A Drainage System for Road Construction on Flat Terrain

Abstract

Structural integrity of flexible road pavements is guaranteed if an effective road drain is provided during construction. On sloppy terrain open concrete drain has sufficient conveyance to get rid of runoff from the road system. The ability decreases as the terrain approaches near flat condition On flat terrain water often ponds on the roadway after a rainfall and conventional road drainage system is hardly effective in discharging the runoff. To address this a trenchless drainage system becomes a suitable option. This involves an engineered open trench backfilled with granular materials. The method is effective and cheaper.

1. Introduction

Road is an indispensible feature of any developing or developed society. It provides a means of communication, within and in-between communities and cities. It can be a single lane road or dual carriage way which aligns or cuts across with one another. During its construction, provisions are made for an effective road drainage system that will sustain the structural integrity of the road pavement. This measure will guarantee the usability and sustainability of the road for an extended period of time. The drain, located on both or either side of the road, provides a means by which runoff from the road surface and adjacent facilities is discharged harmlessly into an engineered or natural outlet.

The effectiveness of the drainage system is a function of the drain invert slope, its size or capacity and the structural disposition of the lining [1]. For a well designed drain the steeper the slope of the invert the more efficient is the discharge up to a maximum critical slope beyond which the flow becomes supercritical and abrasive. However, as the drain invert decreases to a minimum critical slope the discharge becomes subcritical, less efficient and less self cleansing. As this slope approaches a near flat regime, the potential hydraulic head approaches zero. At this stage the drain conveyance becomes paralyzed. Consequently the drainage system will functionally collapse and becomes grossly inefficient in conveying the runoff, thereby resulting to sustained flooding of the area if rainfall persists. In the same vein, the drain may be silted up, vegetation may spring up to cover the surface of the drain, and the stagnant water becomes a breeding ground for mosquitoes and other water borne disease vectors [2]. Apparently malaria infestations and such health related hazards grow exponentially. Some functionally collapsed open drainage systems on a flat terrain in New Owerri district, Imo state Nigeria are shown in Figures 1–4 below, as an illustration.

Figure 1.

A waterlogged open concrete drain overgrown with weeds in new Owerri Nigeria.

Figure 2.

A waterlogged open concrete drain being used as a waste dump site in new Owerri Nigeria.

Figure 3.

A waterlogged open concrete drain serving as mosquito breeding ground in new Owerri Nigeria.

Figure 4.

A stagnant pool of water in an open concrete drain in new Owerri Nigeria.

However, the discussion in this work will principally be directed to an engineered drainage system - a trenchless drain, that can permit gradual disposal of road wash/runoff on a flat terrain, after a rainfall.

2. Flat terrain

The geomorphology of an area is defined by its landforms. The landform could be made up of hills, valleys, lowlands and/or flood plains. The aerial disposition of these land features manifest as the topography of the area. That means, the topography or, in other words, the terrain of an area can be undulating, hilly, steep, gentle or flat.It is the topography of a place that determines its natural drainage pattern and this may guide the networking of man made or engineered drainage systems. However, man can alter, to some extent, the existing natural landforms to fit into his developmental objectives.

A flat terrain is a land form with slopes not steeper than 2% [2, 3]. This type of terrain can occur within a region of greater relief such as hills and mountains or in an undulating or alluvial plain. It can also occur on top of a table land forming an escarpment.

The main characteristic of a flat terrain is water logging after a rainfall. In this regard surface runoff remains within the surrounding since there is no sufficient hydraulic head, amidst other obstructions, to drive the flow along. In which case natural drainage is restricted and flooding becomes overwhelming as long as there is a continuous rainfall. This is apparent during the rainy season.

Human settlement is often attracted to flat terrain, probably because the land would not be subjected to pronounced erosion of the soils, agricultural practice is sustainable and existential construction activities are easier to manage. Particularly, road construction on a flat terrain requires minimal earth cutting and earth embankment.

