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2.2 Empirical Investigations

2.2.7 Cohesive Base Soils

The majority of dam cores are made of cohesive materials such as sandy or silty clays, which is impermeable enough to prevent the loss of water from the reservoir. However, most of design criteria described so far have been developed using non-cohesive base materials. Many designers applied design principles developed for non-cohesive base materials to cohesive bases, assuming that they would be conservative, since the cohesion

of the base soil reduces erosion rates. However, based on observations of dam failures, Vaughan (2000) suggested that the rules for non-cohesive soils were invalid for two reasons. Firstly, the cohesive forces in the clay did not prevent filter failure; rather they allowed cracks to stay open and stable while their walls were eroded by small flow.

Secondly, segregation invalidated reliance on self-filtering to prevent loss of material. Self-filtering is explicitly assumed in the design criteria for non-cohesive soils. Concentrated leaks occur in most embankment dams of all types and sizes without being observed (Sherard and Dunnigan 1985). After a crack develops, erosion of the crack by high velocity flow may occur as shown in Figure 2.4. Eroded particles may be transported to the filter interface. If the filter is fine enough, these particles are captured at the filter interface and form a mud skin over the filter. This low permeability skin or filter cakereduces the flow rate through the crack and prevents further erosion. If the filter is too coarse, eroded particles will pass through the filter and the crack may enlarge as erosion continues.

Figure 2.4Concentrated leak through a crack in a cohesive core (after Sherard et al. 1984b).

Vaughan and Soares (1982) found that cracks up to a certain size and shape in a cohesive material remained stable even when they were flooded. The study suggested that at low flow velocities, slow erosion of these cracks may be accompanied by segregation of the eroded debris within the crack, this segregation may result in only fine particles reaching the filter. In the absence of coarse base soil particles, a self-filtering layer cannot form and the finer particles are continually lost and the crack enlarges, leading to possible piping failure. Based on this, Vaughan and Soares (1982) defined a “perfect filter” to protect a cracked, cohesive base material. The perfect filter will retain the smallest particles that can arise during erosion, even if they arrive at the filter interface after complete segregation.

These smallest particles are the clay flocs that form when the base material is dispersed in the reservoir water. The perfect filter concept is a conservative approach, intended to provide a filter that cannot allow particles to erode. Several studies including Ripley (1982) criticized the design of some filters that would be so fine they may possess some cohesion.

This is unacceptable, as cracks can propagate through the filter. Sherard (1982) in his discussion of the perfect filter concept mentioned that:

• Laboratory tests revealed that filters of clean sand (or gravel sand) with D15 size of 0.5mm conservatively seal concentrated leaks in fine-grained cores.

• Silt-sized particles (30-70 microns) comprise a substantial portion of all fine-grained clayey soils. As these particles are available to seal concentrated leaks, it is not necessary to provide a “perfect filter” to catch clay flocs of 10-20 microns size.

It is now accepted that the “perfect filter” is conservative. There is sufficient evidence suggesting that well constructed fine sand filters can initiate self-filtration and adequately protect most of dam cores (Sherard et al. 1984b).

The ‘critical filter concept’ suggested by Sherard and Dunnigan (1985), adopting the no-erosion filter (NEF) test, was proposed as an alternative approach to the design of filters to seal cracks in cohesive materials. It is a conservative test of filters to seal concentrated leaks through cohesive soils. The empirical design criteria of Sherard and Dunnigan (1985), presented in Table 2.1, are based on the NEF test. In most instances, these design criteria provide a coarser filter than that required by the perfect filter concept. Vaughan (2000) criticized the use of the NEF test to model the cohesive soils, suggesting that the test fails to reproduce the features of crack behaviour in two ways. Firstly, it is conducted in a rigid cylinder. If pieces of clay are washed onto the filter, the filter face can seal. Being rigidly constrained, the filter and the walls of the crack can resist the hydraulic pressure applied, and the filter is defined as a success. In a dam core, a crack can be sustained and re-opened by hydraulic pressure, if this pressure is larger than the total stress. The mechanism in the test is strain controlled, whereas the mechanism in the field is stress controlled. Secondly, the segregation of eroded debris and migration of only fine particles is specifically avoided in the NEF test. Sherard and Dunnigan (1985) conducted several NEF tests at very high hydraulic gradients in vertical apparatus. These tests found the suitability of the filter to retain particles eroded by high velocities, but did not examine the possible segregation at

