Airport Island.
Reef limestone: milk white, off-white; cores are gravels mixed with fragments mostly of 2–8 cm long; local
core are short columnar coral limestones; intergranular porosity is large, and the rock mass is fragmentized.
Reef limestone: milk white, off-white; cores are partially semicircular or circular short columns that are
5–12 cm long, and partial cores are fragments that are 1–3 cm long; framework grains are mostly coral
gravels of 0.5–1.0 cm in diameter (a few are 2.0–4.0 cm in diameter); crystalline calcite poor
cementation,showinghalf-cementation;
intergranular pore develops, the surface of rock cores are rough, and the rock mass is relatively fragmentized.
Reef limestone: creamy yellow, milk white; cores are mostly 10–25 cm long columns, some cores are
30–40 cm long; framework grains are primarily coral gravels 0.5–1.5 cm in diameter, with few coral
fragments 2–4 cm in diameter; multi crystalline calcite is well cemented; biological pores and intergranular
pores are well developed, surface of partial rock cores are uneven, and there are few biological drilling holes;
the core texture is hard, the knocking sound is crisp, not fragile, and the rock itself is relatively complete. This
layer is mainly found near the starting point of the bridge and land area section in Airport Island.
Reef limestone: creamy yellow, milk white; cores are broken fragments with diameter of 3–7 cm, partial
cores are short columns whose lengths are 5–12 cm, and framework grains of the bulky rock cores are mostly
calcareous bioclastic fragments measuring 0.01–0.5 cm in diameter; hyperlobated calcite crystallizes;
intergranular pore develops, cores are fragile and damaged heavily, and there are few biological drilling
holes. This stratum is mainly discovered near the starting point of the bridge.
Reef limestone: creamy yellow, milk white; cores are mostly fragments and gravels 2–5 cm in diameter, with
a few columns 5–8 cm long; partial cores are not completely cemented, and framework grains are mostly
calcareous bioplastic fragments 0.1–0.5 cm long; locally there are mixed coral limestone gravels and bulks,
polycrystalline calcite crystals; intergranular pores develop, the surfaces of rock cores are rough, cores are
fragile and damaged heavily, and there are few biological drilling holes. Only boreholes BH03 and BH04
revealed.
gravelly sand mixed with gravel: milk white; the major component of the sands are calcareous bioclastic
fragments, the grains are inhomogeneous, poorly sorted and graded; the diameters of the gravel are 1–3 cm,
with a few measuring 3–5 cm; rare gravel are short columns, and intergranular pores fully develop, gravel are
fragile, wear heavily, and there are few biological drilling holes. This stratum is mainly discovered in
boreholes in troughs near the deep water slope on the shore of Male.
gravelly sand mixed with cemented sand & gravel:
milk white; most are clastic detritus with diameter between1 and 3 cm; mixed with a few cemented sand &
gravel with diameter of 4–7 cm, intergranular pores fully develop, cores are fragile, and wear heavily; locally
there are many bamboo-like coral branches, 2–4 cm in length. Only borehole BH09 revealed.
Reef limestone: off-white mingled with gray-black; cores primarily display pie or chunky shape, measuring
3–6 cm in diameter; some are gravels 2–5 cm in diameter; a few are short columns 5–9 cm long; a thin gray
biological calcium cementation layer with a thickness of 0.1 cm are commonly seen in coral gravel and
broken branches; the surface of cores is rough, and the cores are relatively broken. This layer is mainly
discovered in boreholes in troughs near the deep water slope of the shore of Male.
Reef limestone: milk yellow, milk white, off-white; cores are mostly short columns or columns 5–25 cm long
and partial long columns measuring 30–50 cm; locally there are gravels of 3–5 cm in diameter; framework
grains are mostly calcareous bioplastic fragments 0.5–1.0 cm in diameter, with few coral gravel 2.0–4.0 cm
in diameter; polycrystalline calcite is crystallized, and well cemented; intergranular pores develop, the
surface of rock cores are rough, and there are few biological drilling holes; locally, there are red algal
cementation; the cores are relatively hard, the knocking sound is crisp, not fragile, the cores are relatively
complete, and strongly cemented.