3. Road construction

Road construction may involve rigid pavement or flexible pavement. Rigid pavement is made up of portland cement concrete base. It could be reinforced with steel bar or could be an ordinary plain concrete slab.Because of its rigidity and high modulus of elasticity, rigid pavement tends to distribute the applied load over a relatively wide area of soil and the major portion of the structural capacity is provided by the slab. Hence the major consideration in the design of rigid pavement is the strength of the concrete. Minor variations in the sub grade and/or base strength have little influence on the structural capacity of the pavement. Provision of drainage assets is therefore not a priority. In flexible pavement the strength is brought about by building up relatively thick layers of sub base, base course and surface coating over a sub grade. These layers are composed of natural aggregates. The manners of load distribution in flexible pavement make it sensitive to moisture variation. Therefore the provision of drainage asset is a priority in the design and construction of flexible pavement.

In flexible pavement design four main features are considered [2]: the pavement structure, pavement materials, cross sectional profile and size of the drain invert.

3.1 Pavement structure

A flexible road pavement structure may comprise the subgrade, subbase, base course and asphalt coating in that order. The subgrade is the in-situ or borrowed earth material spread and compacted in line with the specifications, across the width and lenght of the proposed road to form its foundation. The thickness of the sub grade is semi infinite and the elevation relative to the ground level is dependent on the environmental factors. The subbase is placed over the subgrade and serves as a transition layer to the base course which is the main load spreading layer of the pavement. The uppermost layer is the surfacing or asphalt coating that has a water proofing and abrasion resistant properties.

3.2 Pavement materials

The subgrade is comprised of compacted sandy clay, clayey sand or treated mixtures of sand and silt. The subbase may be made of unprocessed sandy gravel or gravelly sand while the base course is composed of specified thickness of crushed stone or gravel stabilized with cement, lime or bitumen. Then the surfacing is comprised of bitumen and often referred to as asphalt.

3.3 Cross sectional profile

To prevent rainwater retention at the surface of the pavement the cross sectional profile of the road is seen to be cambered towards the drainage inverts on both sides of the road. This allows the road wash to flow towards the side line of the road.

3.4 Drain invert

The drain inverts of the road are positioned along the drainage lines on both sides of the road to convey runoff from the road surfaces towards a sump or discharge basin, down slope. The drainage system could be an open channel or pipe drain depending upon environmental considerations. The conveyance of the drain will be designed based upon rainfall and catchment regimes.

4. Drainage systems

The conventional drainage systems commonly adopted in road construction include open concrete drain and pipe drain. In the design of such a drain some factors which include hydraulic, structural, environmental, sociological and maintenance attributes are considered [4]. The weight attached to each of these factors is a function of engineering judgment and sustainability requirement.

4.1 Open concrete drain

Open drains can be provided on either or both sides of the road. It can be trapezoidal, rectangular or square in shape. The channel can be lined with mass concrete or reinforced concrete. For the drain to be effective the grade must be sufficient to mobilize minimum permissible velocity that will enhance self cleansing and gentle enough not to exceed the maximum permissible velocity that would initiate scouring of the lining. Open drain can easily be maintained in the face of high sediment load or rubbish load in an area. However it is unsightly and may be risky to road users in some cases. But when the invert slope is less than 2% as is the case with flat terrain, the drain can hardly function. In this situation a trenchless drainage system provides a good alternative.

4.2 Pipe drain

Pipe drain is often cylindrical or rectangular in shape, aligned and buried in the subsurface. Drainage through pipes simulates flow through conduits. Pipe drain is mainly suitable for high density and high value residential development. It provides a better appearance and safety than open drain but much more expensive to construct and more challenging to maintain. To function effectively pipe drain must mobilize permissible flow velocities within the range of minimum and maximum. However when the drain invert or the slope of the terrain is less than 2% its conveyance becomes grossly inadequate. Then the alternative and effective choice is the trenchless drain system.

5. Trenchless drainage system

The trenchless drainage system provides a solution for the evacuation of runoff from road pavement situated on flat terrain. It serves as a suitable alternative to open or pipe drains on such an environmental setting.

