Engineering geological characteristics of sea area of Guangdong, Hong Kong and Macao cross-sea bridge

(1. Guangzhou Marine Geological Survey Guangzhou 510760; 2. China Geo University (Beijing) Beijing 100083)

Introduction to the first author: Ma, male, born in 1968, senior engineer, in-service master of engineering, mainly engaged in seismic data interpretation, environmental engineering geology, marine geology and comprehensive research.

Through the detailed analysis of the measured data such as geophysical prospecting, seabed sampling, drilling and field test, it is found that the sea area where Guangdong, Hong Kong and Macao cross-sea bridge is located has unique natural conditions and complex marine engineering geological characteristics. The seabed topography is complex, and there are potential geological disasters such as shallow gas zones, active faults, sand waves, seismic activity, irregular bedrock, buried ancient rivers, scour ditches and underwater shoals. In particular, the Guangdong-Hong Kong-Macao cross-sea bridge is a very large building, and there are many potential geological disasters in the sea area it passes through, which should be paid attention to.

Keywords engineering geological disasters, geological factors, Guangdong, Hong Kong and Macao cross-sea bridge

1 preface

Guangdong-Hong Kong-Macao Cross-sea Bridge is a super-large bridge connecting Shenzhen, Zhuhai, Hong Kong and Macao in Guangdong Province. The sea area of the bridge site is located in Lingdingyang, the estuary of Beijiang River, Xijiang River and Dongjiang River. It is a huge trumpet-shaped bridge extending to the south, and it is a golden waterway reaching five continents and four seas.

Since the 1990s, the economic development on the east and west sides of the Pearl River has been unbalanced. Therefore, it is urgent to strengthen the economic ties between the two sides of the Pearl River. At the same time, Humen Bridge, the only bridge across the two banks, is expected to reach saturation within 5-8 years. As early as 1992, the Zhuhai municipal government put forward the engineering scheme of the Guangdong-Hong Kong-Macao cross-sea bridge. The scale of the cross-sea bridge project is huge and the engineering conditions are extremely complicated, so the engineering geological work is particularly important.

As we all know, in order to ensure the safety of these offshore projects and operations, it is necessary to understand the engineering geological conditions of the seabed and find out the geological factors of potential disasters. Therefore, based on a large number of measured data such as geophysical prospecting, seabed sampling, drilling and field test, combined with the data of surrounding areas, this paper analyzes the seabed topography, shallow strata, sediments and disaster geological factors in the sea area of the bridge site of Guangdong, Hong Kong and Macao, and provides basic geological data and scientific reference for the selection and bridging of the bridge site of Guangdong, Hong Kong and Macao.

2 seabed topography

Lingdingyang is surrounded by the sea on three sides and the South China Sea in the south, which is a drainage depression in the Pearl River Delta fault basin. This is a banded estuary and tidal channel. Due to the interaction of rivers, seawater tides and waves, the inner shore of the bay is shallow and tortuous, with numerous inlets and abrupt headlands, terraces and beaches, islands and sandbars, and Qi 'ao Island. The Pearl River Estuary-Lingdingyang is not only a navigation artery, but also a natural harbor, with 10,000-ton ships sailing freely and the sea water is not frozen all year round.

The seabed topography of Lingdingyang is generally distributed in three beaches and two troughs, which are: Xitan, Dingling Waterway, Zhongtan (Fan Shi Shoal), Fan Shi Waterway and Dongtan from west to east. The underwater terrain trend is influenced by it, and the east-west terrain changes greatly and fluctuates alternately. Isobathymetry is distributed along the channel in NNW-NW direction. The main channel is basically located at the center line of the river bed, and consists of Dingling Channel and Fan Shi Channel, which are naturally scoured and artificially dredged. The water depth is generally 6 ~ 10m, and the Fan Shi waterway to the west of Lujiazui in the harbor is the deepest, exceeding 22m. The rivers on the east and west banks become shallower to 0.2 ~ 2.0m To the east of Panyu New Reclamation, two waterways meet and connect with Longxue Waterway in the north. In some sections of the river, the main channel deviates eastward.