Reef limestone: milk white, creamy yellow, off-white; cores partially are fragments and gravels 2–5 cm in
diameter, and partially are short columns 5–15 cm long, locally containing coral limestone gravels; partially
intercalated gravel limestone;
(continued on next page)
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Table 2 (continued )
Layer No.
Burial conditions of the rock and soil layer (m)
roof depth
top elevation
Lithologic descriptions and distributions
top thickness
minimum-maximum average
(6-3)
15.00–39.00
25.82
69.32 to 45.70
57.29
0.80–14.50
5.88
(7-1)
23.80–65.40
43.40
100.30 to 64.22
78.74
1.10–38.30
14.82
(7-2)
32.00–57.00
45.26
99.60 to 78.10
82.92
0.70–8.70
4.77
framework grains are coral gravels 0.5–1.5 cm in diameter, with a few coral gravel 2–4 cm in diameter;
polycrystalline calcite is medium cemented; intergranular pores develop, and the surface of the cores are
rough; there are few biological drilling holes; core texture are hard, the knocking sound is relatively crisp,
not fragile, and cores are relatively complete; this layer is commonly seen in the bridge area.
Reef limestone: milk white, off-white; cores are fragments and gravels 2–5 cm in diameter; few are short
columns 4–9 cm long; particial cores are not completely cemented, and weakly cemented cores turn soft
when meeting with water and become loose when taken in hand; framework grains are mostly calcareous
bioclastic fragments 0.01–0.5 cm long; particial cores are fragmented or bulky coral limestones; cores are
hard, knocking sound is crisp; leaf-like calcite crystallizes; intergranular pores develop, the surface of rock
cores are rough, cores are easy to wear, and the wear is relatively heavy; there are few biological drilling
holes.
Angular gravel mixed with gravel: grey, off-white, angular gravel are primarily composed of bioclastic
fragments; grains are inhomogeneous, poorly sorted and poorly graded; the grain size of gravel are 1–3 cm in
diameter, a few gravel can be more than 5 cm; the gravels are relatively hard, intergranular porosity
develops, and the surface are rough.
Reef limestone: milk white; cores are 5–13 cm long; a few are 3–5 cm; intergranular pores are well
developed, fragile, heavily worn; the surface are rough, and there are a few biological drilling holes.
Fig. 11. Sketch diagram of engineering geological section.
land subsidence and sea-level fluctuation.
The Maldives islands belong to the morphology of the Indian Ocean
atoll chain, in which multiple atoll are distributed in strings or circles.
Male Island and Airport Island are chain islands in Northern Male Atoll,
east of the Maldives Islands, where the terrain is low and flat especially in
the area where the natural lakes in these islands were artificial filled in
and reclaimed as land. The land elevation of the island is primarily between 1.0 and 1.5 m.
The oceanic trench or trough between the two islands near the bridge
axis displays an approximate width of 1.4 km, and exhibits a broad and
gentle “U”-shape (Fig. 10); both shoals show the development of step
topography, the platform is wide and smooth, and the platform in the
shallow water shows a slope gradient increasing from 0 to 10 that is
inclined towards the submarine trench.
The overall gradient of the submarine slope on the side of Male Island
is between 15 and 20 , locally reaching 25 , and the average width is
approximately 200 m. Two levels of platforms manifest within 90 m of
the submarine slope. The average gradient of the upper section of the
slope is around 26 , locally reaching 30 , with 10–40 m depth; the secondary platform shows a gentle slope gradient of approximately 9 , and
its width is 95–100 m; the average gradient of the lower section of the
slope is around 32 , within 55–90 m depth.