5.1 Concept

The concept behind trenchless drainage system is based on the application of engineered absorption field and grass cover to facilitate runoff infiltration into the subsurface [2]. This aligns with the concept of ‘French drain’ in a general term. The components of trenchless system is highlighted as follows.

Absorption field: An absorption field is an excavation or a trench, backfilled with relatively permeable material when compared with the native or in-situ soil. The excavation creates a wider surface area for the water to infiltrate and percolate in the underlying soil medium [5].

Grasses: The benefits of grasses as an aid to effective drainage of an area had been highlighted [6, 7]. The foliage intercepts raindrops that attempt to fall unto the ground to cause splash erosion. This, in consequence absorbs the terminal energy of the rain drop thereby reducing its erosivity. The foliage also provides an evapotranspiration medium for the system. The stem of the grass in conjunction with the leaves interrupt free flow of the runoff, thereby increasing the flow roughness. This provides suitable opportunity for infiltration to take place. The root system has numerous channels for water infiltration and as well acts as a transition channel for evapo-transpiration. Generally grasses bind soil particles into a matrix and provide numerous infiltration and evapo-transpiration pathways.

A combination of absorption field and grass in an engineered fashion provides a good infiltration sump for road wash. It also restricts migration of composite earth materials, such as silts and fine sand, in the runoff that could block infiltration channels.

5.2 Preliminary studies

The design of trenchless drain requires some fundamental parameters that must be studied and considered. These include rainfall intensity of the area receiving attention, the in-situ or native soil permeability/hydraulic conductivity, properties of the backfill material and the design road width [2].

Rainfall Intensity. The rainfall intensity of the area can be obtained from rainfall studies of the nearest metrological station. For optimal design the value may be determined from maximum daily rainfall data of 10 years return period. Rainfall intensity (I) can be calculated from the expression,

$$\mathrm{I}=\mathrm{Dr}./\mathrm{T}$$E1

where Dr. = depth of rainfall in cm, T = duration of rainfall in min.

In-situ Soil Properties. The permeability or hydraulic conductivity of the in-situ soil is required and this can be obtained through a number of simple field techniques. One of such techniques is the Auger Hole method [8], performed within a relatively shallow depth. A number of such a test is carried out along the length and breadth of the proposed road right- of- way. The soil permeability, K is estimated from the equation,

$$\mathrm{K}=\mathrm{C}∆H/∆T$$E2

Where C = the geometry factor of the auger hole, $∆$H = change in water level in the hole at time interval $∆T.$

Ring infiltration method can also be used in estimating K [9]. It uses the principle of falling head permeameter. The evaluation follows the following equation:

$$\mathrm{K}={\mathrm{A}}_1\mathrm{L}/{\mathrm{A}}_2\left({\mathrm{T}}_2–{\mathrm{T}}_1\right)ln\left({\mathrm{H}}_1/{\mathrm{H}}_2\right)$$E3

Where A₁ = cross sectional area of the observation tube, A₂ = cross sectional area of the soil tube, L = length of the soil tube, H₁ = head at the time T₁, H₂ = head at the time T₂.

In addition to the estimation of soil permeability, other basic properties of the in-situ soil would be studied. These include the grain size distribution, the Atterberg limits, specific gravity, specific weight; etc. They can be used for classification of the soil.

Backfill materials. The materials that could be used to backfill the dug trench must have passed through a series or cycles of abrasion and then become stabilized after the erosive components of the material may have been dissolved and removed. This ensures that the likely cementing agents and organic matters had been washed away. Sands and gravels fall into this category. They can be sourced from alluvial deposits or processed to obtain the required properties.

The backfill must be relatively pervious compared to the in-situ soil. Its porosity can be obtained from laboratory measurements conducted on some samples of the backfill. Other laboratory studies that are required may include grain size distribution, specific gravity, and specific weight. Research has shown [10], that poor graded materials with relatively high values of diameter at 30% passing and relatively high values of uniformity coefficient are preferred as backfill.

