The Conventional Method has been most commonly used for foundation treatment in many hydropower projects in Japan. In the Conventional Method, the water-cement ratio of the grout material is varied from a large number at first for better injection into small cracks, to a smaller number for better injection into larger and wider open cracks. The latest standard for the Conventional Method stipulates a water cement ratio within a range of 10:1 to 0.5:1, and that check holes be drilled in order to evaluate the performance of grouting works (Kudo et al. 1984). This method is popular and common because it is suitable especially for inhomogeneous geological structures, which are characterized by a variety of geological types and a lot of faults formed by ancient geotechnical activities such as folding and fracture movements. On the other hand, the method has some drawbacks due to its very deliberate and elaborate process:
1) low construction speed, and
2) strict requirements.
Since Japan, which is located on the boundaries of several tectonic plates, has inhomogeneous geological structure compared with Northern Europe, which is located on the Eurasian Plate, the Conventional Method has been applied since the early stage of hydropower development and dam construction in Japan. Records show that the method was applied for the first time to the foundation with fault treatment of the Komaki Dam in 1929. Thereafter, the method has gradually become more sophisticated and standardized based on cumulative experiences.
Firstly, the Design Standards for Dams were published by the Japan Commission on Large Dams in 1957 (Japan Commission on Large Dam 1957). Next, the Design Standards for Dam Foundations were published by the Japan Society of Civil Engineers in 1972 (Japan Society of Civil Engineers 1972), which was the first standard for grouting. In 1983, the Technical Guideline for Grouting was published by the former Ministry of Construction (Ministry of Construction River Bureau 1983). And, after a series of experiences and improvements, the updated Technical Guideline with Commentary for Grouting was published by the Ministry of Land, Infrastructure, Transport and Tourism in 2003 (Ministry of Construction River Bureau 2003; Iida 2002; Japan Dam Engineering Center 1987). Accordingly, the method has been matured as a technical standard that fits for the inhomogeneous geology in Japan.
Regarding the grouting process, the split spacing method is generally applied where an interpolated layout of holes along the grouting line is applied regardless of the grouting methods as shown in Fig. 6. The intervals of grouting holes in each step on Site are 24 m for the pilot hole and 12 m for the primary hole, accordingly.
The GIN Method has a set of simple theories represented by total injected volume and grouting pressure to avoid the elaborate work procedure of the Conventional Method. In the GIN Method, a single water-cement ratio and stable slurry are used (Kudo et al. 1984; Japan Commission on Large Dam 1957) based on the test results of viscosity and bleeding, so as to inject into even thin cracks, and a single GIN Value is set to plot the below mentioned GIN Curve when applied even in poor geotechnical characteristics. Therefore, the GIN Method helps reduce project costs compared with the Conventional Method because it is not necessary to repeatedly change the grout mix proportion in a grouting procedure. The GIN Method has been adopted for some recent projects in Lao PDR as shown in Table 1. The termination criteria of grouting treatment is defined as a conceptual formula proposed by Dr. Giovanni Lombardi, which indicates the range of grouting pressure and total injected volume (Lombardi 2003). The GIN Value is determined based on the experimental observation and engineering considerations. The upper limits of grouting pressure and total injected volume are proposed as five (5) limit curves, represented by separate GIN Values as shown in Fig. 7 (Lombardi 2003). Known as “GIN Curves”, the most suitable curve is selected by trial grouting depending on the characteristics of the geotechnical structure. A sample of the termination criteria of curtain grouting in each step is shown in Fig. 8 (Lombardi 2003).
In the design stage of Nam Ngiep 1, the Conventional Method was planned to be applied to the Site. However, the dam excavation work was so prolonged that the remaining works including the curtain grouting work had to be accelerated. Thus, the GIN Method was applied to recover the time and cost.
Modified GIN Method
Yoshizu proposed the “Modified GIN Method” as a new grouting method (Yoshizu et al. 2019), which uses a single mix proportion and injected grouting pressure (IGP) principally based on the GIN Curve, but it also uses the water pressure tests. The Modified GIN Method is a practical variation of the GIN Method in that the grouting process may continue even after it reaches the GIN Curve to enhance the efficiency of the grouting work in the inhomogeneous geology.
The new rule is summarized as shown in Fig. 9 and below.
The grouting process is completed when reaching the Modified GIN Curve with a grouting flow velocity equal to or lower than 0.4 liter/min/m.
The grouting pressure is maintained until the grouting flow velocity falls below 0.4 liter/min/m even after reaching the GIN Curve.
In the event that the grouting flow velocity is over 0.4 liter/min/m when cumulative grouting volume reaches 600 liter/m, grouting is paused and resumed 3 hours later.
