**3.1. Earth core rockfill dams**

#### *3.1.1. The Xiaolangdi ECRD*

The Xiaolangdi ECRD was constructed on the well-known, sediment-laden Yellow River. The thickness of the underlying overburden is approximately 80 m, and it is composed of intricate sand and gravel layers. The dam uses an inclined core wall (low-plasticity loam) as the main anti-seepage barrier, as shown in **Figure 1**. A vertical concrete cutoff wall (1.2 m) was built within the overburden to control the underground seepage. The top of the cutoff wall was embedded into the core wall for 12 m, while its bottom end penetrates the rock surface for at least 1 m. The inclined core was extended using low permeable clayey soils along the top surface of the cofferdam on the upstream side, forming a horizontal blanket that is useful in lengthen the seepage path. The cutoff wall under the cofferdam was elongated into this Practices in Constructing High Rockfill Dams on Thick Overburden Layers http://dx.doi.org/10.5772/intechopen.78547 7

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

blanket. It was assumed that the upstream blanket would connect naturally with sand sediments during long-term operation once the reservoir was impounded.

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

#### *3.1.2. The Changheba ECRD*

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 two main goals in designing seepage control facilities include controlling the hydraulic gradient within overburden layers to ensure the seepage stability of foundation materials and reducing the seepage loss of reservoir water. For overburden layers where excavation and removal are feasible, deposits right beneath the impervious system of the dam body (e.g., clay or asphalt core and toe plinth) can be removed so that the seepage barrier can sit on a firm rock foundation. In cases where deep excavation is impossible, a horizontal, vertical or combined seepage control measure must be employed to meet the above goals. In the design specification for rolled earth and rockfill dams, a vertical seepage barrier that cuts through the overburden layers is recommended over an upstream horizontal measure. This can be evidenced from the cases listed in **Table 1**, in which all dams use at least one cutoff wall. In this section, seepage control techniques used in some of these example dams

The Xiaolangdi ECRD was constructed on the well-known, sediment-laden Yellow River. The thickness of the underlying overburden is approximately 80 m, and it is composed of intricate sand and gravel layers. The dam uses an inclined core wall (low-plasticity loam) as the main anti-seepage barrier, as shown in **Figure 1**. A vertical concrete cutoff wall (1.2 m) was built within the overburden to control the underground seepage. The top of the cutoff wall was embedded into the core wall for 12 m, while its bottom end penetrates the rock surface for at least 1 m. The inclined core was extended using low permeable clayey soils along the top surface of the cofferdam on the upstream side, forming a horizontal blanket that is useful in lengthen the seepage path. The cutoff wall under the cofferdam was elongated into this

the possible anisotropy of engineering properties.

**3. Seepage control techniques**

are reviewed.

6 Dam Engineering

**3.1. Earth core rockfill dams**

*3.1.1. The Xiaolangdi ECRD*

The Changheba ECRD is currently one of the highest rockfill dams under construction in China (**Table 1**). It sits on a thick, three-layer overburden. All three layers, *fglQ*<sup>3</sup> , *alQ*<sup>4</sup> 1 and *alQ*<sup>4</sup> 2 (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. Local liquefiable sand layers are, however, also distributed widely within the *alQ*<sup>4</sup> 1 layer. The dam is located in a high earthquake intensity region, where the peak acceleration for an exceedance probability of 0.02 in 100 years is 3.59 m/s<sup>2</sup> . Sand liquefaction under earthquake 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

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

both the core wall and the filter layer were removed completely. The maximum thickness of the retained overburden under the core wall is about 53 m.

Two concrete cutoff walls were poured with a net distance of 14 m. Both walls penetrate into the bedrock for at least 1.5 m. The main cutoff wall (1.4 m) is located within the dam axis plane and is connected to the core wall by a grouting gallery. The auxiliary cutoff wall (1.2 m) is located upstream of the main wall and embeds into the core wall for 9 m. The core wall is constructed with gravelly soils, where the maximum core material diameter allowed is 150 mm. The percentage of particles finer than 5 mm (P5 ) ranges from 30–50%. Another two strict requirements for the core materials are P0.075 ≥ 15%, and P0.005 ≥ 8%. Curtain grouting was conducted through the preset pipes within the cutoff walls. In particular, curtain grouting under the main cutoff wall was extended to the level 5 m below the relatively impermeable layer (*q* < 3 Lu.)