The width of the platform in the shoal water within 0–10 m deep in
the side of Airport Island is approximately 100–190 m, and the slope
gradient is 0–9 ; the slope section within 10–20 m depth shows a steep
gradient of 30–35 , and the gradient gradually decreases with the
depth increase.
The width of the deep-sea trench or trough in the bottom of “U”shaped is around 500 m, within 45–50 m depth, and the submarine
topography slightly fluctuates.
detector can convert S wave arrival time and vibration waveform into
electrical signals, recorded by recording instrument. The wave velocity of
the stratum between the two detectors can be calculated from the difference of first arrival time of S wave.
During the feasibility study, shear wave velocity testing was taken out
in 3 boreholes. If the formation condition was complex, the shear wave
velocity testing hole will be increased according to the drilling condition.
2.3.7. Borehole TV inspection
The inspection was arranged in 4 boreholes, at the main pier and
auxiliary pier of the bridge.
The observation window of the whole borehole wall imaging system
is 360 (panoramic observation), which can realize the omni-directional
observation of the occurrence of the target body to be observed.
After the test, unrolled images of the whole borehole wall can be
obtained by stitching along the depth. The borehole TV images can help
to determine the integrity and weathering degree of the rock mass by
comparing with the borehole core and the result of the wave velocity test
in the borehole.
In addition to the above field work, experimental analyses of the
physical, mechanical, and pile foundation load-bearing characteristics of
the coral reef rock and soil samples were also performed in the
laboratory.
3. Engineering geological conditions of the site
3.1. Regional geology and geomorphology
The carbonate platform in the Maldives was formed on a volcanic
plateau since the early Eocene Epoch (approximately 55 million years
ago). And for at least 3.5 billion years, no terrestrial sources (mud) were
input in the Maldives. Accordingly, the carbonate platform consisted
mostly of carbonate sediments. The formation of the overall morphology
of the majority of underwater carbonate sediments is the result of both
3.2. Formation lithology
The boreholes on this site are arranged along the bridge sites. After
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Fig. 12. Partial TV images in BH02 borehole.
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Table 3
Distribution characteristics of holes in each borehole.
Borehole
No.
Elevation of the
top of the
hole (m)
Elevation of
the bottom
of the hole (m)
Height
of the
hole (m)
Embedded
depth (m)
Filling conditions
BH04
BH14
BH16
BH20
27.95
28.85
78.80
57.70
28.35
31.25
79.30
58.70
0.4
2.4
0.5
1.0
28
2.3
41
15.4
BH21
BH24
89.70
67.82
71.72
71.33
90.30
69.32
73.32
75.73
0.6
1.5
1.6
4.4
48.1
36.2
40.1
42.1
No fillings, drilling tool drops
No fillings, drilling tool drops
No fillings, drilling tool drops
Between 57.70 and 58.10 m, there are yellow-purple clays; between
58.10 and 58.70 m, there is no filling, and the drilling tool drops
No fillings, drilling tool drops
No fillings, drilling tool drops
No fillings, drilling tool drops
No fillings, drilling tool drops
BH25
characteristics, the engineering geological unit revealed by drilling in the
bridge area can be divided into two major layers, from top to bottom:
upper loose Holocene strata (Q4) and lower Pleistocene lithogenetic reef
limestone strata (Q3). The previous strata can be further divided into
three layers based on the composition of particle and gradation feature;
the latter strata can be further divided into four layers, comprising
fourteen sublayers, according to the degree of cementation and characteristics of particle composition. Descriptions of corresponding lithological characteristics and stratification are listed in Table 2, and an
engineering geological sectional view is illustrated in Fig. 11.
3.3. Unfavorable geological condition
Diagenesis of coral reef limestones is complicated, a relatively large
number of pot holes can be seen from the TV images taken inside the
drilled holes, as well as grooves and cavities formed in a late stage subject
to abrasion (Fig. 12). Coral reef limestones are vulnerable to bioerosion
during their formation process; meanwhile, holes are easily formed under
the comprehensive effect of chemical dissolution and physical erosion.