Road Width. The width of the roadway is an input parameter in trenchless drainage design. It is a reference catchment area of the drain. The size may be obtained from preliminary design specifications or working drawing for the construction. Consideration would be given to the effective width of the road for drainage purposes. This may include areas outside the limit of asphalt coating. However the actual width to be used in the sizing of the drain will, principally, be dependent on the judgment of the drainage engineer.

6. Design of trenchless drainage system

6.1 Design philosophy

The design of a trenchless drain assumes that only the road wash flows towards the sides of the road, and flow down the slope is restricted by the relatively flat nature of the terrain. That means in-flow from outside the road margins is not considered in the formulation of the enabling equation. By synthesizing inflow into the drain and outflow from the drain in the process of infiltration into the adjacent soil, the following equations were obtained [2, 11],

The volume of inflow, Q was given by

$$\mathrm{Q}={\mathrm{k}}_{1}1\left({\mathrm{W}}_{\mathrm{r}}+{\mathrm{W}}_{\mathrm{t}}\right)$$E4

While the volume of outflow, i.e. the infiltration capacity, F of the drain was given by

$$\mathrm{F}={\mathrm{k}}_{2}3{\mathrm{W}}_{\mathrm{t}}\mathrm{K}$$E5

For optimal design, Q = F, and considering a square shaped drainage system, the equation becomes

$${\mathrm{k}}_{1}1\left({\mathrm{W}}_{\mathrm{r}}+{\mathrm{W}}_{\mathrm{t}}\right)={\mathrm{k}}_{2}3{\mathrm{W}}_{\mathrm{t}}\mathrm{K}$$

OR,

$${\mathrm{W}}_{\mathrm{t}}={\mathrm{k}}_{1}1{\mathrm{W}}_{\mathrm{r}}/\left(3{\mathrm{k}}_2{\mathrm{KT}}^{\mathrm{n}}–{\mathrm{k}}_{1}1\right)$$E6

Where W_t = theoretical width of the square drain, 1 = rainfall intensity, W_r = design road width, k₁, k₂ = coefficients representing losses, generally <1, T = time factor, n = standing time of water pool, K = hydraulic conductivity of in-situ soil.

If the rate at which water flows into the drain is assumed to be the same as the rate it infiltrates into the adjacent soil (outflow from the drain), the water standing time, n will be zero i.e. n = 0, and T = 1. Hence Eq. (6) reduces to,

$${\mathrm{W}}_{\mathrm{t}}={\mathrm{k}}_{1}1{\mathrm{W}}_{\mathrm{r}}/\left(3{\mathrm{k}}_2\mathrm{K}–{\mathrm{k}}_{1}1\right)$$E7

However, the factors T, k₁, k₂ are site dependent and can be established empirically. Ideally, where there are no water losses through evaporation or evapotranspiration, k₁ = k₂ = 1

$${\mathrm{W}}_{\mathrm{t}}=1{\mathrm{W}}_{\mathrm{r}}/\left(3\mathrm{K}–1\right)$$E8

W_t is apparently the hypothetical width of the drain and is related to the effective design width, W_d by

$${\mathrm{W}}_{\mathrm{t}}={\mathrm{W}}_{\mathrm{d}}\mathrm{\eta }$$E9

Where η is effective area factor of the backfill and it is the same as its porosity [12]. By substituting Eq. (9) into Eq. (8)

$${\mathrm{W}}_{\mathrm{d}}=1{\mathrm{W}}_{\mathrm{r}}/\left(\mathrm{\eta }\left(3\mathrm{K}–1\right)\right)$$E10

6.2 Design considerations

In the design of the trenchless drainage system specific components are considered. They include the dimensions of the drain, backfill materials, grassing and gang way to user housing.

