[SIE] The total length of dykes of our country is over 13,200 km of dykes, including 10,600 km of river dykes, 2,600 km of sea dykes and 2,500 km of special dykes. In order to meet the flood prevention, these dykes have to be repaired and upgraded regularly, especially in the face of climate change and sea level rise in the near future.

Currently, our dyke system is only resistant to the storm of grade 9, with a maximum tide frequency of  5%. Many dykes are built on thick sand foundations that are easy to be liquefied when subjected to strong earthquakes. In addition, there exist many weak dyke sections that are placed on the soft soil, which is easy to destabilize, and the quality of the filling materials is low due to the manual construction methods traditionally.  Liquefaction research is still very limited domestically. The cause may be due to not many strong earthquakes causing great damage in the territory of Vietnam. On the other hand, it can be clearly seen that the main difficulty in carrying out studies on soil dynamics in general and liquefied in particular is due to the limitations of the ground motion observation network, soil dynamics testing apparatuses. Therefore, in order to evaluate the liquefaction potential of some types of foundation and filling soils of dykes and dams, and dam filling soils containing coarse grain materials when subjected to strong earthquakes; Develop a procedure to analyze, detect and assess the liquefaction potential of some local materials as dam materials and propose solutions to strengthen river dykes and local material dams in case of strong earthquakes, the research team lead by Associate Professor  Nguyen Hong Nam  at ThuyLoi University has conducted a study on the topic of " Studying the liquefaction potential of dykes and dams by local material subjected to earthquake load and stabilization solution " to generate a number of significant changes in perception and technology in the areas of seismic resistance design with liquefion assessment processes, liquefaction design solutions for critical projects in the regions subjected to strong earthquakes. Develop measures to deal with bad scenarios caused by earthquake liquefaction.

Photo: TLU rector welcomed Japanese experts 

After more than 2 years of implementation (4/2013 - 12/2015), the research team has obtained the following results:

- Collecting, synthesizing quite rich data bank in in-situ material dykes/dams, earthquakes, dykes/dams sliding, hydrometeorology, construction materials, Socio economic development planning, technical standards in the field of water resources, construction, seismics.

- Seismic database for the study of liquefaction in Red River dyke (Hanoi) and dams in Dien Bien province which is based on tectonic, earthquake data collection in North Vietnam and neighboring areas. From these data, the research team found:

+ PGA (peak ground acceleration) with earthquake period T = 2475 years at the Red River dyke is in the range of 0.18-0.26g. The PGA value is from 0.18 to 0.26 g appeared at the dyke in the inner Hanoi city. PGA values from 0.24-0.26g appeared in Thuong Tin area. The remaining areas from Trung Ha Bridge to Thanh Tri have PGA values of 0.2g-0.24g.

+Regarding the impact of earthquakes on the ability of liquefaction of dykes attention should be paid to the dyke sections from Km59 to Km117 of right Red river dyke, where the dyke base is located on thick silty layer> 5m, and Km91 - K117 in Thuong Tin dyke area where the dike foundation is placed on 10-20m thick mud layer.  PGA at the area also has a high value of 0.24-0.26g

+ PGA values with period  T = 2475 years for the dams in Dien Bien are in the range of 0.200-0.396g. The largest PGA value found at Hong Khenh dam reaching 0.396g, due to its dam on the western slope, in the impact zone of Lai Chau - Dien Bien fault and near the epicenter of M5.3 Dien Bien earthquake in 2011. The impact of earthquakes on the dam's liquefaction potential in the Dien Bien area is very low due to the erosion terrain.

Photo: Seminar on the development of Seismic database for Red River dyke (Hanoi) and dams in Dien Bien province

