**2.1. Geological investigation**

Core drilling is the most useful subsurface exploration method for investigating the location, extent and constituent makeup of soil and rock strata at a potential dam site. Nonetheless, core drilling becomes increasingly difficult through overburden layers thicker than 40–50 m because [22]: (1) the existence of unpredictable super-large rock particles; (2) frequent borehole collapse; and (3) uncontrollable loss of drilling fluid. The mud or water used in ordinary drilling-with-casing operations can also make the analysis of core grading difficult or inaccurate due to washing away of fine particles. Some special core drilling techniques were therefore used to get high quality cores in the geological investigations at Yele, Aertash, Xiabandi and other dam sites. They include: (1) double-tube swivel type diamond drilling with a proper rpm (revolution per minute) and pressure and (2) special vegetable gum and powder drilling fluid circulated under proper flow rates to protect the bit, the borehole and the core. Until now, the deepest overburden core drilling conducted in China is at Yele ACRD, where the overburden thickness reaches 420 m [15].

can then be used to estimate the density states, bearing capacities, and moduli of the tested layers. This method has been used in geotechnical investigation of almost all dam

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

http://dx.doi.org/10.5772/intechopen.78547

5

**b.** *Plate load tests*. Plate load tests are performed by loading a steel plate at a particular depth and recording the settlement corresponding to each load increment. The load is gradually increased until the plate starts to sink at a rapid rate. The total load on the plate at this stage, divided by its area, gives the value of the ultimate bearing capacity of the tested soil. Assuming an isotropic elastic behavior of the tested soil, the elastic modulus can also be evaluated. In the under-construction Aertash CFRD, plate load tests were performed with a plate of 1.5 m in diameter, and with the maximum reaction force of about 1000 tons [26].

**c.** *In-situ shear tests*. Large-scale in-situ shear tests are widely used in field investigations. A shear box of a specific size is compressed into the overburden and then it is pulled, after applying a designed vertical load upon the enclosed soil, by a jack using a high-strength chain. The applied horizontal force and the displacement of the shear box are recorded, based on which the in-situ shear strength of the tested soil is determined [27]. A special advantage of this method is that it can measure the strength of coarse granular materials under extremely low normal stresses, which is otherwise not so easy in triaxial compres-

**d.** *Pressuremeter tests*. Pressuremeter tests are performed in-situ by placing a cylindrical probe in the ground and then expanding the cylinder to pressurize the soil horizontally. The radial pressure on the soil and the relative increase in cavity radius are measured, from which the in-situ stress strain curve of the soil is derived. This technique is extremely attractive in testing overburden layers because the loading direction is identical to the hydrostatic pressure upon a cutoff wall if it is to be installed. Abundant information can be obtained from this type of tests, such as the in-situ horizontal stress, the pressuremeter modulus, the limit pressure, etc. This technique has been used in many dam sites [22, 28],

**e.** *Wave velocity tests*. The most widely used wave velocity tests include the down-hole test, the suspension logging test and the cross-hole test. The first two methods require only one borehole and evaluate the wave velocities vertically along the borehole wall. The third test requires at least two boreholes and measures the wave velocities within a horizontal plane. Boreholes for down-hole and cross-hole tests should be carefully cased and grouted to ensure good seismic coupling between the geophones and the surrounding soils. Suspension logging, on the other hand, preferably uses uncased holes. All three methods have been applied to the investigation of overburden layers for foundations in many important projects [16, 25]. The velocity results obtained, especially the shear wave velocities, can be used to evaluate the density states, the elastic moduli and the liquefac-

**f.** *Permeability tests*. The permeability coefficients of overburden layers, which are needed to design the underground impervious systems, are determined by various permeability tests. Methods may be selected based on the location of underground water table, the enrichment

and the depth has reached a magnitude of 100 m successfully.

sites [25].

sion experiments.

tion potential of the tested layers.