Dimensioning. The dimension of the drain is dependent upon a number of variables which include rainfall intensity, size and profile of the road, the porosity of the backfill and the hydraulic conductivity of subgrade soil [2, 10, 11]. Consequently for a square drain configuration,

$${\mathrm{W}}_{\mathrm{d}}=1{\mathrm{W}}_{\mathrm{r}}/\left(\mathrm{\eta }\left(3\mathrm{K}–1\right)\right)$$E11

Where W_d = design width/depth of the drain, cm, I = rainfall intensity, cm/sec, Wr = width of the road, η = porosity of the backfill, K = permeability/hydraulic conductivity of the in-situ soil.

For a square drain, in cross section, the width (W_d) is the same as the depth, and the infiltration surface can be taken to be 3W_d. The infiltration surface of the drain is its perimeter short of the upper width of the drain.

If a rectangular channel is to be considered the same infiltration surface determined by Eq. (10) will be used in its dimensioning. In such a case,

$$\text{2d}+\mathrm{w}={\text{3W}}_{\mathrm{d}}$$E12

Where d is the depth of the rectangular channel and w is its width.

For an example:

• If the design width of the square drain is calculated to be 1.5 m based on Eq. (10)

• The infiltration surface = 3 x 1.5 m = 4.5 m.

• For a rectangular drain, if the width, w, of the drain is 1.2 m

• Considering the same infiltration surface of 4.5 m

• The depth, d, of the rectangular drain is calculated from, 2d = 4.5–1.2 = 3.3 m OR d = 1.65 m

That means, the dimension of the rectangular drain, in cross section, will be 1.2 m x 1.65 m as against 1.5 m x 1.5 m for a square drain of the same infiltration capacity when fully mobilized.

The rainfall intensity to be used in the design shall represent a peak condition of at least 10 years return period. This creates room for an optimal design since rainfall is the subject of control in this scenario. Rainfall data from nearest, asymptotic and reliable meteorological station is appropriate.

The width of the road surface (Wr) is measured on site or taken from the construction drawing. If the road is cambered the road wash will flow unto both sides of the road from its centre line, supplying runoff to the drains on both sides. Thus, Wr that could be used in Eq. (10) will be half of the actual design width of the road to be drained.

The value of the porosity (η) of the backfill to be applied in the Eq. (10) could be obtained from laboratory analysis of representative samples using standard methods. Typical laboratory results from various soil types obtained in course of research [10, 11] are presented in Table 1.

Soil	Soil category
Parameters	Mixed sand	Fine sand	Gravelly sand	In situ soil
Water content, %	13.7	18.2	9.3	32
Void ratio	0.13	0.08	0.23	0.37
Porosity	0.12	0.07	0.19	0.27
Bulk wt, kN/m^3	19.2	18.7	17.9	19.8
Dry unit wt, kN/m^3	16.9	15.8	16.4	15.0
Sat unit wt, kN/m^3	20.4	19.5	19.9	20.7
D10, mm	0.3	0.3	0.6	0.26
D30, mm	0.45	0.52	1.2	0.4
D50, mm	0.75	0.72	2.1	0.57
D60, mm	0.82	0.8	2.6	0.73
Cu	2.43	2.67	4.3	2.81
Cc	0.82	1.13	0.9	0.84
USC Class	SP (cmg)	SP (cmf)	GP (gcm)	Clayey sand

Table 1.

Properties of various soil types obtained from laboratory studies.

The in-situ soil permeability, (K) can be obtained from the field using standardized methods [8, 9]. The measurement can be conducted on a number of locations on the right-of-way of the proposed road project. Measurements can also take place when the subgrade of the road pavement has been prepared. The choice is based on site conditions, construction philosophy and/or judgment of the drainage engineer. The in-situ soil properties could be determined to aid in the soil classification. Results obtained on a similar exercise are shown in Table 1.

