Drainage Layers

13.3.6.1 Drainage Layers in the Pavement

The sub-surface drainage system often includes a (permeable) drainage layer in the pavement. Its function is to quickly remove water entering the pavement layers, either through infiltration to the groundwater or to a sub-surface drainage system, before any damage to the road can be initiated. It is common practice to include drainage layers where the groundwater level is high compared to the location of the road, where the subgrade soil has low permeability and on high class roads (Summary of replies to the WATMOVE questionnaire, www. watmove. org).

There are situations where a drainage layer is not considered necessary. Responses to the WATMOVE-questionnaire show that a majority of countries assess the necessity of a drainage layer before deciding to include one. If the groundwa­ter level is low compared to the location of the road, the subgrade soil has a high permeability or if it is a low-class road, a drainage layer may not be included.

There are different national traditions for which layer in the pavement works as the drainage layer, when included. Answers to the questionnaire show that some countries only use one layer as a drainage layer, i. e. the sub-base, whereas other countries mention (e. g.) three different layers. Dependent on the available material and the specific construction, any one or more of these is used as a drainage layer. The questionnaire responses of pavement engineers in many European countries are shown in Fig. 13.7.

An advantage of having the drainage layer just above the subgrade is that it can also act as a capillary break which is highly desirable in cold climate areas to prevent frost-generated water movements from the subgrade into the pavement (see Section 13.3.6). If the drainage layer is placed on the top of subgrade, the

Fig. 13.7 Layer used as primary drainage layer in European pavements. (% of countries indicating that they use this layer. It was possible to indicate more than one layer)
permeability of the granular base and sub-base must be greater than the infiltration rate,[29] so that water can flow freely to the drainage layer.

Dawson (1985) reported that many UK sub-bases (typically containing as much as 10% by mass less than 75 ^m) act more like sponges, absorbing water, than as permeable drainage materials. Jones & Jones (1989) report measurements of coeffi­cient of horizontal permeability in the range 1-60 x 10-3 m/s for typical aggregate sub-bases and Roy & Sayer (1989) report in-situ measurements (by injection) of 2-110 x 10-3 m/s for similar materials. In other granular base course injection tests, Floss & Berner (1989) quote permeability values between 10-6 and 10-4 m/s for sandy gravel aggregates in-situ. Biczysko (1985) tested broadly graded aggregates in the laboratory for their horizontal permeability, obtaining reliable permeability coefficients in the range 2-50 x 10-4 m/s. However, he found that the aggregates with finest gradings (10% of material finer than 75 ^m) were difficult or impossible to saturate — yielding an apparent permeability of 3 x 10-6 m/s — indicating that the lowest values are likely to over-estimate the in-situ behaviour and that substantial

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Bearing capacity measure
ments (e. g. by FWD)

through flow is unlikely to take place. The laboratory values reported by Jones & Jones (1989) and by Biczysko (1985) were both obtained by a specific perme — ameter designed for highway aggregate testing (Department of Transport, 1990) as described Chapter 3, Section 3.1 (Fig. 3.6).

Therefore, it is not sufficient to have a granular base or sub-base layer and assume it will drain. Instead, when the base or sub-base layer is also to act as a drainage layer as in Fig. 13.8, its material must satisfy both the requirements of strength and the permeability requirement for a drainage layer. When the layer is on top of the subgrade, the material may also have to satisfy requirements to act as a capillary break and filter, so that the fine particles of the subgrade soil do not migrate into the drainage layer (see Section 13.3.9).

An overview, resulting from the questionnaire mentioned above, of the require­ments to ensure a successful drainage layer is given in Table 13.1.

Care must be taken if a permeability coefficient and a certain grading envelope are both specified. It would not be difficult to specify one and thereby prevent the other from being achievable. While the relationship between grading and perme­ability can not be precisely defined (see Chapter 2, Section 2.5.1), controlling one will certainly have a large effect on the other.

The performance of the drainage layer does not necessarily stay unchanged with the passage of time. Some countries report that they have noted that the layer might become more or less clogged with time. The fines content might increase caused by degradation of aggregates and/or migration of fines from other layers. This causes decreased permeability and increased frost susceptibility.