- The research team have classified filling and foundation soils for the dykes/dams based on the structure of geology, seismology and hydrogeology. The liquefaction assessment method is carried out using the simplified procedure using SPT standard data and finite element analyses. The results showed that the potential for liquefaction is likely for right Red river dyke and some in-situ material dams in the Northwest area in case of strong earthquakes.
- Based on the results of more than 40 specimens, the research team determined the Young’s moduli and the damping coefficients of the Red river sand, dyke ground and filling soils at K73 + 500 - K74 + 100 at various axial strain amplitudes varying from 10-4 to higher level. For coarse soil samples and cohesive soil samples at hard plastic state, Young's modulus and soil damping coefficient values were not change at strain level of 10-5 . For cohesive soil samples at soft plastic state, it could measure Young’s modulus and damping coefficient values at small strain level of about 10-4 with constant values. At higher strain levels, the Young’s modulus decreased gradually while the damping ratio increased gradually as the axial strain increased. The soil damping ratio varied according to the strain levels, which are common at 5%. The test results show that Young’s modulus and soil damping ratio depend very much on soil type, consolidation stress and soil density. The results of the Young’s modulus at the isotropic consolidation stage show that the Young’s modulus  is proportional to the vertical stress under the exponential rule. Similar results were obtained for sand samples at Pu Don mountain foot, Muong Phang, Dien Bien and sand samples at Loong Luong dam, Muong Phang, Dien Bien.
- Liquefaction curves of dyke/dam foundation soils: Experimental study was carried out on dynamic triaxial apparatus, using sand samples at Ha Noi harbor and foundation soils and filling soils for right Red river dyke (section K73 + 500 - K74 + 100). In this area, the soil consists of sandy soils 3a at different depths, sandy soil 3b, 4c, 4d, 4e. The coarse soil samples were prepared with the same relative density in two loose and medium densely states. Cohesive soils consist of soil layer 2a, filling soil 1d, were tested with undisturbed samples. A series of tests were carried out on a number of samples having the same relative densities as required while changing the axial force amplitudes in constant controlled confining effective stress condition.
- The results of the three axial dynamic experiments exhibited the liquefaction curves of the dyke foundation soil. Fine sand layers in the dyke foundation were very easy to be liquefied when subjected to cyclic loading under undrained conditions. Under the same conditions of relative density, effective confining stress, the small amount of silt content in the sand sample, the greater liquefaction triggering ability. When the soil relative density increased, the number of cycles causing soil to be liquified increased or the soil became harder to be liquefied. The fine grain content of <7% affected the liquefaction curves of sandy soils although the impaction extent was not much. In the coarse soil samples when the excess pore pressure was high, large deformation occurred in the specimen, which was destroyed after a few load cycles. Cohesive soil samples, when subjected to cyclic loading, often exhibited large displacement after several load cycles, while excess pore water pressure increases slowly, after that the strain increased slowly when the number of load cycles increased.

Photo: Training class of using dynamic triaxial apparatus 

- Present research results of a series of dynamic triaxial experiments in undrained conditions on sandy soil samples of Red River in Ha Noi port area and K73 + 500 - K74 + 100 section show that it could be accurately determined the liquefaction characteristics of Red river sand such as variation in pore water pressure, deformation, liquefaction conditions with loading cycles, loading amplitudes. From there, it is possible to construct the liquefaction curves of the Red river sand. Experimental research provided input data to estimate the liquefaction potential of the Red River dyke to withstand strong earthquake loads.  Similar results were obtained for sandy soil samples at the foot of Pu Don mountain, Muong Phang, Dien Bien and foundation soils of Loong Luong dams, Dien Bien. Numerical analyses of cross section of the Loong Luong dam were implemented using QUAKE/W software versions 7 and 2012 of Geoslope company Ltd., Canada. The analysis results show that, with the sand layer upstream of the dam and under the downstream drainage prism, Loong Luong dam could be liquefied in the foundation with the return periods of 475 and 2475 years, possibly causing damage to the dam.

Photo: Field trip to Loong Luong dam, Dien Bien provice 

- Based on the research results, the research team proposed a procedure for analyzing, detecting, and forecasting liquefaction to local material dykes/dams in case of strong earthquakes. Design solutions for liquefaction prevention were synchronous solutions, combining the study of seismic ground movement of dykes and dams, appropriate geotechnical investigation and survey methods, appropriate seismological analysis methods for dams,  liquefaction resistant materials.

The research team also recommended the need to develop maps of safety factors against liquefaction  for river dykes and in-situ material dams to actively cope with possible strong earthquakes; compile technical seismic design standards to prevent liquefying of dykes/dams. Also measures should be taken to prevent the instability of dams in the event of an earthquake, such as the installation of earthquake warning systems in order to move people out of danger.

The full text report of the funded project (No. 12592-2016) can be read at National Agency for Science and Technology Information (NASATI) 

Source: National Agency for Science and Technology Information (NASATI) 

https://www.most.gov.vn/vn/tin-tuc/12296/nghien-cuu-kha-nang-hoa-long-cua-de-dap-bang-vat-lieu-dia-phuong-chiu-tai-trong-dong-dat-va-giai-phap-on-dinh-cong-trinh.aspx