500 HECTARES - HOA PHAT DUNG QUAT STEEL FACTORY

500 hectares - Hoa Phat Dung Quat Steel Factory

1. The project with the largest area

Hoa Phat - Dung Quat iron and steel production complex was built in Binh Dong commune, Binh Son district, Quang Ngai province. The project has a total investment of
VND 52,000 billion, designed with a capacity of 4 million tons a year, the main products are construction steel, high-quality coil steel, and flat products are hot rolled steel coils.
 

The total area of ​​the project is nearly 500 hectares. The factory includes many construction items below the groundwater level, so it is necessary to drill wells to lower the groundwater level. This is the largest project in which the Solar System has been involved in the construction of lowering the groundwater level.

The main work of the contractor includes drying the foundation pit to facilitate easy construction. The purpose of the drywall work is to provide a dry, stable, and safe working environment by using a good system around the excavation and the addition of a drain.

2. Engineering geology

According to construction geological survey documents provided by the construction contractor, the area has 8 basic geological layers:

Layer 1: Leveling soil, crushed rock, weathered rock

Layer 2: (SP) Poorly graded sand, yellow-grey, brown-gray, porous to medium compact

Grade 3: (SC-SM, CL-ML, SM, SP) Dusty sand, mixed sand, poorly graded sand, sometimes clamped with dusty clay, low plasticity, gray-brown, plastic state, and shell

Layer 4: (MH, ML, CL, SC) Plastic dust, plastic dust mixed with sand, mixed sand sometimes mixed with low plastic clay, dark gray, gray; pliable to soft, even with seashells

Layer 5: (SC, SM, SC-SM, CL) Sand mixed with dust, dust, sand mixed in places with low plastic clay; dark gray, gray; plastic state; mixed with seashells

Grade 6a: (SC, SM, CL, CH) Mixed sand, Sand mixed with gravel, dust, low plastic clay; yellow-gray, gray-brown; hard to semi-hard plastic state; a few places and a few stones

Grade 6b: (GP, GW) Remains slope and fully weathered zone: Mixed sand, sand containing many gravels; gray-white, gray-brown; Moderate to tight state

Grade 7: Strongly weathered zone, with a few lumps of gray-yellow, gray-white mixed with mixed clay.

Grade 8: Multi-granite in gray-blue, gray-white

Groundwater aquifer (non-pressurized aquifer) is soil layers 2,3 (from fine-grained to coarse-grained sand), the thickness of the aquifer is not uniform in the survey area.

In the category of Thermal Power Cooling Tower, the current ground level is about -0.0m above cos 00. The current groundwater level appears at a depth of about -0.5m above the ground (-0.5m above cos. 00), the bottom of the hole dug is about -6.8m deep compared to cos 00. So the work of lowering groundwater needs to lower -6.8m of water to maintain the groundwater level 0.5m deeper than the excavation core.

3. New groundwater pumping techniques

3.1 Design of point-well drying system

To control the groundwater level, point well technology is backed up by a system of ditches and submersible pumps. The Contractor should construct free systems and protection pipes for the electrical installation of the submersible pump. 

The sequence of installation and operation of the point well system is as follows:

• The groundwater level after lowering is 0.5m lower than that of the dug hole;
• Surveying the layout lines of point wells and placement of submersible pumps;
• Drilling, installation, and operation of point wells;
• Installation and commissioning of pumping equipment (Surface vacuum pump and submersible pump); 
• Install drain pipes and drain tanks;
• Operate, maintain and test point well systems;

Point wells are installed by drilling using steel pipes, high-pressure pumps, and suitable supporting equipment such as forklifts or mobile cranes. The point wells are connected by 150mm diameter PVC pipes and connected to a vacuum pump. Electricity supplied to the vacuum pump is taken from the contractor's grid. The drained water is led to the drainage pit by 150mm diameter plastic pipes.

During the excavation process, the point well system is backed up by submersible pumps when needed.