Lingdingyang is a trumpet-shaped estuary, and the larger geomorphic unit in the bay is three beaches and two troughs, on which many small geomorphic types are developed. Lingdingyang submarine landforms mainly include gullies, sand waves, depressions and shoals. The main channels are two of the two channels in Santan, the west channel-Dingling channel and the east channel-Fan Shi channel. The two waterways trace back to the vicinity of Jiaomenkou to form a large waterway, connecting Longxue Waterway and Chuanbi Waterway to Humen. The terrain in the ditch is uneven, with depressions of different sizes living in it and small sand waves in the NE direction. Influenced by the sand coming from Jiaomen and the forced siltation in Xitan, the channel from Caoxi to Dingling is seriously silted and the degree of shrinkage is shallow. Gully belongs to the landform type of natural and artificial interaction. Due to the scouring and silting of rivers and tidal currents, a waterway is formed at the entrance. Due to human needs (navigation or flood discharge, etc.). ), dredging or reclamation activities on or around the original trench not only changed the face of the trench, but also changed the surrounding hydrodynamic environment.

On the west bank of Lingdingyang, many waterways, such as Humen, Jiaomen, Hongqimen and Hengmen, discharge incoming water and sand. Due to the differentiation of water flow and the discharge of alluvium, there are many shoals, sandbars or gullies outside the water crossing. In coastal harbors, soft silt is often filled to facilitate reclamation and breeding. Lingdingyang exports include Qi 'ao Island and Neilingding Island.

3 shallow geophysical characteristics and sequence

According to the 3.5 kHz shallow profile and single channel seismic profile, three reflection sequences, A, B and C, are divided according to the characteristics of reflected waves (Figure 1).

Layer A: It is a horizontal layer with weak reflection energy and good continuity, and it is a parallel and integrated covering reflection structure.

The thickness of this layer varies greatly, ranging from 0 to 0 ~ 26.4 m. Generally speaking, the thickness near the coast and islands is small, and the thickness in offshore and river areas tends to increase. The thickness of the Inner Lingdingyang Ocean in the north is the largest, and this layer is missing near Okawashima in the east.

Layer B: It is a set of continuous reflection layers with medium-low frequency, medium amplitude and medium-low, disordered and valley-filled, with basically parallel and nearly parallel reflection structures. Layer B is widely distributed in the whole region, which is unconformity contact with layer A. The top surface of layer B has been seriously eroded, and the bottom surface is undulating bedrock, which is in overlying relationship with the underlying stratum.

The partial reflection of layer B is disordered, not layered, and the reflected energy is strong and weak, and the stratum fluctuates, which has the characteristics of valley-filled continental deposition. It may be an abnormal area with active erosion and deposition, and a small area of river erosion can be seen locally.

Layer C: It is a set of weak reflection layer with medium and low frequency, medium amplitude and low continuity, with random reflection structure and bedrock surface.

According to the reflection characteristics inside layer C and the distribution of drilling, land and nearby island strata, it is considered that layer C is mainly composed of bedrock weathering and bedrock. There are three kinds of bedrock in Shenzhen-Hong Kong-Zhuhai-Macao sea area: one is granite, mainly the granite of Yanshan Phase III () and Phase IV (); Second, tertiary sedimentary rocks, mostly tertiary sedimentary sandstone, Cretaceous gravelly coarse sandstone and silicified breccia; Third, metamorphic rocks, Sinian and pre-Paleozoic granite gneiss.

The buried depth of bedrock varies greatly, from 0 to -64. 1m, generally becoming shallower near the coast and islands, and getting deeper in the offshore and river channels.

Figure 1 Single channel seismic profile shows sequence, fault and buried ancient river channel.

Figure 1 Ancient rivers and faults buried under the sea

The sediments in layer A exposed by drilling are mainly clayey sand and sand-silt-silty clay. According to the reflection characteristics of shallow profile, combined with the analysis of regional geological data, seabed sampling and borehole data, the geological age of layer A is sediments gradually accumulated since the transgression of the late Holocene ice age, and the reflection sequence of layer A is mainly Holocene shallow-sea facies deposits, but it is partially influenced by rivers and has channel deposits. The lithology is mainly clayey sand, silty clay and biological debris such as shells.