Holes exist throughout the entire coral reef stratum, and the height of
holes revealed by boreholes are mostly lower than 1.0 m; a few relatively
large holes are influenced by the early-stage reef diagenetic process, and
are also the result of further enlargement of holes under the influence of
succeeding dissolution. Consequently, comprehensive analysis assessed
that the holes in the current site belong to moderate development.
Concealed holes are common exist in the stratum studied within the
scope of this investigation. Specifically, they were found in boreholes
BH04, BH14, BH16, BH20, BH21, BH24, and BH25, etc. In these boreholes, drilling tools happen sudden drop. Yellow-purple clays were only
found in borehole BH20, and filled half of the hole. Other boreholes
showing no filling result in severe water-loss phenomena during drilling.
In particular, boreholes BH24 and BH25 in Pier 23 display relatively
large holes within 71–75 m depth. The possibility that these holes are
connected in the plane cannot be excluded, which requires sufficient
attention from designers. The distribution characteristics of holes inside
boreholes are listed in Table 3.
Fig. 13. Loading system of large direct shear apparatus.
combining the stratum revealed by the holes drilled and the previously
data collected, comprehensive analysis of the major engineering stratum
layers in Male Island and Airport Island are found to consist of layers of
loose Holocene coral mixed with sands or sands mixed with coral, upper
and middle Pleistocene reef-flat limestone, and deep-lagoonal weakly
cemented or uncemented coral mixed with sands or sands mixed
with coral.
According to the age, lithological, and engineering geological
4. Geotechnical engineering performance testing of the site
The geotechnical properties of coral sand and reef limestone in field
area are tested in laboratory and field. Field tests include standard
penetration test, dynamic penetration test and wave velocity test. And
the indoor rock-and-soil tests include natural water content test of coral
sand, large direct shear test, undrained consolidated triaxial test, saturation density test of reef limestone, standard point load strength index
and saturated uniaxial compressive strength test. Test sample size, test
environment, test methods and test results are as follows.
4.1. Indoor rock-and-soil tests
4.1.1. Natural water content test of coral sand
Coral sand sample were collected in plastic bags in the field. In the
Fig. 14. Vacuum saturation for specimen.
306
C.Q. Zhu et al.
Ocean Engineering 146 (2017) 298–310
Fig. 15. Saturated uniaxial compression strength test of reef limestone.
Table 4
Physical and mechanical property indexes of coral sands.
Index
Natural water content (%)
Large-scale direct shear test
Consolidated-undrained
triaxial test
C (kPa)
ϕ ( )
C (kPa)
ϕ ( )
Table 7
Saturated uniaxial compressive strength of reef limestones (MPa).
Sample size
Range
Mean
33
6
6
3
3
3.3–22.6
9.9–103.3
38–58
151.2–400.7
21–34
15.4
56.9
47
255.2
27
Table 5
Saturation density of reef limestones (g/cm3).
Type of Rock and Soil
Frequency
Variable coefficient
Range
Mean
(4-1) Reef limestone
(4-3) Reef limestone
(5-1)a Reef limestone
(5-1)b Reef limestone
(5-1)c Reef limestone
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel
mixed with gravel
(7-2) Reef limestone
9
3
4
2
4
10
5
12
10
16
0.04
2.02–2.35
1.97–2.73
2.37–2.46
2.07–2.37
2.25–2.31
2.13–2.52
2.16–2.43
2.06–2.45
2.14–2.37
2.04–2.46
2.21
2.31
2.40
2.22
2.29
2.34
2.26
2.25
2.23
2.20
0.05
0.05
0.04
0.07
1
laboratory, the natural water content tests of coral sand samples of
different depth were carried out by the drying method.
Coral sand were dried at 105 C in an oven for not less than 6 h,
Table 6
Strength index of standard point load of reef limestones (MPa).