Geophysical exploration methods, such as electrical and electromagnetic methods, seismic procedures, gravity techniques, magnetic methods, and so on, are now increasingly used in dam engineering. These techniques are mostly used to locate the interface between overburden and bedrock and to detect weak layers. Geophysical techniques generally does not directly measure the parameters desired for designing purpose. The vast majority of objectives is inferred from the known geologic data and measured geophysical contrast [23]. That is to say, an inverse solution is sought usually in geophysical exploration, and in most cases, it is the most likely but not necessarily the unique conclusion. Assumptions used in interpreting geophysical contrasts, such as the distinct subsurface boundaries, the homogeneity of materials and the isotropy of material properties, are also, in many cases, at variance with the reality, which may lead to inaccurate and misleading conclusions. Therefore, geophysical methods are almost always used in combination with irreplaceable core drilling. Thereby, results obtained by different methods can be verified mutually and a most reliable judgment can be made.

#### **2.2. Geotechnical tests and interpretation**

While geological explorations give overall information on the overburden layers, geotechnical tests and their interpretation yield more relevant parameters for designing. However, systematic laboratory experiments with overburden materials are usually unrealistic due to difficulties in obtaining high-quality undisturbed samples. Although some techniques do exist for sampling (e.g., in-situ freezing [24]), they are generally expensive and only applicable to shallow layers. Therefore, measurement of engineering properties of overburden materials relies more upon in-situ tests as exemplified as follows:

**a.** *Heavy and super-heavy dynamic penetration tests*. For layers with high relative densities, heavy or super-heavy dynamic penetration tests are usually used, in which a cone-tipped probe is driven into the ground by a 63.5 or 120 kg weight dropped freely from a height of 76 or 100 cm. The number of blows needed to drive the probe into the tested layer for 10 cm is registered and an average penetration per blow is calculated. The data gathered can then be used to estimate the density states, bearing capacities, and moduli of the tested layers. This method has been used in geotechnical investigation of almost all dam sites [25].

**2.1. Geological investigation**

4 Dam Engineering

where the overburden thickness reaches 420 m [15].

**2.2. Geotechnical tests and interpretation**

relies more upon in-situ tests as exemplified as follows:

Core drilling is the most useful subsurface exploration method for investigating the location, extent and constituent makeup of soil and rock strata at a potential dam site. Nonetheless, core drilling becomes increasingly difficult through overburden layers thicker than 40–50 m because [22]: (1) the existence of unpredictable super-large rock particles; (2) frequent borehole collapse; and (3) uncontrollable loss of drilling fluid. The mud or water used in ordinary drilling-with-casing operations can also make the analysis of core grading difficult or inaccurate due to washing away of fine particles. Some special core drilling techniques were therefore used to get high quality cores in the geological investigations at Yele, Aertash, Xiabandi and other dam sites. They include: (1) double-tube swivel type diamond drilling with a proper rpm (revolution per minute) and pressure and (2) special vegetable gum and powder drilling fluid circulated under proper flow rates to protect the bit, the borehole and the core. Until now, the deepest overburden core drilling conducted in China is at Yele ACRD,

Geophysical exploration methods, such as electrical and electromagnetic methods, seismic procedures, gravity techniques, magnetic methods, and so on, are now increasingly used in dam engineering. These techniques are mostly used to locate the interface between overburden and bedrock and to detect weak layers. Geophysical techniques generally does not directly measure the parameters desired for designing purpose. The vast majority of objectives is inferred from the known geologic data and measured geophysical contrast [23]. That is to say, an inverse solution is sought usually in geophysical exploration, and in most cases, it is the most likely but not necessarily the unique conclusion. Assumptions used in interpreting geophysical contrasts, such as the distinct subsurface boundaries, the homogeneity of materials and the isotropy of material properties, are also, in many cases, at variance with the reality, which may lead to inaccurate and misleading conclusions. Therefore, geophysical methods are almost always used in combination with irreplaceable core drilling. Thereby, results obtained by different methods can be verified mutually and a most reliable judgment can be made.

