Subsidence Research Situation in South Saigon Area

Army

4, about 32 km long, width

g max 130m, area

narrow

75m wide. [14] .

b. Hydrological regime of rivers, canals and streams

The South Saigon area has a low terrain so it is strongly affected by the irregular semi-diurnal tide regime, with a large amplitude of up and down twice a day with 2 approximately equal peaks and 2 quite different bottoms. Every month, the tide has 2 high tides (spring tides) and 2 low tides (neap tides) in 24 hours and 50 minutes. Table 1.1 and Figure 1.2 below show the level of tidal fluctuations in the period 1990 - 2014.

Table 1.1: Average highest water level in 3 months October, November, December at Phu An and Nha Be stations from 1990 to 2014

Average highest water level in 3 months (10, 11, 12) (cm)

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1995

Source: Southern Hydrometeorological Station

Highest average water level in three months (October, November, December) at Phu An and Nha Be stations

180

160

140

120

100

80

60

40

20

00

y 1 = 1.4403x 1 + 119.11

R 1 ² = 0.7507

y 2 = 1.5087x 2 + 120.03 R 2 ² = 0.8593

Year

Nha Be Station

Phu An Station

Linear (Nha Be Station) Linear (Phu An Station)

Water level (m)

The highest tides of the month are on the 1st, 2nd, 3rd and 15th, 16th, 17th; the highest tides of the year are in the months of October, November, and December. The tidal regime in the Nha Be area (the part adjacent to and affecting the research area) is observed at the Nha Be station on the Dong Dien canal and the Phu An station on the Saigon river, figure 1.2.

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

Source: Southern Hydrometeorological Station Figure 1.2: Highest average water level in the three months of October, November, December at the station

Phu An and Nha Be

1.2. Geological conditions

1.2.1. Geological characteristics

In the study area, there are strata from Mesozoic to Cenozoic. Belonging to Mesozoic with Middle Jurassic sediments, La nga J 2 ln formation, Upper Jurassic - Lower Cretaceous J 3 k 1 eruption . Belonging to Cenozoic with Neogene, Pleistocene and Holocene sediments. The soil layer of the study area in this thesis belongs to the Holocene weak soil layer, quite thick, about 6.5 - 35.0 meters and Pleistocene layer [14] . In addition, to further assess the impacts on the Holocene soil layer, it is necessary to consider the Pleistocene sediments underneath.

Pleistocene: Cu Chi Formation (Q 1 3 cc ).

The Cu Chi Formation lies unconformably on the Thu Duc Formation (Q 1 2-3 tđ ) and is unconformably covered by sediments of the Binh Chanh Formation (Q 2 1-2 b c). The sediments in this area are mainly coarse-grained, with the following 3 types of origin:

River origin: gray-white silty clay sand sediment, gray kaolinite clay pebbles. Mainly distributed in Thu Duc and a range in North Cu Chi. Consists of 3 layers: the top layer is loose gray sand, the middle layer is iron Laterite, the bottom layer is gravel, crushed gravel mixed with a little white Kaolinite clay. River and sea origin: in the borehole documents in Cu Chi district, the sediment consists of gray silty clay sand containing pebbles, gravel. Sea origin: composition of gray-black clay sand containing a little gravel. Distributed in the West of Binh Chanh and Southeast of Can Gio. [14]

Holocene system (Q 2 ): Modern Holocene sediments are classified into an incomplete system, consisting mainly of fine-grained sediments distributed widely in the area. Including: Binh Chanh formation (Q 2 1-2 bc ), Can Gio formation (Q 2 2-3 cg ) and modern alluvial sediments (Q 2 3 ). [14]

Binh Chanh Formation (Q2 1-2 bc ) :

The main component is blue-gray clay containing plant remains, saltwater algae and pollen spores of early-middle Holocene age. The sediment is of marine origin. Appears at a depth of about 28m, distributed mainly in the West - South of Nha Be district and exposed in a small part in Tan Hung ward, District 7, thickness 15-20m. Has the following physical indicators: thickness

void ratio from 37.9 – 50%; yield limit from 32 – 48%; plastic limit from 20 – 27%; compression coefficient from 0.12 – 0.17 cm 2 / kg. [14]

Can Gio Formation (Q 2 2-3 cg ):

Composition: gray-green clay, peat. Formed during the late middle Holocene marine regression stage. Sediments are of river and sea lagoon origin. Exposed entirely in the research area of ​​District 7 and Nha Be, thickness from 5 - 20m. Has the following physical and mechanical indicators: porosity from 50.8 - 63.6%; liquid limit from 31 - 56%; plastic limit from 25 - 34%; high compression coefficient from 0.11 - 0.21 cm 2 / kg; most of them are in liquid state. [14]

Modern alluvial sediments (Q 2 3 ):

Composition: mainly sand, humus, plant mud. Mixed river and sea origin, the main component is dark gray sand and mud. Sediments here are formed in tidal river environments. Distributed mainly along and in the riverbeds (Sai ​​Gon, Nha Be), canals (Kinh Te, Thay Tieu),... Thickness from 3 - 5m. [14]

In this study, all Holocene sediments were grouped together and called the weak soil layer for settlement survey. According to Nguyen Giang Nam (2016), a model was created for the weak soil layer (with SPT value < 5) with a thickness of up to 35.5m. [13, 4]

1.2.2. Geomorphological characteristics

The South Saigon area is a low plain with an average elevation ranging from 0 to 2m. The terrain is divided into 3 main types:

Low plain – accumulation of mixed river-sea materials: height from 1-2m, composed of silty clay and sand sediments of mixed river-sea origin belonging to the Can Gio formation (Q 2 2-3 cg), distributed in almost the entire area of ​​Nha Be district and District 7. The plain area is located in a high alluvial area, has a flat surface, is not frequently flooded and is strongly divided by a system of rivers and canals. [12,13]