Frequency
Variable
coefficient
Range
Mean
(4-1) Reef limestone
(4-2) Reef limestone
(4-3) Reef limestone
(5-1)a Reef limestone
(5-1)b Reef limestone
(5-1)c Reef limestone
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel mixed
with gravel
(7-2) Reef limestone
40
18
10
37
35
10
70
37
105
29
122
0.61
0.60
0.35
0.43
0.57
0.58
0.39
0.56
0.43
0.34
0.46
0.5–12.3
0.9–7.1
1.8–5.2
0.5–5.5
0.6–7.3
0.1–3.9
0.6–8.4
0.7–8.5
0.9–6.3
1.0–4.8
0.0–6.0
3.5
2.9
2.8
2.3
2.5
1.9
5.0
3.3
2.8
2.7
2.4
28
0.55
0.9–7.7
2.6
Frequency
Variable
coefficient
Range
Mean
(4-1) Reef limestone
(4-3) Reef limestone
(5-1)a Reef limestone
(5-1)b Reef limestone
(5-1)c Reef limestone
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel
mixed with gravel
(7-2) Reef limestone
7
3
23
4
5
2
21
24
7
7
0.64
0.54
0.47
0.77
0.30
2.8–22.2
7.1–12.8
3.3–26.3
2.7–14.1
3.1–11.9
5.2–13.7
2.8–29.8
2.8–18.3
1.1–13.3
3.7–7.2
10.7
10.1
10.2
7.5
5.9
9.4
10.6
8.4
5.5
5.2
17
0.87
2.2–34.6
10.4
0.63
Table 8
Statistical table of the standard penetration blow count.
2.12
Type of Rock and Soil
Type of Rock and Soil
Type of Rock and Soil
Frequency
Variable
coefficient
Range
Mean
(3) gravelly sand mixed with gravel
(4-1) Reef limestone
(4-2) Reef limestone
(5-2)a gravelly sand mixed
with gravel
(5-2)b gravelly sand mixed
with gravel
(5-3) Reef limestone
(7-1) Angular gravel mixed
with gravel
13
1
4
2
0.24
12–29
18–32
22–23
21
76
25
22
33–41
37
39–64
60
53
2
1
7
0.14
drying to constant weight. The experiment used 2 parallel tests and took
the arithmetic average.
Table 9
Statistical table of the conic dynamic sounding test blow count (N63.5).
307
Type of Rock and Soil
Frequency
Range
Mean
(1) Fill soil
(3) gravelly sand mixed with gravel
(4-1) Reef limestone
(4-2) Reef limestone
(5-3) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
4
3
2
3
5
2
3
6–10
11–14
14–125
10–38
12–26
28–35
22–41
7
12
69
26
20
31
32
C.Q. Zhu et al.
Ocean Engineering 146 (2017) 298–310
Taking the sample of drilling coral sand P5 ¼ 66.8% as standard (P5
was the mass percentage of the coral sand in which particle size was
greater than 5 mm in the sample), the direct shear tests of P5 ¼ 54.86%
and P5 ¼ 76.96% were carried out at a shear rate of 4 mm/min. The dry
density of the prepared sample was 1.37 g/cm3, and the moisture content
was the natural water content.
In the test, the vertical load of 1 kN was adopted to make the components reach intimate contact, and then the vertical loading rate was
controlled to 1 kN/s, until the required load was reached, respectively,
50 kPa, 100 kPa, 200 kPa and 400 kPa. The sample was consolidated
until there was no displacement in the vertical direction. Then the bolts
were removed, and sheared. The test was stopped when the relative
displacement between the upper and lower shear box reached 140 mm.
4.1.3. Undrained consolidated triaxial test
Coral sands taken from the BH13 borehole were applied to the test.
Due to limited samples, the coral sands were divided into three parts:
shallow layer, middle layer and deep layer. Before the test, sands in a
particle size more than 10 mm would be removed. Then the sands was
prepared into solid cylinder samples of diameter 61.8 mm and height
120 mm. Shear rate in the test was 1 mm/min, confining pressure were
200 kPa, 400 kPa and 800 kPa.