#### *3.1.3. The Luding ECRD*

The 84-m Luding ECRD was built on an overburden with a maximum thickness of 148 m. It is among the deepest overburden layers used as foundations of a rockfill dam in China. The complex soil and rock strata is shown in **Figure 3**. Four main layers can be observed: the *fglQ*<sup>3</sup> layer, the *prgl* + *alQ*<sup>3</sup> layer, the *al + plQ*<sup>4</sup> layer, and the *alQ*<sup>4</sup> 2 layer. Basic properties of these layers are listed in **Table 2**. The third sub-layer of the *prgl* + *alQ*<sup>3</sup> layer consists mainly of fine and silty sands, and therefore has a relatively low deformation modulus and a low bearing capacity. Sand lenses also exist in the second sub-layer of the *prgl* + *alQ*<sup>3</sup> layer and the first sub-layer of the *al + plQ*<sup>4</sup> layer.

a suspended cutoff wall was designed in the river center with the bottom end located at an

<sup>2</sup> 2.15–2.25 2.00–2.10 50–60 0.50–0.55 28–30 0 1–10 × 10−2 0.10–0.12 Note: *ρ* = natural density; *ρ*<sup>d</sup> = dry density; *E*<sup>0</sup> = deformation modulus; *R* = allowable bearing capacity; *ϕ* = friction angle;

grouting gallery. The maximum height of the wall is 110 m, and the underlying unsealed overburden has a thickness of 40–50 m. Two rows of grouting pipes (ϕ 114 mm) were preset in the cutoff wall for grouting the bedrock, and two additional rows outside the wall for grouting the unsealed overburden. Both curtains extend into the bedrock, that is, the rock curtain reaches the level where *q* < 5 Lu., and the overburden curtains penetrate the rock for at least 2 m.

When high-quality clayey soils are difficult to obtain to construct an ECRD, an ACRD is an appropriate alternative. Asphalt is a highly plastic and impermeable material and has a good

layer. The cutoff wall was connected to the clay core by a

**Shear strength Permeability**

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

9

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

 **(MPa)** *R* **(MPa)** *ϕ* **(°)** *c* **(MPa)** *k* **(cm/s)** *J***<sup>c</sup>**

elevation of 1200 m within the *fglQ*<sup>3</sup>

**Layer Density Modulus and bearing** 

**)** *ρ***d (g/cm3**

*c* = cohesion; *k* = coefficient of permeability; *J*<sup>c</sup> = allowable hydraulic gradient.

**Table 2.** Basic properties of the overburden layers in Luding ECRD.

**Figure 4.** The segments of seepage control barriers in the Yele ACRD.

*ρ* **(g/cm3**

**capacity**

1: *fglQ*<sup>3</sup> 2.20–2.30 2.05–2.15 55–65 0.55–0.65 30–32 0 2–4 × 10−2 0.12–0.15 2-2: *prgl + alQ*<sup>3</sup> 2.05–2.15 2.00–2.05 40–50 0.35–0.45 26–28 0 1–5 × 10−3 0.20–0.25 2-3: *prgl + alQ*<sup>3</sup> 1.60–1.70 1.40–1.60 18–22 0.12–0.16 15–18 0 1–10 × 10−3 0.25–0.36 3-1: *al + plQ*<sup>4</sup> 2.10–2.20 2.05–2.10 45–55 0.40–0.50 29–31 0 5–10 × 10−3 0.15–0.18