Drilling reveals that layer B is a set of clayey silty sand, fine sand-coarse sand (including gravel) and silty clay-clay, which is mainly composed of continental deposition and erosion, and locally deposited by land and sea. According to the fluctuation and erosion characteristics of the top boundary R 1 interface, compared with the stratum data drilled by the Lingdingyang section bridge, according to the dating of 14C, the annual average of the B-layer sample is greater than 15000 a(B.P). Combined with regional lithology and paleontological data, it can be considered that it is caused by the erosion of layer B in the late sedimentary period, so layer B is inferred.

4 engineering geological characteristics

There are four types of surface sedimentary soil, namely: flowing mud, silt, muddy soil, silt mixed with sand or sand mixed with silt.

The micro-penetration bearing capacity of seabed topsoil is 15.5 ~ 52. 1 kPa, with an average of 30.6kPa, and the undrained shear strength of vane is 2.8 ~ 1 1.6 kPa, with an average of 6.7kPa.

The cohesive force (triaxial shear resistance) of seabed topsoil is 0.3 ~ 18.6 kPa, with an average of 8.8kPa. From Qi 'ao Island to Neilingding Island, from Neilingding Island to Dachan Island, it changes greatly, being 1.6 ~ 10.0 kPa.

The friction angle (triaxial shear resistance) of seabed topsoil is 2.3 1 ~ 14.9, with an average value of 4.88. From Qi 'ao Island to Neilingding Island, and from Neilingding Island to Dachan Island, there are great changes, ranging from 3.7 to 10.2.

The natural water content of topsoil is 27.6% ~11%,with an average of 76.7%. From the north of Neilingding Island to the north of Dachan Island and Qi 'ao Island, the area changes greatly, ranging from 43% to 95%, and the overall change trend is gradually decreasing from the shore to the center of the river.

The natural porosity ratio of seabed topsoil is 0.70 1 ~ 2.86 1, with an average of 2.0 16. From the north of Neilingding Island to Dachan Island and near Qi 'ao Island, the changes are slightly larger, which are 1.0 ~ 2.2 and 1.6 ~ 2.5 respectively, and the overall change trend is gradually decreasing from the shore to the center.

The compressibility of topsoil is 0.44 ~ 3.380 MPa- 1, and the average value is 1.55MPa- 1. The area north of Qi 'ao Island varies greatly, ranging from1.0 to 2.2 MPa-1.

To sum up, in Qi 'ao Island-Neilingding Island-Dachan Island, the cohesion, friction angle, natural moisture content, natural void ratio and compressibility of seabed topsoil change greatly, while other areas change gently. The general trend of natural water content and natural void ratio also shows a gradual decrease from the shore to the center of the river.

The engineering geological layers on the seabed are:

(1) coverage

A Holocene marine silt, gray-black-gray, plastic, saturated, rich in organic matter, with a thickness of 6.0 ~ 25.0m.

B clay, with variegated colors such as brownish yellow, orange red, gray and white, irregular variegated structure, mainly plastic, is the weathering product of intermittent deposition period. Only found in the east and west, with a thickness of1.5 ~ 5.6m..

C silty soil, Holocene marine deposit, dark gray, gray-black, flowing plastic-soft plastic, widely distributed in the whole region, with an average thickness of10.0m.

D. The sand layer developed in the late Late Pleistocene, consisting of silty sand, fine sand, medium-coarse sand, gravel sand, round gravel and pebbles, with poor sorting, staggered transition, often lenticular, unequal thickness, wedge-shaped output, and sequence structure with fine upper part and coarse lower part. The sand layer is mostly medium-dense to dense, and the upper part is slightly dense to medium-dense. The thickness of both banks is 10 ~ 15m, which becomes thinner and pinches out. The maximum thickness of the middle section is 24.0 ~ 37.0m.

(2) bedrock

It consists of Yanshanian granite, Paleozoic granite gneiss, Sinian granite gneiss, Cretaceous gravelly coarse sandstone, silicified breccia and fractured granite. The lithology is complex and changeable, which is obviously influenced by structural faults in the area. The uniaxial saturated compressive strength is 25.0 ~ 106.0 MPa. The height difference of bedrock surface at the east and west ends fluctuates greatly, and the buried depth is below 0 ~ 45.0 m, and the buried depth in the middle is mostly 55.0~60.0m m.

