Trích yếu luận án Tiến sỹ kỹ thuật của NCS. Nguyễn Anh Tú

Tên tác giả: Nguyễn Anh Tú. Tên Luận án: "Nghiên cứu giải pháp tường hào đất - bentonite - xi măng để nâng cao khả năng chống thấm cho đập đất". Chuyên ngành: Xây dựng công trình  Thủy. Mã số: 62 58 40 01. Đơn vị đào tạo sau đại học: Viện Khoa học Thủy lợi Việt Nam

TÓM TẮT NHỮNG ĐÓNG GÓP MỚI CỦA LUẬN ÁN

1. Ngoài tác dụng ổn định thành vách hố đào, dung dịch bentonite c̣òn làm tăng khả năng chống thấm vùng đất được bentonite xâm nhập. Hệ số thấm của đất sau khi có dung dịch bentonite xâm nhập giảm khoảng 10 lần so với đất nguyên trạng. Từ đó tác giả đề xuất cần thiết phải bổ sung thêm vùng đất có bentonite xâm nhập và màng vỏ sét trong quá trình tính toán ổn định.

2. Đề xuất được giải pháp chống mất vữa dung dịch bentonite trong trường hợp gặp tầng cuội sỏi bằng cách pha cát mịn vào dung dịch bentonite với tỷ lệ 10% khi gặp tầng cuội sỏi có độ rỗng <50%.

3. Đề xuất được cấp phối vật liệu Đ-B-X hợp lý để làm tường hào có hệ số thấm k≤1.10^-6 cm/s phù hợp với điều kiện thi công thực tế ở Việt Nam, điều đó mang lại hiệu quả kinh tế - kỹ thuật

PhD Thesis abstract

a) Introduction:

Name of author:       Nguyễn Anh Tú

Thesis title:   Study on soil – bentonite - cement cut-off wall for improvement of seepage control in earth dams

Principle of thesis: Construction of Hydraulic works;   Code:     62 - 58 - 40 - 01

Name of training unit: Vietnam Academy of Water Resources Research

b) Content:

Thesis objectives and subjects:

- Study subjects:

+ Soil – Bentonite – Cement materials for cut-off walls in earth dams;

+ Process of penetrating and filter cake formation at the slurry wall.

- Study objectives:

+ Study to determine appropriate aggregation of soil-bentonite-cement mixtures to use for cut-off wall in earth dams with seepage coefficient k≤1.10-6 cm/s.

+ Study the process of penetrating and filter cake formation at the slurry wall in soils, rocks and gravels with different void ratios.

+ Study the anti-seepage capacity of soil samples with bentonite slurry penetrated then recommending if bentonite-penetrated soil should be added in the process of calculating for seepage control and stability of dams.

- Scope of study:

+ Pressure of bentonite slurry in tests in this study has the height, H≤6m, and the maximum penetrated length is 80cm.

+ Soils with low clay content, high seepage coefficient: (K>=10-4cm/s)

Applied study methods:

- Synthetic method: Studying domestic and international materials and references in related subjects, summarized and learning lessons from projects actually applied bentonite materials in Vietnam, etc.

- Experiment study: Applying the experiment, measuring method (in the laboratories, on field) for determining properties and mechanic-physic properties of soil-bentonite-cement materials, penetration process of bentonite slurry in soils and gravel layers. 

- Physical modeling:  Using the physical model to study behaviors of cut-off wall in time, possibility of shrinkage, crack of cutoff wall at work.

- Numerical modeling: Using the finite element method to analyze impacts by filter cake to flow, seepage coefficient, and top flow line in dam body.

Key outcomes and conclusions:

- Practical significance:

+ The outcomes of the study shall be applied practically in calculating the seepage control and stability of earth dams. The aggregations of soil-bentonite-cement materials for cutoff walls recommended by this study can be used in seepage control works in any project where they have any evidence of seepage-based instability, or can be used in new construction works that need cutoff walls due to high permeability of embankment materials and lack of low permeability materials for seepage control

+ In addition to the seepage control capacity, the studied mixture has high shear strength, so it can be used for repairing, upgrading and improving the stability of existing dams, dikes, etc.

+ The study introduces methods to minimize the loss of bentonite soil solution while the construction of trench encounters with a gravel layer.

- Scientific significance:

+ Confirming that the technology of seepage control for earth dams with soil-bentonite-cement cutoff walls is not only a cost-saving solution (reducing 23.3% of cost for materials compared with the cement-bentonite cutoff wall method that is being applied in Vietnam), but also an achievement in applied technical aspect in the current practice (increased capacity of seepage control in cutoff wall materials; making use of soil excavated for cutoff walls, etc.)

+ Confirming that the bentonite slurry not only works on stabilization of trench wall during construction but also helps increase anti-seepage capacity for the area of soils where bentonite slurry has penetrated in. Then, the author recommends that it is necessary to enlarge the area of soils where bentonite can be penetrated and the filter cakes for stability analysis. 

+ Pointed out the impacts of cement on improvement of shear strength, meanwhile, increase of seepage coefficient of soil – cement – bentonite mixture.

- Thesis outcomes:

1. The Thesis has studied and found out the relations between the penetration processes of bentonite solution into cut-walls responding to different void ratios of soils, in range of 41%, 43%, 45%. The seepage coefficient of soil measured when the soil has been penetrated by bentonite reduces approximately 10 times compared with the soil in primitive status.

2. The Thesis has studied and recommended the solution for minimizing bentonite slurry lost in case the slurry trench encounters with a gravel layer with void n<50% by adding fine sand up to 10% into the bentonite slurry.

3. The Thesis has studied and recommended the aggregation of soil-betonite-cement materials with the proportion: 62% of soil, 4% of bentonite, 3% of cement and 31% of water for making cut-off walls. The seepage coefficient attained is k≤1.10-6cm/s in current construction conditions in Vietnam, meanwhile, the cost-saving and technical efficiency are also ensured.

4. Using SEEP/W software for seepage and stability analysis, the dam that is considered with all 3 factors: cut-off wall, bentonite penetrated soil area, and filter-cake will have higher capacity for seepage control and higher stability compared with the case of only 1 factor, i.e. cut-off wall. Seepage discharge reduces 26.57% and the safety factor increases from Kminmin = 1.110 to Kminmin = 1.181 compared with the case of only 1 factor, i.e. cut-off wall.

 Summary of achievements of the thesis

1. In addition to the effectiveness as stabilizing the cut-walls, bentonite solution increases the capacity of seepage control in the area where soil is penetrated by bentonite.  The seepage coefficient of soil measured when the soil has been penetrated by bentonite reduces approximately 10 times compared with the soil in primitive status, therefore, the author recommends including the area of bentonite-penetrated soil and filter-cake  in the process of stability calculation.

2. It is recommended that the solution for minimizing bentonite slurry lost in case the cut-wall encounters with a gravel layer with void ratio n<50% is to add fine sand up to 10% into the bentonite solution.  

3. It is recommended appropriate proportion of soil-bentonite-cement materials for making cut-off walls with the seepage coefficient k≤1.10-6cm/s in current construction conditions in Vietnam, and thus this is really a cost-saving and technically efficient.