Engineering geological characteristics of breakwater site outside the Pearl River Estuary

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

About the author: Ye Guanghui, male, born in 1950, engineer. 1977 Graduated from Chengdu Institute of Geology, majoring in oil and gas exploration. Now he is engaged in comprehensive research on petroleum geology, seismic data interpretation, marine geology and engineering geology.

According to the lithologic characteristics of borehole 18 of the breakwater site outside the Pearl River Estuary, it is divided into 7 layers from top to bottom. According to these drilling results and test data, through comprehensive analysis, it is considered that the bearing layer of buildings should be medium-coarse sand mixed with silt layer and medium-coarse sand mixed with clay layer, and the granite residual soil layer is the best bearing layer of buildings.

Keywords: engineering geology outside the single bearing stratum

1 Geological survey of dike site

Wailingding Island is located at the northernmost tip of Wanshan Islands in the Pearl River Estuary, with a superior geographical position, facing Hong Kong across the sea. Shek Chung Bay is located in the north of Wailingding (Figure 1), only 4 ~ 5 kilometers away from Hong Kong waters, and many fishing boats from Guangdong, Hong Kong and Macao have been berthed and inhabited here all the year round.

Shi Chong Bay is a bedrock bay with an approximate "ox yoke" shape. According to the topographic features of the seabed, the bay can be divided into two areas: the slope area and the flat area. The slope area is located on the isobath from the bay shore to 13m. In the slope area, the northeast coast is steep and the southwest coast is gentle. From 13m isobath to 17m isobath, the terrain is flat.

Shi Chong Bay has a natural barrier of Dingling Peak, which is a bedrock coast with low water depth and sedimentation rate (about 0.08cm/a), so it can be described as a natural harbor. When the breakwater is completed, it will be an ideal haven.

The bedrock exposed by drilling is consistent with the rocks exposed on the island, belonging to Yanshanian granite (), slightly weathered-moderately weathered. During Q 1-2, the granite in the area was strongly weathered to form a layer of residual soil (ⅵ layer). Subsequently, the exploration area gradually sank, forming a shallow land depression, which accepted the deposition of surrounding weathered materials. During this period, the water in the depression changes alternately from shallow to deep to shallow, forming medium-coarse sand mixed with clay (layer V), clay (layer IV) and medium-coarse sand mixed with silt (layer III).

In the late Q3, the exploration area sank further, and seawater invaded, which became an intertidal environment of land-sea interaction, and a set of fine sand (layer II) containing a lot of biological debris was deposited. Since then, the present appearance of Shi Chong Bay has gradually formed, and a set of blue-gray silt (I layer) containing biological debris has been deposited.

2 Engineering geological characteristics of embankment site

There are 18 boreholes at the breakwater outside the Pearl River Estuary. According to the soil (rock) layer structure of the dike site, it is (Figure 2) from top to bottom: silty soil, silty fine sand, medium-coarse sand mixed with silty soil, clay, medium-coarse sand mixed with clay, residual soil (sandy cohesive soil) and slightly-moderately weathered granite.

Table 1 Comprehensive Statistical Table of Formation Lithology and Main Physical and Mechanical Properties of Soil

Figure 1 Drilling Location Map

Figure 1 Drilling Location Map

The first layer: silt

Blue-gray, containing biological debris. According to the analysis of representative samples (data are shown in the attached table, the same below), the water content of silt is 63.2%, the porosity ratio 1.752, and the liquid index 1.64, all of which are in a saturated flow plastic state. The thickness of this layer is 4.6~7. Om, the lateral change is relatively stable. The slope along the coast has become a deposit of shell fragments. According to the main index water content of sample analysis, the basic bearing capacity of silt is 50kPa, and if the regression correction coefficient is 0.75, the standard value is 37kPa.

Layer Ⅱ: silty sand.

It is bluish-gray, contains a lot of biological debris, is loose, and has a thickness of 0.8~2.4m, with great lateral variation. Shell debris is deposited in hole D9, medium-fine sand is deposited in hole D 12 in the northeast bank slope, and hole D 1 and hole D2 in the southwest bank slope are missing.

Main indicators of sample analysis: the porosity ratio is between 0.8 19- 1.373, with an average value of 1.04, and the liquid index is between 0.8- 1.77, with an average value of1.

According to the analysis results of five samples, the regression correction coefficient ψf=0.7 123, the bearing capacity reference value f0 is 180kPa, and the standard value is 128kPa.

Layer ⅲ: medium coarse sand mixed with silty sand.

It is grayish white and yellowish brown, containing gravels, with a diameter of 2 ~ 3 mm, slightly dense ~ loose and saturated, with a layer thickness of 0.5~3.4m, and the thickness varies greatly. Borehole D 12 in the northeast bank slope belt is gravelly sand, while borehole D 1 and D2 in the southwest bank slope belt are missing.

Main indicators of sample analysis: the porosity ratio is between 0.462 ~ 1.6 19, with an average of 0.77, and the compressibility is between 0.02 1 ~ 0. 165, with an average of 0.056.

According to the analysis results of three samples, the regression correction coefficient ψf=0.6 12, the bearing capacity reference value f0 is 250kPa, and the standard value is 150kPa.