**)** *E***<sup>0</sup>**

**3.2. Asphalt core rockfill dams**

*3.2.1. The Yele ACRD*

4: *alQ*<sup>4</sup>

The dam uses a clay core as the anti-seepage barrier stabilized by the rockfill shoulders. The maximum diameter allowed for the core materials is 100 mm. Other restrictions imposed on the core materials are P5 ≥ 90%, P0.075 ≥ 60%, and P0.005 ≥ 15%. Repeated compaction near the optimum water content (±2%) produces a barrier with a coefficient of permeability less than 5 × 10−7 cm/s. A vertical concrete wall (1.0 m) was designed to cut off the foundation seepage water. The cutoff wall penetrates into the bedrock at both the left and right abutments of the dam. However, the overburden near the center of the canyon is so thick (148 m) that the current technology limits the capacity for constructing such a high underground wall. Therefore,

**Figure 3.** The maximum cross section of the Luding ECRD.


Note: *ρ* = natural density; *ρ*<sup>d</sup> = dry density; *E*<sup>0</sup> = deformation modulus; *R* = allowable bearing capacity; *ϕ* = friction angle; *c* = cohesion; *k* = coefficient of permeability; *J* <sup>c</sup> = allowable hydraulic gradient.

**Table 2.** Basic properties of the overburden layers in Luding ECRD.

a suspended cutoff wall was designed in the river center with the bottom end located at an elevation of 1200 m within the *fglQ*<sup>3</sup> layer. The cutoff wall was connected to the clay core by a grouting gallery. The maximum height of the wall is 110 m, and the underlying unsealed overburden has a thickness of 40–50 m. Two rows of grouting pipes (ϕ 114 mm) were preset in the cutoff wall for grouting the bedrock, and two additional rows outside the wall for grouting the unsealed overburden. Both curtains extend into the bedrock, that is, the rock curtain reaches the level where *q* < 5 Lu., and the overburden curtains penetrate the rock for at least 2 m.

#### **3.2. Asphalt core rockfill dams**

#### *3.2.1. The Yele ACRD*

both the core wall and the filter layer were removed completely. The maximum thickness of

Two concrete cutoff walls were poured with a net distance of 14 m. Both walls penetrate into the bedrock for at least 1.5 m. The main cutoff wall (1.4 m) is located within the dam axis plane and is connected to the core wall by a grouting gallery. The auxiliary cutoff wall (1.2 m) is located upstream of the main wall and embeds into the core wall for 9 m. The core wall is constructed with gravelly soils, where the maximum core material diameter allowed is 150 mm. The per-

for the core materials are P0.075 ≥ 15%, and P0.005 ≥ 8%. Curtain grouting was conducted through the preset pipes within the cutoff walls. In particular, curtain grouting under the main cutoff

The 84-m Luding ECRD was built on an overburden with a maximum thickness of 148 m. It is among the deepest overburden layers used as foundations of a rockfill dam in China. The complex soil and rock strata is shown in **Figure 3**. Four main layers can be observed: the *fglQ*<sup>3</sup>

and silty sands, and therefore has a relatively low deformation modulus and a low bearing

The dam uses a clay core as the anti-seepage barrier stabilized by the rockfill shoulders. The maximum diameter allowed for the core materials is 100 mm. Other restrictions imposed on the core materials are P5 ≥ 90%, P0.075 ≥ 60%, and P0.005 ≥ 15%. Repeated compaction near the optimum water content (±2%) produces a barrier with a coefficient of permeability less than 5 × 10−7 cm/s. A vertical concrete wall (1.0 m) was designed to cut off the foundation seepage water. The cutoff wall penetrates into the bedrock at both the left and right abutments of the dam. However, the overburden near the center of the canyon is so thick (148 m) that the current technology limits the capacity for constructing such a high underground wall. Therefore,

layer, and the *alQ*<sup>4</sup>

2

layer. Basic properties of these

layer consists mainly of fine

layer and the first

wall was extended to the level 5 m below the relatively impermeable layer (*q* < 3 Lu.)

) ranges from 30–50%. Another two strict requirements

the retained overburden under the core wall is about 53 m.

layer, the *al + plQ*<sup>4</sup>

layer.

