E stimating cement take and grout efficiency on foundation improvement for Li-Yu-Tan dam
Yang , Chau-Ping
Department of Civil Engineering, Chuag-Hau University,
30 Tung Shiang, Hsinchu, Taiwan, 30067
Fax: +886-3-5372188; E-mail address:
Abstract
The cement take needed for dam foundation improvement with grout-curtain is difficult to estimate due to the complexity of the rock foundation and the great number of Lugeon tests involved in the analysis. Therefore, this study adopted the mean method, the linear regression method, and the back-propagation neural network (BPN) method to analyze the grout-curtain construction data of the Li-Yu-Tan dam, Taiwan, in order to estimate the cement take needed. The samples analyzed included data from 3,532 grout sections. The data from the first half of the grout-curtain construction were used to derive the parameters of the predictive schemes, and then the second half of the grout-curtain construction’s data were used to test the accuracy of those schemes. The accuracy levels estimated by these three methods on gross cement take were 71.8%, 59.8%, and 75.3% for the mean method, the linear regression method and the BNP method respectively. All accuracy levels estimated by these three methods were higher than the original design level of 43.4%. Furthermore, the efficiency of the grout improvement in the studied cases were confirmed by observing the changes of the distribution curve of the following each grout sequence. The method proposed is intelligible and can be applied in other situations.
Keywords: Dam foundation; Grouting; Cement take; Estimation; BPN
1. I ntroduction
Taiwan is located on a sedimentary rock with rugged terrain and complicated geological properties. The bedrock inherently has discontinuities such as faults, folds, beddings, joints, and fractures, which are the major factors that affect the engineering properties of rock foundations such as permeability, shear strength, and deformation. When a dam is located on bedrock that has unknown discontinuities, the underlying foundation needs to be improved to raise its engineering properties and ensure a watertight reservoir. Using cement grouting to improve bedrock has been quite common (Baker, 1982; Jaroslavl, 1989; Houlsby, 1990; JSIDRE, 1994), and there are numerous examples of its application to the engineering of dam-foundation improvement (Ewert, 1985; Weaver, 1991). However, since the dam foundation is below the surface of the ground, the expense for cement grouting is the most difficult construction expense to estimate. The expense of cement grouting mainly includes the operational part and the material part. The expense of materials is calculated based on the cement take. Then the expense of the grouting operation is determined based on the material’s expense. Therefore, it is necessary to study various methods of estimating the cement take of the grouting based on actual construction data.
In general, the status of the discontinuities in the dam foundation is indirectly expressed by the determined from the Lugeon tests. The information gained from the Lugeon tests can also be used to design the water to cement ratio and the injection pressure used in the grouting process. Eq. (1) is the definition of the. Generally speaking, if a dam foundation has a high, it will have more discontinuities with high permeability and more cement take is needed for the grout improvement.
== (1)
Where is the water take (), is the standard injection pressure (981), is the injection time (), is the injection pressure used (), is the length of grout section ().
The is the best physical parameter to express the status of discontinuities in a dam foundation. Theoretically, it is quite difficult to define the relationship between cement take and the(Yamaguchi and Matsumoto, 1989; Hirota et al., 1990). Additionally, when researchers estimate the cement take needed for a new dam foundation from past experiences, they still encounter the problems of different geological properties for the proposed dam site. For example, the cement take designed for the improvement of the foundation of the Li-Yu-Tan dam, Miao-Li County, Taiwan, was 50. However, the average reading of cement take from the construction records of Li-Yu-Tan dam was 115 (Taiwan Water Resources Agency, 1993). This difference resulted in a doubling of the amount of gross cement take from what was required in the original design. This experience illustrates the difficulty in cement take estimation.
Consequently, this study focuses on the practical application of cement take estimation by adopting the mean method, the linear regression method, and the back-propagation neural network (BPN) method to analyze the construction data from the grout- curtain improvement of the Li-Yu-Tan dam’s foundation, and indicate how to estimate the cement take needed. The efficiency of the grouting for this dam site is also addressed in this article.
As shown in Fig.1, the Li-Yu-Tan dam is located at about 500in the upper stream of the Jing-San brook, a tributary of the Da-An river, which is in the mountainous regions of northwestern Taiwan. The dam is a zone-type-earth-dam with a height of 96, a bottom width along the foundation of the river of about 500and a gross volume of 3,700,000. The major terrain includes gravelly terra rossa and some riverbank outcrops. There are no faults or obvious folds on either side of the river. The major discontinuities in the foundation of the dam site are dozens of developed shear zones. Most shear zones are distributed in the right side of the abutments of the dam with slips of 2~5above. (Taiwan Water Resources Agency, 1986a).
2. F actors affecting cement take
Theoretically, there are many factors that affect the cement take needed for improving dam foundations. Moreover, since some factors may have combined effects, it is not possible to clearly define the role of each factor. Some factors that can be categorized or quantified are the strata, zone of dam foundation, depth of grout section, injection pressure, and the.
2.1. Strata
This category covers properties such as the rock layers, the nature of discontinuities, the rock strength, the mineral components, and the cementation. All of these properties may have combined effects on cement take. If a dam foundation consists of different rock layers, it may have more hidden discontinuities. Shallow bedrock tends to have a high density of cracks or openings and is subjected to grout leakage and hole collapse. If a rock foundation has little strength, the grout hole will be less independent. The disadvantages of bedrock mentioned above increase the amount of cement take needed for grout improvement.