The fourth layer: clay

Gray, plastic and wet, with a thickness of 0.6 ~ 3.9m This layer is only distributed in holes D 1 ~ D6 and D 12.

Main indicators of sample analysis: the porosity ratio is between 0.711~1.705, with an average value of 1.08, and the liquid index is between 0.5 and 0.97, with an average value of 0.69.

According to the analysis results of four samples, the regression correction coefficient ψf=0.72 13, the bearing capacity reference value f0 is 180kPa, and the standard value is 130kPa.

The fifth layer: medium and coarse sand mixed with clay.

It is grayish white and contains gravel. The gravel is 2 ~ 3 mm in diameter, plastic, medium dense and slightly wet. The layer thickness is 0.7~3.4m, and the lateral variation is great. D 1 hole and D2 are missing in the southwest bank slope of this layer.

Main indicators of sample analysis: the porosity ratio is between 0.524 and 0.904, with an average value of 0.68, and the liquid index is between 0.39 and 0.66, with an average value of 0.47.

According to the statistics of the analysis results of four samples, the regression correction coefficient ψf=0.73 12, the bearing capacity reference value f0 is 280kPa, and the standard value is 200kPa.

Layer ⅵ: Residual soil (sandy cohesive soil)

It is reddish brown to yellowish brown with brown stripes, and is strongly weathered residual soil of bedrock. Hard plastic to plastic, slightly wet, this layer is missing in D 1 hole and bank slope D2.

Main indicators of sample analysis: Pore ratio is between 0.534 and 0.972, with an average value of 0.69, and liquid index is between 0. 14-0.84, with an average value of 0.476.

According to the analysis results of five samples, the regression correction coefficient ψf=0.742 1, the reference value f0 of bearing capacity is 320kPa, and the standard value is 230kPa.

Fig. 2 Geological profile of borehole in breakwater area of Shek Chung Bay outside Dingling.

Fig. 2 histogram of engineering geological drilling holes

Layer 7: Slightly-moderately weathered granite.

Brown-black, gray-white, brown-red and other variegated granite are Yanshanian granite, which is slightly-moderately weathered, not easy to break and hard. Drill only in holes D 1, D2 and D 12. The basic value of bearing capacity is greater than or equal to 4000 kPa.

3 Conclusion

1) The geological profile of drilling holes in the breakwater site is silt, silty fine sand, medium coarse sand mixed with silt, clay, medium coarse sand mixed with clay and residual soil from top to bottom, and the bedrock is Yanshanian granite (). Generally speaking, engineering mechanical properties get better with depth.

2) The first layer is silt, and the engineering geomechanics condition is poor, so it cannot be used as the bearing layer of any building.

3) Layer II is silty sand, loose, with porosity ratio of 0.8 19 ~ 1.373, average 1.04, and liquid index of 0.8 ~ 1.77, average1. Loose sand (especially silt) tends to be dense under the action of external force. If the sand is saturated with groundwater, this densification will lead to a sudden increase in pore water pressure, which will reduce the pressure originally transmitted through the contact point of sand particles, and the load pressure acting on sand particles will all transition to neutral pressure. This will destroy the structure of the sand body. When the effective pressure disappears completely, the sand body will easily reach the liquefied state, leading to ground cracking, dislocation, landslide, uneven settlement and other foundation instability phenomena. Therefore, this floor cannot be used as the load-bearing floor of any building.

4) The third layer, medium-coarse sand mixed with silty sand. The bearing capacity is 150kPa. The porosity is 0.462 ~ 1.6 19, with an average of 0.77, and the compressibility is 0.02 1 ~ 0. 165, with an average of 0.056. Loose, high compressibility. Its underlying layer is a locally distributed plastic clay layer. These factors are easy to cause uneven settlement of buildings. Therefore, these unfavorable engineering geological conditions must be fully considered when considering this layer as the bearing layer of the building.

5)ⅳ layer, clay layer, only distributed in local areas, should not be used as the bearing layer of buildings.

6)ⅴ layer, medium coarse sand mixed with clay, bearing capacity of 200kPa, is suitable as the bearing layer of buildings.

7) Layer VI, residual soil (sandy clayey soil), is sandy clayey soil formed by strong weathering of bedrock, with large bearing capacity, and is suitable as the bearing layer of buildings.

Reference materials and materials

Chen Xizhe. 1989. Foundation of soil mechanics. Beijing: Tsinghua University Publishing House.

, Xue,, Feng,, etc. 1996. Evaluation of geological disasters in the northern South China Sea and submarine engineering geological conditions. Nanjing: Hohai University Press.

Hebei Provincial Construction Committee. 1977. Code for engineering geological investigation of industrial and civil buildings. Beijing: China Building Industry Press.

Engineering geological characteristics of Lingdingyang levee in the Pearl River Estuary

Ye Guanghui

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: 18 dike boreholes are located in Shi Chong Bay, Lingding Island outside the Pearl River Estuary. According to the lithologic characteristics, seven dam layers are divided from shallow to deep seabed. According to a large number of drilling results, test data and comprehensive analysis, the following conclusions can be drawn: medium coarse sand mixed with silt layer and medium coarse sand mixed with clay layer can be used as the compression layer of foundation, and muddy clay layer is the best choice as the compression layer of foundation.

Keywords: Lingding Island outside the engineering geological compression layer