Alluvial flats accumulate mixed river and sea materials: height from 0-1m. Composed of sediments including clay, sand, and silt of river and sea origin, mainly distributed in Nha Be and District 7 along the river valley. This is a modern sedimentary area (District 2-3 ) and belongs to low alluvial flats , the alluvial surface is flat and narrow, so it is frequently flooded at high tide and is strongly separated by the river and canal system. [13]

Man-made terrain: The surfaces of Ho Chi Minh City in general are located in areas with concentrated urban development and dense population, strongly affected by humans. These are construction works, roads, mineral exploitation works, canals, mounds, landfills, etc. These activities gradually change the original terrain surface, especially for low-lying plain areas such as the South Saigon area. [19,17] These low-lying areas will naturally develop a natural vegetation cover. When leveling to build houses (level 4), this layer of vegetation is also buried, and that is a sign to distinguish the boundary before leveling. Some areas with high terrain, also known as mounds, are often the place to grow crop seeds or fruit trees, so they often have patchy characteristics, which is also a sign of raising the boundary of different layers (Tran Danh Thuy, 2017). [15]

1.3. Socio-economic characteristics

1.3.1. Population

The study area includes: District 7, part of Binh Chanh district (Binh Hung commune, Phong Phu commune), part of Nha Be district (Phuoc Loc commune, Phuoc Kien commune, Nha Be town)... Therefore, the assessment of socio-economic conditions will encounter some difficulties. Below is a table showing population fluctuations in the study area during the period 1999-2015.

Table 1.2: Population changes in District 7, Binh Chanh, and Nha Be from 1999 - 2015

Year

Source: [22.5]

From Table 1.2, it can be seen that the population of District 7, Nha Be District, and Binh Chanh District has been increasing from 1999 to 2015. Due to the following reasons: first, the rapid urbanization in the inner city and the orientation of residential development to the South. Second, the rapid increase in population combined with the rapid increase in immigration requires an increase in the demand for construction of housing, offices, and urban areas.

1.3.2. Education and training

With the increasing number of immigrants to the city, the issue of education and training has become a top concern. Therefore, the construction of training facilities from primary to university is focused on to serve education well. Schools located in the outskirts of the city center such as Ton Duc Thang University, Police University, RMIT University, kindergartens, primary schools (16 primary schools in District 7), junior high schools (7 schools in District 7), high schools (3 schools) ... [3] . In addition, the district also has a continuing education center and a vocational training center.

1.3.3. Traffic system

South Saigon has a very important geographical position with a strategic position for exploiting water and road traffic, is the southern gateway of the city, is a bridge opening the development direction of the city to the East Sea and the world as well as a bridge connecting the Eastern provinces with the Western provinces.

Major traffic axes passing through this area are: Nguyen Van Linh Avenue, Pham Hung Street, Nguyen Huu Tho, Huynh Tan Phat, National Highway 50, etc. These routes make it easier to travel from the city center to South Saigon. In addition, the Saigon River surrounds the East with a specialized port system, transferring goods abroad and vice versa, which is very convenient for the development of trade and the transportation of goods and passengers to neighboring areas [1] .

1.3.4. Economy

Ho Chi Minh City is one of the major economic centers of the country, contributing 31.8% of the state budget (first 6 months of 2016) [3] . With such great attraction and strategic location as above, this area has the conditions to attract domestic and foreign investment.

Especially real estate activities with the immigration situation make the city's population increase rapidly (8,247,829 in 2015 [14] ). Typically, large residential areas such as: Trung Son residential area, Dai Phuc Green Villas residential area, Tan Quy Dong residential area, An Phu Hung residential area, ... In addition, there are urban areas, high-rise buildings.

such as: Phu My apartment complex, Riverside Residence, Central W3 Tower - Himlam urban area,... mainly concentrated in District 7 and the east of Nha Be area.

In addition, restaurants and shopping malls are also developed along major roads. In addition, Tan Thuan Export Processing Zone in District 7 is one of the largest and most effective export processing zones in the city. [1]

1.4. Situation of subsidence research in South Saigon area

The South Saigon area has a typical Holocene weak soil layer with a fairly large thickness. When calculating the placement of construction works and infrastructure on this weak soil, it is necessary to carefully calculate the safety as well as the possibility of subsidence. Therefore, research topics on the possibility of subsidence in the area as well as the construction of a typical geological map in the area are very necessary. The construction of a typical geological map and the results of subsidence research will help planners, managers, and construction designers to be more proactive in the management and prevention of incidents.

Currently, there are relatively few research topics on subsidence in the area. In 2006, authors Phan Thi San Ha and Le Minh Son used the Kriging interpolation method with borehole data to survey the distribution rules of Holocene weak soil layers in Ho Chi Minh City with the topic "Application of Kriging interpolation method to survey the distribution of Holocene weak soil layers in the inner city of Ho Chi Minh City". The research results are iso-thickness diagrams and interpolation error diagrams of weak soil layers in the inner city of Ho Chi Minh City [2] . After comparing this result with the iso-thickness diagram of weak soil layers established by the Southern Geotechnical Engineering Federation in 1988 [6] and the actual survey results, it was shown that the research results of the authors are relatively accurate. The authors concluded that although the interpolation results are not highly accurate, they still help managers and planners have an overview of the distribution of soft soil layers in the study area and the Ordinary Kriging interpolation method not only provides interpolation results but also interpolation error results, thereby assessing the reliability of the interpolation results.

In addition to the research results from scientific articles, according to the announcement of the Department of Natural Resources and Environment (TN-MT) of Ho Chi Minh City, the area with the most severe subsidence in 2015