Fig. 16. Laboratory acoustic wave test on reef limestone samples.
Table 10
Shear wave velocity of rock-soil layer inside boreholes, Vs (m/s).
Type of Rock and Soil
Mean
BH2
(1) Fill soil
(3) gravelly sand mixed
with gravel
(4-1) Reef limestone
(4-2) Reef limestone
(5-1)a Reef limestone
(5-1)b Reef limestone
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel
mixed with gravel
(7-2) Reef limestone
Statistical value
BH13
BH20
198
BH30
216
207
228
475
398
364
606
626
432
633
628
587
589
228
321
458
615
626
691
270
597
432
633
636
587
565
558
614
610
4.1.4. Saturation density test of reef limestone
Reef limestone samples of diameter 50 mm and height 100 mm were
took vacuum pumping saturation for at least 4 h, reaching the absence of
bubbles. Then let the samples stand for at least 4 h at atmospheric
pressure in the original container (Fig. 14). After saturation, took out the
samples and dried the surface water until no water dripped, took the
quality of the saturated samples, accurate to 0.01 g.
4.1.5. Saturated uniaxial compressive strength test
Reef limestone samples taken from boreholes were processed into
standard solid cylinder samples of diameter 50 mm and height of
100 mm. After saturation, RMT multi-function rock servo test machine
was carried out to test the ultimate stress, elastic modulus and Poisson's
ratio of reef limestone samples under axial pressure (Fig. 15).
The physical and mechanical property indexes of coral sands are listed in Table 4. Statistical values of the saturated density, strength index of
Point Load Test and saturated uniaxial compressive strength for reef
limestones are shown in Tables 5–7. Statistics indicate that the mean of
saturated density of reef limestones is between 2.12 and 2.40 g/cm3, and
the discreteness of the experimental values for each reef limestone layer
is relatively small. But, the layered statistical results of strength index of
Point Load Test and saturated uniaxial compressive strength show relatively large variability, and the variable coefficient ranges between 0.30
and 0.87, indicating a relatively significant unevenness of the strength of
reef limestones.
610
Table 11
Longitudinal wave velocity of rock-soil layer inside boreholes, Vs (m/s).
Rock type
Mean
BH2
(1) Fill soil
(3) gravelly sand mixed
with gravel
(4-1) Reef limestone
(4-2) Reef limestone
(5-1)a Reef limeston
(5-1)b Reef limestone
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel
mixed with gravel
BH13
BH20
1,042
BH30
987
1,014
1,355
2,468
1990
1747
3,319
2,876
2,146
3,019
2,530
2,587
2,959
1,355
1,512
1795
3,364
2,876
2,718
1700
3,274
2,146
3,019
2,613
2,587
3,283
2,259
2,635
Statistical
value
4.2. In-situ tests inside the boreholes
The results of standard penetration test (SPT) blow count and conic
dynamic sounding test blow count in coral reefs shown in Tables 8 and 9
indicate the relatively large discretenesses in the ground. In particular,
the minimum and maximum of the SPT blow count is respectively 12 and
76. The conic dynamic sounding test blow count also displays the same
discreteness. With the increase of stratum depth, the experimental value
shows an increasing trend.
4.1.2. Large direct shear test
During the large direct shear test, the length, width and height of the
shear box were 500 mm, 500 mm, 410 mm respectively (in which the
shear seam is 10 mm in the height).
The baffles on both sides of the shearing box were connected with the
upper and lower shearing box through the bearing and the pulley, and
the 10 mm shearing joint was generated between the upper and lower
shear boxes, so that the influence of the friction on the experimental
result was eliminated (Fig. 13).