**Figure 3.** The maximum cross section of the Luding ECRD.

layers are listed in **Table 2**. The third sub-layer of the *prgl* + *alQ*<sup>3</sup>

capacity. Sand lenses also exist in the second sub-layer of the *prgl* + *alQ*<sup>3</sup>

centage of particles finer than 5 mm (P5

*3.1.3. The Luding ECRD*

8 Dam Engineering

layer, the *prgl* + *alQ*<sup>3</sup>

sub-layer of the *al + plQ*<sup>4</sup>

When high-quality clayey soils are difficult to obtain to construct an ECRD, an ACRD is an appropriate alternative. Asphalt is a highly plastic and impermeable material and has a good

**Figure 4.** The segments of seepage control barriers in the Yele ACRD.

adaptability to uneven deformation. The Yele ACRD sits on a thick overburden as shown in **Figure 4**, with an extremely thick overburden at the right abutment. There are five main layers under the dam, as divided by the solid curves in **Figure 4**. The first (*Q*<sup>2</sup> 1 & *Q*<sup>2</sup> 2 ), third (*Q*<sup>3</sup> 2−1), fourth layers (*Q*<sup>3</sup> 2−2) are mainly composed of weakly cemented gravel materials, while the second layer (*Q*<sup>3</sup> 1 ) is composed of a mixture of gravel and hard clay. The relatively high fifth layer (*Q*<sup>3</sup> 2−3) is mainly composed of silty loam. The second layer (*Q*<sup>3</sup> 1 ) forms a relatively impermeable barrier in the foundation, the permeability coefficient of which is less than 2.2 × 10−5 cm/s and the allowable hydraulic gradient reaches 10.4. These features were fully used in designing the foundation impervious facility.

The seepage control measures for this dam are divided, from the left bank to the right, into a number of different segments as described below. Curtain grouting was conducted within the gallery in the left river bank (0−150.00–0 + 007.275) to an elevation of 2574.5 m, with a maximum depth of 80 m. From 0 + 007.275 to 0 + 150.00, a concrete cutoff wall (1.0 m) was built into the bedrock for 1.0–2.0 m and curtain grouting was conducted into the weakly weathered rock. The maximum height of the cutoff wall in this segment is 53 m. The third segment starts from 0 + 150.00 until 0 + 308.00, and has a suspended cutoff wall (1.2 m), with its bottom end penetrating the second layer (*Q*<sup>3</sup> 1 ) for at least 5 m. The height of the cutoff wall in this section ranges from 25 m to 74 m, and curtain grouting was not conducted. From 0 + 308.00 to 0 + 414.00, two layers of concrete cutoff wall were constructed separately. The lower cutoff wall (1.0 m) was cast within the gallery, while the upper wall was constructed from the slope surface. Curtain grouting was conducted into the second layer (*Q*<sup>3</sup> 1 ) for at least 5 m. The fourth segment (0 + 414.00–0 + 610.00) uses a similar combination of two layers of cutoff wall (1.0 m) and a curtain grouting. The lower cutoff wall was cast to an elevation of 2500 m in the gallery, beneath which a curtain grouting embedding the second layer for at least 5 m was used to cut off the seepage water. The maximum depth of the curtain grouting in this fifth segment is about 120 m. Reinforced concrete was used for the top of the cutoff wall at an elevation of 2639.50–2654.50.0 m.

A concrete cutoff wall was constructed, with the bottom inserted into the bedrock within shallow bank slopes. At the deepest locations in the center of canyon, concrete was poured from an elevation of 2803 m to an elevation of 2888 m at an ascending speed of 2.0–7.5 m/h, forming an 85-m high-suspended concrete cutoff wall (1.0 m). Four rows of curtain grouting were constructed to extend the impermeable system into the bedrock, including a row of curtain grouting upstream of the cutoff wall and two rows downstream. The middle curtain grouting was performed through the pipes preset in the cutoff wall. The main (inner) curtain grouting penetrates the bedrock for 10 m, and the outer three rows for at least 5 m. The permeability

The Aertash CFRD, currently under construction, is the highest dam of its type filled upon thick overburden layers. The alluvial foundation materials can be broadly divided into two

weakly cemented gravel materials. The total thickness of the overburden layers reaches 94 m, as shown in **Figure 6**. The basic properties of both layers are given in **Table 3**. In general, both gravel layers are in medium dense states and have relatively high strength and deformation moduli. The permeability, however, is also very high and the discontinuous grading makes