5 Main geological hazard factors

Submarine geological hazard factors refer to geological factors that have some direct or potential danger to the construction and safety of offshore structures under the seabed and strata (Feng Zhiqiang et al., 1995). The analysis results show that the main geological hazards in this area are shallow gas, active faults, sand waves, seismic activity, irregular bedrock, buried ancient rivers, gullies and underwater shoals (Figure 2). They all have direct or potential dangers to offshore structures.

5. 1 Shallow gas (reflection fuzzy area)

Shallow seabed gas is mainly distributed in shallow sediments in estuaries and shelf areas, which is not only a common geological phenomenon, but also a very dangerous geological factor of marine disasters. According to the investigation, shallow gas is distributed in the southeast coast of China and the alluvial plain of the Yangtze River basin, such as Jiangsu, Zhejiang, Anhui, Shanghai, Fujian, Guangdong, Hubei, Hunan and other places (Ye Yincan et al., 2003; Chen Shaoping et al., 2004).

Fig. 2 Schematic diagram of the distribution of potential geological hazard factors in Shenzhen, Hong Kong, Zhuhai and Macao.

Fig. 2 Distribution map of potential geological hazard factors in Lingdingyang area

The shallow gas in the Pearl River Estuary is mainly biogenic, and its main components are methane, carbon dioxide, hydrogen sulfide, nitrogen and ammonia. Under the pressure of overlying water, soil and rock, shallow gas mostly migrates upward along faults or fractures. When shallow gas exists in the form of gas in sediment, the gas in sediment changes the mechanical properties of soil in sediment, which reduces its strength and loose structure, destroys the original stability of soil and reduces the supporting force of basement. Under the action of external load, gas-bearing sediments will creep, which may lead to settlement, lateral or rotational sliding, resulting in the final imbalance of buildings on them and inclined collapse. Shallow gas in layered reservoirs has a large gas content and a certain pressure. Once the platform leg is inserted on it, it will damage the equipment lightly, and even cause a "blowout" accident during drilling, which is very harmful. During the exploration and development of offshore oil and gas resources in the Gulf of Mexico, the British North Sea, the Indonesian Java Sea, the Alaska Sea, the Persian Gulf and the Caribbean Sea, due to the lack of investigation and understanding of shallow gas, certain disaster losses have been caused (Feng Zhiqiang et al., 1995).

The sediments in the Pearl River Estuary are thick and mainly composed of terrigenous clastic sediments rich in organic matter, especially argillaceous sediments, which are rich in humic organic matter, which is beneficial to the generation of biogas (biogas) under biodegradation. This kind of gas may be captured by the near source of underwater channel sand bodies, delta sand bodies and other reservoirs and gathered on the shelf, and may freely diffuse between regions without long-distance migration, forming a large range of gas-bearing sediments.

Shallow profile and single-channel seismic records show that the interlayer reflection of gas-bearing sediments is chaotic, the reflection wave with good continuity is suddenly interrupted, the in-phase axis is sometimes hidden or blurred, accompanied by blank zones, which are columnar, cystic, banded or irregular (Figure 3). The gas-bearing characteristics of such sediments are found at different water depths. This is due to the increase of gas content in the stratum, which slows down the seismic propagation speed, and the rapid attenuation of reflected wave energy leads to the formation of acoustic blank zone on the profile, that is, shallow gas appears as "reflection fuzzy zone" on the profile (Feng Zhiqiang et al., 1995). Where a large number of shallow gas overflows, the seabed topography is often uneven, and sonar records are mostly pockmarks.

Shallow gas is closely related to the ancient river channel. Abnormal seismic reflection often occurs in the ancient river channel, that is, acoustic waves are absorbed or severely shielded, resulting in reflection blank zones and regions, which are gas-bearing deposits. Sediments and fillings in ancient rivers are mainly terrigenous debris, which is rich in organic matter. Rivers are transported, piled up and buried quickly. With the change of water system and lithofacies palaeogeographic conditions, organic matter may evolve into methane and biogas under certain thermal metamorphism or biological action. These gases diffuse into river sediments, or gather in river sand bodies to produce cavitation and become gas-bearing strata.