Yoshizu raised the issues to be solved for better application of the method as below (Yoshizu et al. 2019). Figure 10 shows the average Lu and cement take per meter (hereinafter, “unit cement volume” or UCV) in each step on Site. Theoretically, Lu and UCV should decrease as the steps progress. The results of Block 1 (see Fig. 1) show theoretical behavior (see Fig. 10(a)). However, the UCV increases in the final step of Block 2 (see Fig. 10(b)). As shown in Fig. 11, the geology in Block 1 is generally simpler, harder and more impermeable than that in Block 2. It is considered that the main causes of this are that 1) large-scale fractures are newly formed or developed by over-pressurized grouting just before the grouting in the final step, and/or 2) Block 2 has an extremely steep topography on the surface, which is supposedly derived from toppling phenomenon. As a result, the vertical cracks perpendicular to the dam axis in Block 2 might be difficult to grout from the grouting holes that were vertically drilled parallel to the vertical cracks, and hence not improved in the earlier steps. According to Figs. 10(c) and 10(d), it is found that the UCV of Blocks 13 and 14 is similarly larger in the final step than in the early steps as in Block 2.
In order to further pursue the reason, parameters such as Critical Grouting Pressure (CGP) and UCV are analyzed in each block. The average CGP and UCV of the sandstone, mudstone and weak layers in Blocks 12, 13, 14 and 15 are shown in Fig. 12. It is assumed that there are few cracks in Blocks 12 to 15 because they are located in the riverbed where there are few of the large-scale fractures seen in Block 2. However, comparing the respective values shown in Fig. 12, the UCV in the weak layers is larger than that in the sandstone and mudstone though the CGP in the weak layers is smaller than that in sandstone and mudstone. The reason for this is assumed that the weak layers are so sensitively affected by the geotechnical actions that the fractures might have easily induced.
To pursue the possible reason of the fractures inducement in the weak layers, the step-by-step transition of the Lu and the UCV are as follows. The degree of grout improvement in weak layers by the Modified GIN Method is investigated by Progress Management Method (“PMM”) for grouting. As shown in Fig. 13, the horizontal axis represents the difference in Lu (⊿Lu) and the vertical axis represents the difference in UCV (⊿UCV) at the transition of each step. For example, when Lu decreases from 40 to 30 and UCV decreases from 300 to 200 through a step, (⊿Lu; -10, ⊿UCV;= -100) is plotted. That is, the chart of PMM indicates that the grouting is proceeding appropriately when the data concentrates in the third quadrant and moves to the center as the step progresses. Figure 14 shows the transition of Lu and UCV in each step determined by PMM for Blocks 12 to 15 that had weak layers that were grouted by the Modified GIN Method. Figure 14(a) indicates that Lu and UCV did not improve well with tertiary hole grouting because the data are scattered in all of the quadrants. Figure 14(b) indicates that both Lu and UCV show some improvement by quarterly hole grouting because the data are concentrated in the 3rd quadrant. Figure 14(c) indicates that Lu and UCV get larger in the final step because the data moves to the 1st quadrant again. This means that cracks might develop due to over-pressurized grouting.
Next, the CGP are analyzed in detail. Too high of an IGP may cause harmful damage to the dam foundation, therefore the IGP should not exceed the CGP too much. However, it is common practice to set IGP slightly higher than the CGP as shown by the green line in Fig. 15, so as not to leave any voids in the cracks in the dam foundation. It should be noted that the Modified GIN Method has a weak point in that the IGP might in certain cases not reach the CGP, as occurred in Blocks 1, 2, 13 and 14 as shown in the red area in Fig. 15.
Originally, the CGP was simply supposed in accordance with the depth of the grouting holes. However, as shown in Fig. 16, its variation is too large compared with the highest envelop of the CGP against depth (H). Thus, it was concluded that the CGP should be correctly determined by conducting water pressure tests stage by stage. It is considered that the injection may have been paused or terminated before the void was sufficiently filled in the intermediate step due to a relatively lower IGP than CGP in some stages. It is also considered that the injection volume may unhelpfully have increased in the final step due to cracks developing as a result of IGP becoming much higher than CGP as shown in the blue area in Fig. 15. As shown in Fig. 15, IGP was significantly higher than CGP in 110 stages, equivalent to 65%, where the injection volume may impractically have increased in the final step due to cracks developing because of the excessive IGP. To solve the issue, a new method should have been developed whereby a proper IGP is set based on the CGP obtained from the water pressure test in each stage. Applicable conditions of this new method and the GIN Method are also proposed depending on the geological situation. Therefore, by carefully regulating the IGP according to geotechnical conditions, it is possible to improve permeability of the dam foundation effectively.