Reinforced concrete face slabs are used to retain the reservoir water and a deep concrete wall (1.2 m) penetrating the rock foundation is designed to cut off the underground seepage. The thickness (*t*) of the concrete face is *t* = 0.4 + 0.0035*H*, where *H* is the depth measured from the top of the face slabs. The concrete face slabs are connected to the concrete cutoff wall by a toe plinth and two horizontal linking slabs. The maximum height of the cutoff wall is 90 m, with the top 10 m reinforced by steel rebar. Curtain grouting is conducted under the cutoff wall into the bedrock to a level where *q* < 5 Lu. The depth of curtain grouting ranges from

) mainly consists of gravel materials inlayed by boulders, the

Practices in Constructing High Rockfill Dams on Thick Overburden Layers

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11

) is constituted mainly by

restriction on the curtain grouting is *q* < 5 Lu or *k* < 10−4 cm/s.

**Figure 5.** The overburden and seepage control barrier in the Xiabandi ACRD.

thickness of which ranges from 4.7 to 17.0 m. The lower layer (*alQ*<sup>2</sup>

**3.3. Concrete faced rockfill dams**

layers. The upper layer (*alQ*<sup>4</sup>

them vulnerable to seepage failure.

17 to 69 m.

*3.3.1. The Aertash CFRD*

#### *3.2.2. The Xiabandi ACRD*

The Xiabandi ACRD was constructed mainly with gravel materials collected from the riverbed. The thickness of the foundation overburden reaches 148 m, which is almost twice the dam height (78 m). The distribution of the deposited layers is shown in **Figure 5**, where three main influential layers can be seen. The lowest layer (*fglQ*<sup>3</sup> 1 ) mainly contains glacial gravel particles 2–8 cm in diameter. The thickest layer (*glQ*<sup>3</sup> ) mainly consists of coarser grains such as boulders and rubble, and it has local bridged structures distributed widely throughout and has a very complicated lithology. Enclosed within the *glQ*<sup>3</sup> layer is an almond thick sand lens (*fglQ*<sup>3</sup> 2 ) which mainly consists of medium and fine sand, silty loam and silty sand. No high-quality clayey soils are found within 60 km of the dam site, and cement, steel and other necessary construction materials would also have to be imported from places even far away (320 km). The transportation condition to the dam site is rather severe at the time of designing. Traffic interruption is often caused by heavy snows in winter while in summer the flood originated from melting ice and snow often results in debris flow accidents. Because of these natural conditions, using too much steel and cement should be avoided. The dam site, on the other hand, is rich in good aggregate for asphalt concrete. Therefore, an asphalt core is used as the impervious system of the dam.

Practices in Constructing High Rockfill Dams on Thick Overburden Layers http://dx.doi.org/10.5772/intechopen.78547 11

**Figure 5.** The overburden and seepage control barrier in the Xiabandi ACRD.

A concrete cutoff wall was constructed, with the bottom inserted into the bedrock within shallow bank slopes. At the deepest locations in the center of canyon, concrete was poured from an elevation of 2803 m to an elevation of 2888 m at an ascending speed of 2.0–7.5 m/h, forming an 85-m high-suspended concrete cutoff wall (1.0 m). Four rows of curtain grouting were constructed to extend the impermeable system into the bedrock, including a row of curtain grouting upstream of the cutoff wall and two rows downstream. The middle curtain grouting was performed through the pipes preset in the cutoff wall. The main (inner) curtain grouting penetrates the bedrock for 10 m, and the outer three rows for at least 5 m. The permeability restriction on the curtain grouting is *q* < 5 Lu or *k* < 10−4 cm/s.

