Drainage Design Assignment

The main access to a large rural farming community in a developing country is provided by an engineered earth road. However this is often subjected to flooding and impassability which prevents access during several months of the year, making the community vulnerable and isolated. Costs of significant agricultural production in the area are also correspondingly affected. As a result the government is set to upgrade the road to an all season road.

Your organisation has been invited to undertake studies to determine a suitable drainage scheme for the new road. You are also required to provide an indication of the new road's geometrical design and some calculations associated with a suitable pavement structural design.

Available Data

The information provided to you is of low quality, and so will need to state clearly any assumptions you make.

1. The existing road is an engineered earth road

2. The new all season road is expected to carry 1.5 million standard axles over its design life (10 years).

3. Possible horizontal alignments are shown on logographical maps (Figure 1 and Figure 2). Select one of these or provide justification for an alternative.

4. The horizontal curve is 1500 metres in radius with a 3% crossfall

5. Topographic maps (of low quality) shown in Figure 1 and Figure 2

6. The nature of the surface soils is silty clay with light natural vegetation covering. Any imported material will be sandy clay. Subgrade CBR's are 12% and 18% respectively un-soaked and 4% and 8% soaked.

7. The road surface is to receive a thin bituminous surface overlying a base and sub-base, characteristics of which are to be recommended by this study.

8. The sealed pavement width is 7.4 metres plus two 1.8 metre unsealed shoulders which comprise of crushed stone.

9. Run off and stream flow is as follows:

• Run off from the verge - calculate using the shown catchment areas
• Run off from the road - calculate using 'i' of 75 mm/hr for 1 in 5 year and 95 mm/hr for 1 in 10.
• Stream flow at chain age 240 metres - 8 m3/sec (1 in 10 yr flow)
• Stream flow at chain age 390 metres - 23 m3/sec (1 in 10 yr flow)
• 5 m3/sec dry season flow.

10. When determining the coefficients of run off for the various surfaces you need to bear in mind that although future land use for the catchment area are undecided, housing development in the vicinity of the new alignment should be considered.

Design Steps

i) Choose a vertical alignment for your horizontal alignment so that it contains some cutting and some fill sections tying into the existing grades at the ends of the project (given in some cases). A flood crossing is to be considered for the crossing at 390 metres.

ii) Provide a pavement design solution. You may use an analytical design approach or utilize (with appropriate justification) existing empirically founded catalogue methods.

iii) Establish typical cross sections of the road for cuts, fills and culvert/flood crossings. Indicate cross falls, side slopes, back slopes and any necessary drain facilities. The position and direction of the longitudinal drainage can be shown schematically on a plan for run off determination. Off road catchments for local drainage are to be calculated. The initial designs can be changed or modified later after more detailed design. Assume that the slopes will be resistant to slip but not necessarily surface erosion.

iv) Design hydraulic drainage treatments which have the capacity to cater for each of these discharges. Apart from a piped system for the village and the culvert or flood crossing, you have options in the type of drainage facility that can be provided, e.g. open or closed side drains, kerbed or un-kerbed paved carriageway.

v) Velocity of flow and rainfall impact may also be an important design parameter. Make appropriate checks. Protection works may be needed, e.g. at the outlet to the culvert.