As shown in Fig. 2, the strata in the dam site vicinity are northeastwards and meet the river valley at 28~34 degrees. All the strata are leaning towards the upper stream at 30~34 degrees. The strata of the Li-Yu-Tan dam’s foundation include clean sandstone (CS), mudstone (MS), and alternations with sandstone and shale (AL). The major formation of clean sandstone contains quartz sand, which has a tensile strength of about 1,050 and a coefficient of permeability about . However, since quartz sand has a poor cementation quality, the seepage paths are more likely to cause a loss of fine material. Mudstone contains different amounts of mud; therefore, its tensile strength ranges from 1,140 to 2,010, and the average coefficient of permeability is. If the mudstone has good cementation and low permeability, it is considered as the bedrock layer because of the better engineering properties. Alternations with sandstone and shale have intertwined clean sandstone and shale or mudstone and shale in small alternating thickness. The thickness of mud accumulation between layers can reach 30above. On the surface layer, seepage paths can form that cause deterioration of the shale into fragments or even seams. The width of fragments is about 20~30above.
2.2. Zone of dam foundation
Runoffs flush weak parts of the ground to form river valleys. When the pressure of ground is relieved, riverbanks will move inwards, and tensile fractures will occur in the banks. This development will result in more cracks on the upper half of the dam abutments and induce greater permeability. For this reason, the cement takes needed for the grout improvement in the right zone, left zone, and the valley are different. This research has divided the dam foundation into the riverbed, the left upper zone, the left lower zone, the right upper zone and the right lower zone, as shown in Fig.3 and Fig. 4, according to the tunnel locations for the grout-curtain construction. However, because the riverbed has been dug to the level of fresh bedrock with a permeability lower than 10, there are only a few in-place grout holes. Thus, the analytical extent of this research covers only the left upper zone, the left lower zone, the right upper zone, and the right lower zone. The shaded part in Fig. 4 is the outcome of the grout-curtain in the Li-Yu-Tan dam’s foundation. For the shallower parts, grouting can be performed from the top, but, in the deeper areas, the grouting will have to be performed from tunnels.
2.3. Depth of grout section
In a rock layers deeper into the underground, the cracks are narrow and comparatively do not take in grout because of the greater tectonic stresses in lower elevation. When the tectonic stress is taken into consideration, the depth of the grout section is considered as one of the factors that affect cement take. As to the grout-curtain construction in the Li-Yu-Tan dam, the diameter of the grout holes was 3.8 and the greatest vertical depth of a grout hole was limited to 50. Inside of each grout hole, there were several grout sections, and the grout process was conducted from the bottom to the top of the grout hole. If the depth of the grout section was smaller than 30, the grout section length was 5. When the depth of a grout section was greater than 30, the section length was 10.
2.4. Injection pressure
The injection pressure is the major technical factor affecting cement take. If during the grouting process, the operator increases injection pressure to fill the cracks with more grout, this action may cause the loosening and cracking of bedrock. As a result, the extent of the grout area may become larger. Theoretically, the injection pressure should be smaller than the tectonic stress corresponding to the depth of a grout section, which is obtained from the hydraulic fracturing test. Moreover, the injection pressure should be smaller than the tensile strength of the rocks (Kutzner, 1985; Shibata, 1989). In Taiwan, the field of dam engineering, considers that the injection pressure is determined based on the principle of additional pressure increasing about 30 per meter depth. The injection pressure adopted for the grout-curtain construction of the Li-Yu-Tan dam was 150 to 1200 from top to the bottom of the grout hole (Taiwan Water Resources Agency, 1986b).
2.5.
The is the only physical parameter that the researcher could obtain to evaluate the multiple factors that affect cement take. This value shows the degree of permeability in the dam foundation. Basically, in grout improvement, a dam foundation that has a high requires more cement take.
3. D ata analysis
In the Li-Yu-Tan dam’s grout-curtain construction, the grout holes were of the split-spacing type. Split-spacing means that the grout holes were arranged in the sequence of primary holes, secondary holes, tertiary holes, and quaternary holes. Supplementary holes may be added to enhance the locations with more discontinuities in the bedrock or near the holes that required more cement take. Basically, the arrangement of grout holes was based on the quality of bedrock. In the Li-Yu-Tan dam, the grout holes were arranged at intervals of 1to 3. When the grouting process of a specific hole lasts for 60 minutes, but the amount of cement take does not reach 70, the grouting for this section should be stopped. Finally, the drill inspection holes used for performing the Lugeon test to check the permeability of the dam foundation were improved. The process of grouting in each grout section was arranged in the following sequence: drilling, washing, water testing, and grouting. During water testing, the Lugeon tests need to be performed to obtain the , which gives the permeability of that specific grout section, and determines the water to cement ratio.
Table 1 lists the data analyzed for 469 grout holes and 3,532 grout sections. Each grout section had data such as zone, sequence, hole depth, length of grout section, rock nature,, injection pressure, and cement take. All of the data were collected from the inspection chart of the grout-curtain construction for the Li-Yu-Tan dam in 1993. Then, all the data were entered into an Excel application program for calculations before the analysis began.
For the convenience of analysis, this study has adopted the symbol to represent the of a specific grout section. In addition, because the lengths of the grout sections analyzed were not the same (between 5 and 10), the cement take of a grout section was divided by its length to obtain the cement take per unit length (). There were three reasons to use cement take instead of cement mortar take to define . First, the voids in the cracks were filled by solid cement. Secondly, the major material expense in grout construction is the quantity of cement. Thirdly, many documents related to grouting refer to cement take in place of cement mortar take (Ennto, 1988; Tano, 1988). Generally speaking, a grout section with a higher needs a greater amount of for grout improvement.
4. E stimation method of cement take
4.1. The mean method