#### **3.3. Concrete faced rockfill dams**

#### *3.3.1. The Aertash CFRD*

adaptability to uneven deformation. The Yele ACRD sits on a thick overburden as shown in **Figure 4**, with an extremely thick overburden at the right abutment. There are five main

impermeable barrier in the foundation, the permeability coefficient of which is less than 2.2 × 10−5 cm/s and the allowable hydraulic gradient reaches 10.4. These features were fully

The seepage control measures for this dam are divided, from the left bank to the right, into a number of different segments as described below. Curtain grouting was conducted within the gallery in the left river bank (0−150.00–0 + 007.275) to an elevation of 2574.5 m, with a maximum depth of 80 m. From 0 + 007.275 to 0 + 150.00, a concrete cutoff wall (1.0 m) was built into the bedrock for 1.0–2.0 m and curtain grouting was conducted into the weakly weathered rock. The maximum height of the cutoff wall in this segment is 53 m. The third segment starts from 0 + 150.00 until 0 + 308.00, and has a suspended cutoff wall (1.2 m), with its bottom end penetrat-

25 m to 74 m, and curtain grouting was not conducted. From 0 + 308.00 to 0 + 414.00, two layers of concrete cutoff wall were constructed separately. The lower cutoff wall (1.0 m) was cast within the gallery, while the upper wall was constructed from the slope surface. Curtain grouting was

uses a similar combination of two layers of cutoff wall (1.0 m) and a curtain grouting. The lower cutoff wall was cast to an elevation of 2500 m in the gallery, beneath which a curtain grouting embedding the second layer for at least 5 m was used to cut off the seepage water. The maximum depth of the curtain grouting in this fifth segment is about 120 m. Reinforced concrete was

The Xiabandi ACRD was constructed mainly with gravel materials collected from the riverbed. The thickness of the foundation overburden reaches 148 m, which is almost twice the dam height (78 m). The distribution of the deposited layers is shown in **Figure 5**, where three

as boulders and rubble, and it has local bridged structures distributed widely throughout

high-quality clayey soils are found within 60 km of the dam site, and cement, steel and other necessary construction materials would also have to be imported from places even far away (320 km). The transportation condition to the dam site is rather severe at the time of designing. Traffic interruption is often caused by heavy snows in winter while in summer the flood originated from melting ice and snow often results in debris flow accidents. Because of these natural conditions, using too much steel and cement should be avoided. The dam site, on the other hand, is rich in good aggregate for asphalt concrete. Therefore, an asphalt core is used

) which mainly consists of medium and fine sand, silty loam and silty sand. No

1

used for the top of the cutoff wall at an elevation of 2639.50–2654.50.0 m.

main influential layers can be seen. The lowest layer (*fglQ*<sup>3</sup>

and has a very complicated lithology. Enclosed within the *glQ*<sup>3</sup>

particles 2–8 cm in diameter. The thickest layer (*glQ*<sup>3</sup>

2−2) are mainly composed of weakly cemented gravel materials, while

) for at least 5 m. The height of the cutoff wall in this section ranges from

) for at least 5 m. The fourth segment (0 + 414.00–0 + 610.00)

1

) mainly contains glacial gravel

layer is an almond thick sand

) mainly consists of coarser grains such

) is composed of a mixture of gravel and hard clay. The relatively high

1 & *Q*<sup>2</sup> 2 ), third

) forms a relatively

1

layers under the dam, as divided by the solid curves in **Figure 4**. The first (*Q*<sup>2</sup>

2−3) is mainly composed of silty loam. The second layer (*Q*<sup>3</sup>

(*Q*<sup>3</sup>

10 Dam Engineering

2−1), fourth layers (*Q*<sup>3</sup>

1

used in designing the foundation impervious facility.

1

the second layer (*Q*<sup>3</sup>

ing the second layer (*Q*<sup>3</sup>

*3.2.2. The Xiabandi ACRD*

lens (*fglQ*<sup>3</sup>

2

as the impervious system of the dam.

conducted into the second layer (*Q*<sup>3</sup>

fifth layer (*Q*<sup>3</sup>

The Aertash CFRD, currently under construction, is the highest dam of its type filled upon thick overburden layers. The alluvial foundation materials can be broadly divided into two layers. The upper layer (*alQ*<sup>4</sup> ) mainly consists of gravel materials inlayed by boulders, the thickness of which ranges from 4.7 to 17.0 m. The lower layer (*alQ*<sup>2</sup> ) is constituted mainly by weakly cemented gravel materials. The total thickness of the overburden layers reaches 94 m, as shown in **Figure 6**. The basic properties of both layers are given in **Table 3**. In general, both gravel layers are in medium dense states and have relatively high strength and deformation moduli. The permeability, however, is also very high and the discontinuous grading makes them vulnerable to seepage failure.