4.3. Wave velocity test
In order to study the relationship between the features of integrality
and structure of reef limestones and geotechnical strength. Wave velocity
tests in situ in four boreholes and laboratory acoustic wave test on reef
limestone samples were put into effect (Fig. 16). The results
(Tables 10–12) reveal that the shear wave velocity (Vs) of each rock-soil
308
C.Q. Zhu et al.
Ocean Engineering 146 (2017) 298–310
Table 12
Longitudinal wave velocity of reef limestones, Vs (m/s).
Index
Dye condition
Saturated condition
Borehole
Frequency
Range
Mean
Frequency
Range
Mean
BH2
BH13
BH20
BH30
20
3,565–5,070
4,631
13
3,544–4,654
4,248
13
4,034–4,871
4,650
12
3,018–4,737
4,219
12
3,402–5,084
4,561
7
3,431–5,192
4,343
7
3,504–5,505
4,655
Fig. 17. Friction resistance characteristic test for rock-socketed pile.
Fig. 18. End resistance characteristic test for rock-socketed pile.
layer is within 207–633 m/s and the longitudinal wave velocity (Vp) is
between 1,014 and 3,319 m/s. The longitudinal wave velocity (Vp) of
saturated reef limestone samples measured indoors were approximately
4,622 m/s. According to the results of wave velocity measurements, the
following conclusions can be reached: (1) The longitudinal wave velocity
(Vp) increases with increase of burial depth. (2) The longitudinal velocity
(Vp) shows small in those reef limestones having large porosity and many
holes. (3) The integrity index of reef limestones in the proposed site is
0.30, and the rock mass is categorized to be fragmentized according to
related terms in the Chinese National Standard “Code for Investigation of
Geotechnical Engineering” (GB50021-2001) (Ministry of housing and
urban-rural development of the people's republic of China, 2009).
rolling in late-stage. They are now in slightly dense or medium dense
status, thus can be used directly as foundation bearing layer for
connection road in the future. Other section of the foundation soil due to
non compaction, most of them are in loose or slightly dense status, with
relatively low strength, high compressibility and poor uniformity. So the
method of vibrating compaction or mechanical compaction was needed
to strengthen and improve the compactness and uniformity of the foundation. Of particular note, road foundation founded on reef flat or outer
coral reef flat should be treated using the same method above even if the
thickness of the overlying unconsolidated sediments was not large.
5. Selection of foundation type
As a large scale sea-crossing bridge, the bearing load of the proposed
bridge foundation is relatively large. Consequently, the type of pier
foundation overmatching natural foundation or enlarged pier foundation
with high load-bearing capacity must be selected. Moreover, the bored
pile group foundation is the most ideal type for proposed construction
after taking into account key indicators such as topography, geology,
construction factors, and the environment.
Coral reefs are a special biogenetic rock and soil medium. Owing to
properties such as brittleness, porosity, and low material strength, associate with the relatively outdated and not rigorous investigative
5.2. Proposed bridge
5.1. Connection roads
The foundation of connection roads can be formed by filling. The
filling soil layer, reef gravel layer, or gravelly sand mixed gravel layer can
be treated as the foundation bearing layer, and the filling materials can
be coral fragment, bioclastic gravels or bioclastic sands. In the overlapped section between connection road and the current road, the
foundation soils are compacted both by mechanical methods and vehicle
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C.Q. Zhu et al.
Ocean Engineering 146 (2017) 298–310
Table 13
Suggestion values of design parameters for coral reef rock-soil layer.