low velocities. Thus, the NEF test cannot predict field behaviour if segregation of debris is possible.

In order to study flow at low velocities, Maranha das Neves (1989) developed a ‘crack erosion test’, where water flows at varying velocities over the flat surface of a soil sample and then through a filter, to examine the behaviour of the simulated crack and filter. The following observations were made.

• There was no visible segregation during transport of the eroded debris, i.e. all the eroded material is transported and there is no preferential movement of fines;

• When erosion occurs, low flow velocities (2cm/s) are sufficient to transport sand-sized particles to the filter surface, thus enabling self-filtration to occur at the filter interface;

• Before self-filtering is established, even the most conservative filters were not able to retain fine particles.

Maranha das Neves (1989) concluded that segregation at low flow velocities is not a problem in filter design; the NEF test is suitable for determining successful filters for cohesive soils and the Sherard and Dunnigan (1985) criteria can be adopted for the design of filters for most cohesive base soils.

The design criteria of Sherard and Dunnigan (1985) have been shown to have limitations, particularly when applied to broadly-graded and gap-graded cohesive materials. Khor and Woo (1989) conducted a number of NEF tests on sandy clays. The study assumed that

particles coarser than fine sand cannot be relied upon to help seal the filter, and that it is necessary to provide a protective filter that will retain the fines but not necessarily the clay floc-sized particles.

Indraratna et al. (1996) studied the filter requirements for a cohesive, lateritic soil, a typical residual soil of Thailand and other parts of South East Asia. This material lacks much of the silt-sized particle fraction usually present in other natural soils. The experiments involved forcing slurry of base soil into the filter under high pressure, to examine the retention of clay flocs. Based on these observations, Indraratna et al. (1996) revealed that the following particle size ratios are appropriate for this material.

Ford85= 50 to 60µm: D15/d85<5 to 5.5 (2.14) Ford85= 60 to 80µm: D15/d85<4 to 5 (2.15) The retention ratios D15/d85(Equations 2.14 and 2.15) for filtration of a lateritic soil are considerably lower than those proposed by Sherard and Dunnigan (1985), for fine soils (i.e.

D15/d85≤9). This is most likely because the lateritic soil is low plasticity soil and behaves like a cohensionless soil in relation to filtration. These lower safe ratios suggested that the current filter criteria for cohesive fine soils may not be universally applicable and testing of proposed combinations is usually necessary. The current practice of using concrete sand as a filter material to protect fine-grained soils reflects general indistinctness in determining correct filter materials to protect cohesive soils, for example, the Sherard and Dunnigan (1985) requirement,D15<0.7mm, for sandy silts and clays.

More recently, statistical analyses carried out by Foster and Fell (2001) found that the criterion D15/d85”9 corresponds to a 50% chance of erosion whereas for a 10% chance, the ratio is 4.60. Unlike the observations of Sherard et al. (1984b), where the study found that the base soil-filter combinations with retention ratios much higher than 9 were successful, Foster and Fell (2001) and Sherard and Dunnigan (1989) found that the failure boundary varies in between 6 to 14. However, both the studies recommended D15/d85”9 as filter criterion as a mean value of two limits. The former also recommended that for highly dispersive base soils with more than 85% fines,D15/d85”6.4 be used whereas for base soils with fines content in between 35 to 85%, D15 ”0.5mm is more appropriate. The study ignored the effect of filter gradation and used only densely compacted filters.

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