Reinforced concrete face slabs are used to retain the reservoir water and a deep concrete wall (1.2 m) penetrating the rock foundation is designed to cut off the underground seepage. The thickness (*t*) of the concrete face is *t* = 0.4 + 0.0035*H*, where *H* is the depth measured from the top of the face slabs. The concrete face slabs are connected to the concrete cutoff wall by a toe plinth and two horizontal linking slabs. The maximum height of the cutoff wall is 90 m, with the top 10 m reinforced by steel rebar. Curtain grouting is conducted under the cutoff wall into the bedrock to a level where *q* < 5 Lu. The depth of curtain grouting ranges from 17 to 69 m.

**Figure 6.** The maximum cross section of the Aertash CFRD.


The dam uses upstream concrete face slabs as the seepage barrier, the thickness of which is determined by *t* = 0.3 + 0.003*H*. The toe plinths on the left and right bank slopes sit on bedrock, with both consolidation and curtain grouting performed underneath. The toe plinth built on the riverbed is located directly on the gravel layer, removing only the surficial loose deposits (1–2 m). Dynamic compaction was, however, performed to enhance the relative density and modulus of the materials beneath the toe plinth. A concrete wall (1.2 m) inserting the bedrock was constructed to cut off the foundation seepage. The cutoff wall was also connected by two horizontal linking slabs and the toe plinth to the upstream concrete face slabs, forming a closed impervious system. Curtain grouting was also performed under the cutoff wall into the

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13

There are other types of rockfill dams and sluices built on overburden layers. Reviewed above are three main kinds of rockfill dams used in water conservancy and hydropower engineering. All the dams in operation reviewed above function well without abnormal performance and major accidents. It could be remarked, in a general sense, that constructing high rockfill dams upon thick overburden layers is technically feasible. Using one or two vertical cutoff wall(s) embedding into the bedrock layer is an effective measure to control the underground seepage. In the case that the underlying overburden layers are extremely thick, a suspended cutoff wall extended by several rows of curtain grouting seems to be a feasible and effective choice.

A reliable connection between the seepage control components within a rockfill dam and its overburden foundation is a prerequisite for a successful impervious system. Connection zones are weak places that require special design considerations. In this section, connection

techniques used in different types of rockfill dams are briefly introduced.

bedrock until the designed level was achieved.

**Figure 7.** The maximum cross section of the Chahanwusu CFRD.

**3.4. General remarks**

**4. Connection techniques**

**Table 3.** Basic properties of the overburden layers in Aertash CFRD.

For concrete faced rockfill dams, it is possible to construct the dam first and then continue with the construction of the cutoff wall, or vice versa. Finite element analyses can be used to optimize the construction sequences. In the current case, the concrete cutoff wall is planned to be built after the dam is filled to a certain elevation. The linking slabs will be cast before reservoir impounding. Connecting the top of the concrete cutoff wall to the linking slabs will also be finalized before impounding.

#### *3.3.2. The Chahanwusu CFRD*

The Chahanwusu CFRD is another high dam (110 m) built mainly with gravel materials, as shown in **Figure 7**. The dam sits on sand and gravel overburden layers with a maximum thickness of about 47 m. Three layers can be observed in **Figure 7**: the upper sand and gravel layer with an average thickness of 19.2 m; the medium-coarse sand layer with an average thickness of 5.9 m; and the lower sand and gravel layer with an average thickness of 11.2 m. Both of the sand and gravel layers have similar engineering properties. The average relative density is 0.85 and the average coefficient of permeability is 6.68 × 10−2 cm/s. The middle sand layer has an average relative density of 0.92 and a permeability coefficient of 4.27 × 10−2 cm/s. Therefore, all foundation layers are in relatively dense states. The dam is located within a region of high earthquake intensity, with a design horizontal acceleration of 2.31 m/s<sup>2</sup> . However, liquefaction within the medium-coarse sand layer is considered impossible.