Figure 3 shows the reflection blur area in the shallow part.

Fig. 3 Soil layer containing gas

A large shallow gas zone and several small shallow gas zones have been discovered along the Pearl River Estuary, with a total area of about 420km2, among which the shallow gas in the west of Lingdingyang is the most widely distributed. The shallow gas area is located in the west of Lingdingyang Waterway, and descends from Dongsimen along the Waterway to the south of Guishan Island, but the buried depth and gas-bearing layer thickness are unknown. Generally speaking, shallow gas is mainly distributed in Bamen area of the Pearl River, and the ancient channels, branches and floodplains formed under the channels are widely distributed, mainly occurring in Quaternary sediments, and the silt layer is the caprock.

5.2 Active faults

In marine engineering, it is generally defined as a fault that has been active since the late Pleistocene. The formation reason is the stratum dislocation caused by crustal activity and sedimentation, which leads to the different thickness of sediments in the two plates.

Ground dislocation caused by faults and its accompanying ground deformation will often destroy buildings built across faults or nearby buildings, and at the same time, faults will also cause excessive uneven settlement of seabed, which is extremely harmful to marine engineering.

A shallow normal fault (figure 1) that has been active since Quaternary has been found in the central part of this area, which is located at14 45' 00 "~14 50' 00" e, 22 25' 30. It extends to the northwest with a length of 7km, and the fault is within 25m from the seabed. When the bedrock is cut, the Quaternary part on it is dislocated, with a fault distance of 7-25m, which increases from northwest to southeast with an inclination angle of 50-80. Drilling also revealed the existence of this fault.

According to the data of drilling and regional geological structure, after NEE dived into Lingdingyang of Wuhua deep fault zone, it can be divided into Kyubi no Youko Ridge fault and Henggang-Luohu fault, and cut through the bedrock of the bridge site.

A. Kyubi no Youko Ridge Fault: This fault starts from Henggang, Shenzhen in the east, intermittently extends in NEE, passes through Shekou, exits Chiwan, obliquely cuts the axis of the bridge in the northwest of Neilingding Island, and goes straight into Tangjiawan, Zhuhai, with a direction of 45 ~ 60 northeast. The fault plane is inclined to the southeast with an inclination angle greater than 70.

B. Henggang-Luohu fault: this fault starts from Henggang in the east, and NEE extends to Luohu, which is basically parallel to the south bank of Shenzhen Bay, obliquely cuts the axis of the outer bridge of Lanjiaozui Port, runs through Lingdingyang, crosses the north side of Qinheng Island, and then extends westward, cutting the bridge site with the NW-trending Longtoushan fault, Baining-Shawan fault and Qi 'ao-Guishan Island fault.

This area is a faulted basin in the Pearl River Delta, where many groups of faults meet, and the intersection zone of active faults is where strong differential movements occur, often accompanied by earthquakes and causing secondary geological disasters.

5.3 Shapo

Sand waves are formed by the hydrodynamic action of sandy seabed. When hydrodynamic conditions change, especially under the action of storm surge, the shape and distribution of sand waves will change and move. When seismic activity occurs, vibration may lead to liquefaction of sand. The migration, activity and transformation of sand waves not only directly affect the anchorage, but also cause great harm to the engineering facilities on it. The migration of sand wave will also cause serious threats to the engineering facilities in front of it, such as burial, impact and dragging, so it is extremely important to study the moving direction and speed of sand wave.

On the geophysical profile, the seabed sand wave shows a continuous sawtooth reflection with strong amplitude, and the secondary reflection wave of the seabed is strong, and the seabed with sandy structure forms a reflection shield below it on the shallow profile; Through the analysis of the opposite sonar image, it shows a regular reflection of black and white.

There are many seabed sand waves in this area, which are mainly distributed along the grooves. Generally, the wave height is less than 1m, and the wave crest direction is mainly NE direction, which is nearly orthogonal to the water flow direction. Their existence shows that the seabed sediment movement is strong and the seabed stability is poor. When a typhoon or hurricane causes a storm surge, the shape and distribution of sand waves may change and shift.