Submersible pumps should be installed with protection such as perforated PVC pipes and placed in boreholes. The borehole wall is filled with washed gravel to prevent soil particles from entering the well. The ditches are connected to the borehole to channel surface water into the pit.

The drains do not need to be wide, but deep enough to allow water to drain into the well. The trench is about 0.4 m wide and at least 0.5 m deep and inclined 1% to the drainage pit. The gravel filter with a diameter of 10-20 mm, acts as a filter for groundwater.

3.2 Installation process of point well system

3.2.1 Gathering materials

The main contractor carries out the preparation work, ensures that the good construction area is not buried, and marks the good construction area.

If necessary, the main contractor must provide a drilling permit before drilling. The main contractor must agree with the construction supervisor's plan to assemble and install the well. The main contractor must remove obstacles that may interfere with the good drilling process. The main contractor needs to ensure space for drilling equipment to operate smoothly in a good construction area.

The main contractor shall ensure that all overhead power cables in the good construction area are decommissioned and provide SSFG Groundwater with a permit to carry out work below such lines.

3.2.2 Drilling method

Point well pumps need to be located near the water supply and not higher than 3m above the water surface. The suction nozzle of the pump must always be placed in the water. When the equipment is ready, the drilling work is carried out. The nozzle is pressed directly down to the marked positions. Water pressure from the nozzle will help drill holes in the soil easily.

When the required depth is reached, the pump speed will decrease and the nozzle will be removed from the well. The point well pipe is then placed into the borehole. The tube will be raised and lowered so that water can enter from below. Then, use filter gravel to fill the space between the pipe and the borehole. The process is repeated for the other pits.

3.2.3 Rotary drilling method

The rig is placed in a predetermined drilling position.

The drill will rotate at a low speed combined with the circulating fluid, drilling deep into the soil.

The strut can be used to widen the borehole, making the drilling process more convenient

When reaching the depth, proceed to pump and wash the well to end the drilling process.

3.2.4 Installing the filter tube

After drilling is completed, the filter pipe is placed in the borehole

3.2.5 Construction of filter gravel hotel

Fill in the space between the borehole and the incoming filter with gravel

3.2.6 Check the good system

Each pump works in conjunction with the damper, pressure regulator, and pressure sensor separately, creating and maintaining an airtight environment during pumping. Then the valves will be opened so that water from the wells can enter the drainage system. When all valves connected to the water collection pipe are closed, it is necessary to check for leaks in the water collection pipe system, the quality of water pumped out from the well; If the well works well, then after about 3 hours the pump sucks up the water quality is relatively good.

Once the vacuum is stabilized, repeat the process with the remaining pumps until all pumps are in operation.

After a period of time when the pump lowers, the groundwater level decreases, and in some wells, water shortage will occur. Those wells will be closed by an automatic control valve system.

3.2.7 Point well system operation

During the commissioning phase, the system will be supervised by a field engineer. The designated person will take care of DAILY MAINTENANCE. Including:

• Check the vacuum pump
• Fuel pump or generator, if needed.
• Oil and care for equipment cooling water.
• Make sure the drainpipe and tank are not clogged or damaged.
• Check the backup pump and generator periodically.
• Record groundwater level and flow rate (on V-notch).

4. Experience in pumping down groundwater

The solar system through the project has the opportunity to learn from experts in groundwater, for the first time to contact an international company with the same field of operation, to learn some special construction experience, the most accurate methods of analysis and calculation of groundwater lowering.

The methods of lowering groundwater are rarely applied in Vietnam such as well points, drainage ditches, multi-layer wells...

The software applied to calculate and lower the groundwater level by good point overtime was first applied for analysis. Previously, the problems of pumping to lower the groundwater level by the good point method were mainly done by empirical or analytical formulas. However, the formulas are only suitable for homogeneous and isotropic strata, while the construction site stratigraphy is quite complex, the application of the software brings more precise and detailed analysis.