**Figure 7.** The maximum cross section of the Chahanwusu CFRD.

The dam uses upstream concrete face slabs as the seepage barrier, the thickness of which is determined by *t* = 0.3 + 0.003*H*. The toe plinths on the left and right bank slopes sit on bedrock, with both consolidation and curtain grouting performed underneath. The toe plinth built on the riverbed is located directly on the gravel layer, removing only the surficial loose deposits (1–2 m). Dynamic compaction was, however, performed to enhance the relative density and modulus of the materials beneath the toe plinth. A concrete wall (1.2 m) inserting the bedrock was constructed to cut off the foundation seepage. The cutoff wall was also connected by two horizontal linking slabs and the toe plinth to the upstream concrete face slabs, forming a closed impervious system. Curtain grouting was also performed under the cutoff wall into the bedrock until the designed level was achieved.

#### **3.4. General remarks**

For concrete faced rockfill dams, it is possible to construct the dam first and then continue with the construction of the cutoff wall, or vice versa. Finite element analyses can be used to optimize the construction sequences. In the current case, the concrete cutoff wall is planned to be built after the dam is filled to a certain elevation. The linking slabs will be cast before reservoir impounding. Connecting the top of the concrete cutoff wall to the linking slabs will

*alQ*<sup>4</sup> 0.80–0.85 2.23–2.23 40–50 0.60–0.70 37.0–38.0 0 0.29 0.10–0.15 *alQ*<sup>2</sup> 0.83–0.85 2.18–2.20 45–55 0.65–0.80 37.5–38.5 0 5.00 0.12–0.15

**Shear strength Permeability**

 **(MPa)** *R* **(MPa)** *ϕ* **(°)** *c* **(MPa)** *k* **(cm/s)** *J***<sup>c</sup>**

The Chahanwusu CFRD is another high dam (110 m) built mainly with gravel materials, as shown in **Figure 7**. The dam sits on sand and gravel overburden layers with a maximum thickness of about 47 m. Three layers can be observed in **Figure 7**: the upper sand and gravel layer with an average thickness of 19.2 m; the medium-coarse sand layer with an average thickness of 5.9 m; and the lower sand and gravel layer with an average thickness of 11.2 m. Both of the sand and gravel layers have similar engineering properties. The average relative density is 0.85 and the average coefficient of permeability is 6.68 × 10−2 cm/s. The middle sand layer has an average relative density of 0.92 and a permeability coefficient of 4.27 × 10−2 cm/s. Therefore, all foundation layers are in relatively dense states. The dam is located within a region of high

. However, liquefac-

earthquake intensity, with a design horizontal acceleration of 2.31 m/s<sup>2</sup>

tion within the medium-coarse sand layer is considered impossible.

also be finalized before impounding.

**Layer Density Modulus and bearing** 

**Figure 6.** The maximum cross section of the Aertash CFRD.

*D***<sup>r</sup>** *ρ***d (g/cm3**

Note: *D*<sup>r</sup> = relative density.

12 Dam Engineering

**capacity**

**)** *E***<sup>0</sup>**

**Table 3.** Basic properties of the overburden layers in Aertash CFRD.

*3.3.2. The Chahanwusu CFRD*

There are other types of rockfill dams and sluices built on overburden layers. Reviewed above are three main kinds of rockfill dams used in water conservancy and hydropower engineering. All the dams in operation reviewed above function well without abnormal performance and major accidents. It could be remarked, in a general sense, that constructing high rockfill dams upon thick overburden layers is technically feasible. Using one or two vertical cutoff wall(s) embedding into the bedrock layer is an effective measure to control the underground seepage. In the case that the underlying overburden layers are extremely thick, a suspended cutoff wall extended by several rows of curtain grouting seems to be a feasible and effective choice.