5.4 Irregular shallow bedrock

On the geophysical profile, irregular shallow bedrock mainly shows that its interface reflection is mostly cone-shaped or spike-shaped strong reflection, and its internal reflection is fuzzy and non-hierarchical, and the reflection form is random fluctuation, and some diffracted waves can be seen.

For engineering construction, bedrock is a good bearing stratum, but if the surface of bedrock is uneven and the height difference is large, the bearing capacity will be different because of its heterogeneity with surrounding rock lithology.

Irregular shallow bedrock is widely distributed in the area, and the depth of irregular bedrock surface is-14.4 ~-67.3 m, which fluctuates greatly. East to Okawashima, southwest to Qi 'ao Island, and north to Neilingding Island, the depth is small and changes greatly. In some places, the exposed seabed becomes a reef.

5.5 Buried ancient river course

On the single-channel seismic profile, the bottom boundary of buried ancient channel (Figure 1) presents a strong reflection of continuous fluctuation, and the internal chaotic phase is braided channel deposition. Some bottom interface reflection waves are concave, and the internal reflection is a bit messy, which is gravel packing; Part of it is weak reflection and is formed by argillaceous filling. On the shallow profile, we can see the characteristics of depression and continuous strong reflection at the bottom of the river, and the structure of internal filling is clear. We can also see the characteristics of lateral accretion, top accretion, cyclicity and oblique bedding of filling. The sediments in the buried ancient river channel are quite different from the surrounding rock lithology, and the bearing capacity is obviously uneven, which has potential harm to marine engineering facilities that cannot be ignored.

The sediments and fillings in the ancient river channel are mainly coarse clastic gravel, which has large porosity, fast water circulation between layers and strong permeability. After long-term erosion and scouring, the stratum is easy to cause local collapse under the action of overlying load, destroying the original structure of the stratum and causing instability of the basement.

The longitudinal cutting depth of the ancient channel is different, and the lateral sedimentary facies changes rapidly. There are completely different mechanical supports at close range, such as riverbed sand and muddy sediments on the floodplain, which obviously have different shear strength. Soft clay sediments are prone to creep under the action of uneven compaction or gravity and seismic force, causing landslides and geological disasters.

Sediments and fillings in ancient rivers are mainly terrigenous debris, which is rich in organic matter. Rivers are quickly transported and piled up, and they will be quickly buried. They may evolve into methane and biogas under certain thermal metamorphism or biological action. These gases diffuse into river sediments, or gather in river sand bodies to produce air pockets, which will become shallow gas-bearing strata and form geological disasters.

Buried ancient rivers are developed in this area, and there are ancient rivers in layer A and layer B. Some rivers in these two layers are self-contained, and most of them are superimposed and developed for a long time. The riverbed moved many times, forming a large river sediment system. It is difficult to divide specific rivers, and its scale and trend cannot be described in detail. Some rivers are directly exposed to the seabed, and there are often shallow submarine troughs, which shows that the hydrodynamic effect is strong. This kind of river course will directly bring trouble to the project.

5.6 groove

Trench is formed by erosion and erosion of surface sediments on the seabed. It is mainly distributed on narrow islands on both sides of the strait, where the tidal current or current is relatively fast, which is an unfavorable condition that ocean engineering should avoid or must deal with. On various geophysical survey data, it is shown that the waveform of submarine reflection wave is obviously distorted, the reflection interface suddenly breaks or sinks, and the two sides are symmetrical, which is quite different from the surrounding terrain.

The development of Lingdingyang gully in the Pearl River Estuary is controlled by topography. Gully is a large-scale gully, and Lingdingyang gully is developed. There are obvious traces of manual excavation in the ditch, and the height and slope of the ditch change greatly. Steep scour ditches may form steep ridges and may be accompanied by landslides. Underwater scour grooves are easy to develop near islands, and are often accompanied by irregular bedrock. Gully can be said to be a large scour trough, which can form a waterway, but it has obvious constraints on offshore engineering.

5.7 Underwater Shoal

The formation of underwater shoal is a kind of underwater deposit, which is formed in an environment with rich supply of nearshore sediments and weak hydrodynamic conditions. When the hydrodynamic conditions change, especially under the action of storm surge, the shape and distribution of shoals will change and move. The migration, movement and transformation of shoals not only directly affect the anchorage, but also cause great harm to the offshore engineering facilities on it, and also pose serious threats to the engineering space in front of it, such as burial, collision and towing. There are many shoals in the bridge area, and the elevation difference with the surrounding terrain is 1 ~ 3m.