While geological explorations give overall information on the overburden layers, geotechnical tests and their interpretation yield more relevant parameters for designing. However, systematic laboratory experiments with overburden materials are usually unrealistic due to difficulties in obtaining high-quality undisturbed samples. Although some techniques do exist for sampling (e.g., in-situ freezing [24]), they are generally expensive and only applicable to shallow layers. Therefore, measurement of engineering properties of overburden materials

**a.** *Heavy and super-heavy dynamic penetration tests*. For layers with high relative densities, heavy or super-heavy dynamic penetration tests are usually used, in which a cone-tipped probe is driven into the ground by a 63.5 or 120 kg weight dropped freely from a height of 76 or 100 cm. The number of blows needed to drive the probe into the tested layer for 10 cm is registered and an average penetration per blow is calculated. The data gathered


of underground water, and the hydraulic conductivity of the concerned layer. In principle, pumping tests or water injection tests are conducted to evaluate the permeability of highly permeable overburden layers, while pump-in tests are used for relatively less permeable bedrock layers [25]. Permeability tests are indispensable for almost all dam projects.

Most of in-situ geotechnical investigation techniques listed above require high-quality predrilled boreholes. Unfortunately, this becomes increasingly difficult when the thickness of the overburden at the potential site exceeds 50 m. Uncertainty exists in all foundation conditions, and therefore designing and constructing an underground impervious system within a thick overburden is a very challenging task. Adequate geological and geotechnical investigations are undoubtedly the only way to improve design confidence in these systems. It is also important for designing engineers to fully assess the reliability of investigation results, including factors such as the size effect in plate load tests, the field draining condition in pressure meter tests, the influence of underground water on compressive wave velocities, the influence of drilling fluid layer adhering to the borehole wall on measured permeability coefficients, and the possible anisotropy of engineering properties.

blanket. It was assumed that the upstream blanket would connect naturally with sand sedi-

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

http://dx.doi.org/10.5772/intechopen.78547

7

During the design-phase for the Xiaolangdi ECRD, China had no experience in building such high rockfill dams on 80-m overburden layers, making this project a particularly difficult challenge. A number of alternative design proposals were also considered, including the complete removal of the overburden under the core wall and the use a horizontal impervious blanket without the permanent cutoff wall. Lessons learned from previous cases and, more importantly, technological advances in cutoff wall construction resulted in the final chosen design. The thickness of the cutoff wall was determined based on the allowable hydraulic gradient of concrete materials, the available equipment and stress–strain and seepage analyses results. Conventional concrete with a 28-d strength of 35 MPa was used for the main cutoff wall, while plastic concrete and high-

pressure rotary jet grouting were used to construct the temporary cofferdam cutoff wall.

China (**Table 1**). It sits on a thick, three-layer overburden. All three layers, *fglQ*<sup>3</sup>

Local liquefiable sand layers are, however, also distributed widely within the *alQ*<sup>4</sup>

exceedance probability of 0.02 in 100 years is 3.59 m/s<sup>2</sup>

**Figure 2.** The maximum cross section of the Changheba ECRD.

The Changheba ECRD is currently one of the highest rockfill dams under construction in

The dam is located in a high earthquake intensity region, where the peak acceleration for an

condition is therefore a potential problem for this dam. The existence of these sand layers may also cause uneven deformation of the dam. To avoid these adverse risks, sand layers beneath

 (shown in **Figure 2**) consist mainly of coarse gravel materials and therefore have relatively high deformation moduli and bearing capacity, but also exhibit high permeability.

, *alQ*<sup>4</sup> 1 and

> 1 layer.

. Sand liquefaction under earthquake

ments during long-term operation once the reservoir was impounded.

**Figure 1.** The maximum cross section of the Xiaolangdi ECRD.

*3.1.2. The Changheba ECRD*

*alQ*<sup>4</sup> 2