Stratum
(1) Fill soil
(2) Reef stone
(3) gravelly sand
mixed with gravel
(4-1) Reef limestone
(4-2) Reef limestone
(4-3) Reef limestone
(5-1)a Reef limestone
(5-1)b Reef limestone
(5-1)c Reef limestone
(5-2)a gravelly sand
mixed with gravel
(5-2)b gravelly sand
mixed with gravel
(5-3) Reef limestone
(6-1) Reef limestone
(6-2) Reef limestone
(6-3) Reef limestone
(7-1) Angular gravel
mixed with gravel
(7-2) Reef limestone
Dynamic
penetration
blow count
(count/10 cm)
Saturation
density ρsat
(g/cm3)
Strength index
of Standard
point load (mpa)
Saturated uniaxial
compressive
strength frk (mpa)
Characteristic
value of foundation
bearing capacity
fak (kpa)
Standard value of
ultimate side
friction resistance
qsik (kpa)
120–180
/
150–200
15–25
/
45–50
22
450–500
350
300
800
550
400–450
150–220
100–120
80
70
220–240
150–180
80–120
40–55
37
230–250
58–65
Standard penetration
blow count
(count/30 cm)
7
21
12
76
25
69
26
2.21
2.31
2.40
2.22
2.29
60
20
31
32
53
3.5
2.9
2.8
2.3
2.5
1.9
10.7
10.1
10.0
7.5
5.9
2.34
2.26
2.25
2.23
2.20
4.9
3.3
2.8
2.7
2.4
9.4
10.6
8.4
5.5
5.2
400–500
650–700
450–550
350–400
400
90–140
180–200
110–170
90–120
90–100
2.12
2.6
10.4
450–500
120–150
structure with developed pores and varied cementation or diagenetic grade. Therefore, local experience should be taken into account for the selection of design parameters for coral reef
limeston.
4). Because of the lack of experience in the design and construction of
large pile foundation in this area, the design parameters of the pile
foundation for the reef limestone are limited to use in a preliminary design stage. It is suggested that the testing of the bearing
capacity of field pile foundations should be performed to fully
understand the load-transfer mechanism of the pile body before
the actual construction of pile foundations, so as to provide accurate design parameters for the pile parameters and the optimization of pile bearing stratum.
methodology of coral reefs, several engineering accidents have happened
in the worldwide caused by coral reef rock and soil medium. In 1968,
during the construction of the Arabian gulf ocean platform, the pile
foundation was free falling 7.5 m when the pile was going through the
cemented calcareous soil layer (Mcclelland, 1988). In 1982, a surprisingly low pile resistance took place in the northwest continental shelf of
Australia, at the time, a diameter of 1.8 m base pile was free falling 60 m
under its own weight (King and Lodge, 1988). These indicate that the
friction of the piles can not meet the design requirements.
The Maldives has never mounted such a large engineering and construction project before, and the local experts are lack of experience in
the engineering properties of coral reefs, especially regarding the properties of pile foundations of coral reefs.
In view of this, test device researching on the property of pile foundation engineering were specially developed. And experimental studies
(Figs. 17 and 18) on the property of foundation bearing of reef limestones
(Zhu and Meng, 2015) were carried out (Table 13).
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Grant nos. 41372318, 41330642 and 41372316), the National
Key Technology Support Program (Grant no. 2014BAC01B01), the National Program on Key Basic Research Project of China (Grant no.
2013CB956104 and 2012CB026103).
6. Conclusions and suggestions
1). Coral reefs are a special type of rock and soil medium. The
physical and mechanical properties and related relationships of
coral reef rock and soil in the Maldives were analyzed using
investigative methods such as field drilling and geophysical prospecting, combined with in situ measurement methods such as
standard penetration tests inside boreholes, dynamic penetration
tests, and wave velocity tests inside boreholes, as well as indoor
tests of water content, density, point-load bearing, uniaxial
compressive strength, wave velocity of reef limestones, and
bearing capacity of piles. The investigative methods adopted in
this project along with the findings of the investigations can be
applied and used as a reference for similar engineering projects.
2). The wave velocity test results indicate that the integrity of coral
reef rocks at the proposed engineering site is categorized as
fragmentized.
3). The suggested values of the design parameters of each stratum
proposed by the author are primarily based on the test results of
coral limestones. On-site investigation and testing found that
mechanical indicators like strength and deformation of the test
samples are significantly better than the actual rock bodies, which
is even more apparent for reef limestones that possess an uneven
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