6 discussion

The sea area where Guangdong, Hong Kong and Macao cross-sea bridge is located has unique natural conditions and complex marine engineering geological characteristics. The seabed topography is distributed in three beaches and two troughs, and the main landform types are: trough, sand wave, depression and shoal; There are four types of surface sedimentary soil: flowing mud, silt, muddy soil, silt mixed with sand or sand mixed with silt; Below the seabed are silt, clay, muddy soil, sand layer and bedrock; There are potential geological disasters such as shallow gas, active faults, sand waves, seismic activity, irregular shallow bedrock, buried ancient rivers, scouring gullies and underwater shoals, which are potential threats. Of course, these potential geological disasters are not imminent, and may be induced under the influence of fault activity, earthquake or bad weather.

The thick layered, fluid plastic and highly compressible muddy soft soil layer in the bridge site area has the characteristics of low strength, high compressibility and high sensitivity, which may produce thixotropy under vibration, and its engineering properties are extremely poor, which is not conducive to engineering construction; There is earthquake liquefaction problem in silty sand and fine sand layer; Rock residual soil and fully weathered rock disintegrate in water.

The great difference of deep weathering trough of bedrock and the change of buried depth of fresh bedrock are not conducive to the selection of engineering foundation and bearing stratum, especially for the heavy-load, durable, safe and reliable cross-sea bridge, which has to go deep into bedrock to select bearing stratum, which increases the difficulty of foundation engineering.

Lingdingyang facing the South China Sea is one of the landing points of typhoons and tropical storms, which is also one of the most serious natural disasters in the region. Especially extreme wind load is not conducive to high-rise buildings or long-distance and long-span suspended buildings.

Prevention is the main way to deal with geological disasters. First, find out the causes, distribution and development laws of various geological disasters, make necessary monitoring and forecasting for some potentially dangerous geological disasters, so as to avoid their occurrence, or formulate effective measures to curb the formation and development of disasters. For gradual geological disasters, we should strengthen the research on the formation and development law of disasters.

1) various geological disaster factors such as large active faults. Because these geological disasters can't be controlled, the project must be carried out carefully.

2) Measures can be taken to remove small and inactive restrictive geological conditions, such as blasting to remove exposed or shallow bedrock at the bottom.

3) For some small-scale geological disaster factors in the process of energy accumulation, artificial methods can be adopted to induce them to occur in advance, reduce energy and enhance stability.

4) For some small-scale geological disaster factors, the reinforcement method can be adopted in the case of short construction period to make the project go smoothly.

refer to

Sun, Shen Chuanbo, etc., 2004. Analysis of shallow gas accumulation conditions in Hangzhou Bay area. Marine Geology and Quaternary Geology, 24 (2): 85 ~ 88.

, Feng, Xue, etc. Geological hazards in the northern South China Sea and evaluation of submarine engineering geological conditions. Nanjing: Hohai University Press, 5 ~ 123.

Ye Yincan, Pan, et al. 2003. Genesis, occurrence characteristics and harm to engineering of shallow seabed natural gas. East China Sea Ocean, 2 1 (1): 27 ~ 36.

Engineering geological characteristics of Guangdong, Hong Kong and Macao Bridge

Ma Shengzhong 1, 2 Chen Yanbiao 1 Chen Taihao 1

(1. Guangzhou Marine Geological Survey, Guangzhou, 510760; 2. China Geo University, Beijing, 100083

Abstract: According to geophysical exploration, acoustic survey, core sampling and geotechnical test, it is found that the offshore of Hong Kong has special natural conditions and complex seabed topography and geomorphology. Geological disasters such as earthquake, landslide collapse, underground river, fault, sand wave, shallow gas and possible sand liquefaction will bring potential dangers to this area. Especially on the edge of continental shelf and slope in the study area, deep slope may cause potential geological disasters. Pay attention to the risk factors.

Keywords: engineering geological hazards, Guangdong, Hong Kong and Macao Bridge