6.3 Backfilling

Backfilling along the length and breadth of the trench is done with suitable materials. Such materials may be sourced from alluvial deposits and treated, if necessary, to satisfy the design requirements. Suitable materials must have been stabilized after long transportation in fluvial regime amidst dissolution and abrasion of original rock. Granite is not recommended. From research finding [10, 11], gravelly sand is most recommended, followed by fine sand and then mixed sand in that order. The results of such evaluations are graphically illustrated in Figures 5 and 6. The diameter at 30% passing (D₃₀) and Uniformity coefficient (Cu) of a suitable material must be relatively high, Table 1.

Figure 5.

Drawdown versus time curve with varying backfill materials.

Figure 6.

The regression graph for varying backfill materials.

6.4 Grassing

Grass species suitable for use are locally available in abundance and the type to be introduced can be decided by the horticulturist attached to the project. The grass is provided and nourished as a cover over the backfill. The roots of the grasses serve as the fiber reinforcement to the loose backfill and as well provide infiltration channels into the fill. This may prevent clogging of the interstices by oxides and clays suspended in the runoff. In addition, it adds to the esthetic value of the drain.

6.5 Gang way

Situations may arise where the trenchless drain must be bridged. This is mainly in residential areas in which case crossings to serve as entrances to housing units must be provided. This may require placement of precast concrete slabs over the drain and will be limited specifically on the required position.

6.6 Comprehensive outline

In special consideration of all the input variables, a hypothetical design of trenchless drainage system on a road section is illustrated in Figure 7 while the plan of the sketch drawing is shown in Figure 8. The alignments and relative locations of the features are defined in the drawing.

Figure 7.

Hypothetical design section of a trenchless drainage system on a road profile.

Figure 8.

Plan view of a hypothetical trenchless drainage system in residential district.

7. Construction method

The construction of trenchless drain in a road project simply involves trenching, backfilling and grassing.

7.1 Trenching

This requires the digging of trench along the length of the road on either side or both sides of it in accordance with the specification. Either of motorized excavator or manual labour can be used in the exercise depending upon availability, cost and size of the project. The excavated materials can be disposed as waste or reused depending upon the properties of the soil.

7.2 Backfilling

Alluvial sands that can serve as backfill are often locally available. The material could be brought to site by trucks and processed to satisfy the design specifications. It is also possible to lift material that was naturally sorted to meet the requirement. The material can then be dumped and spread in full dimension of the trench to its design level. Densification of the fill is not encouraged. The backfill serves as a transmission medium to the in-situ soil.

7.3 Grassing

The last stage in the construction exercise is the introduction of grasses. Specific species of grasses are obtained and planted over the backfill in a specified manner. To sustain its survivability recommended nurturing procedure must be followed. The grass draws in the water into the backfill.

8. Comparative advantages

The benefits of the Trenchless drain over conventional methods in road networking on flat terrain can be assessed using the following parameters.

8.1 Construction cost

Open concrete drain is constructed of a mixture of aggregates and cement at a design ratio, and sometimes reinforced with iron bars. For a similarly sized trenchless drain the only construction material is sand or gravel put in place with no mandatory skill. A comparative cost analysis for the various dimensions and composition of drains using local market prices was carried out and the results are illustrated in Figure 9. It is observed, for example, that for a given dimension, a reinforced concrete drain is about four times (4x) more costly than a trenchless drain constructed of sand aggregate, outside labour cost. Open concrete drain requires properly designed culvert at each crossing as against slab, if a trenchless drain is constructed. A culvert of known dimension is 10 times more costly than slab of the same size, [11].

Figure 9.

Cost comparison for various sizes of drains and component materials.

8.2 Maintenance cost

A concrete drain requires regular cleansing for free flow of water to be sustained. But because of high cost of labour and equipment for the evacuation of the attendant waste, the maintenance could be done ones in a year and preferably at the onset of the rainy season. Comparatively trenchless drain requires grass cutting when overgrown to maintain its environmental friendliness. The maintenance is easily done at a desired time and at a relatively very cheap operational cost.

8.3 Aesthetics

Open concrete drain presents an unsightly view especially when it is not properly maintained. Sometimes it serves as an easy waste dump site for uninformed and carefree inhabitants. Comparatively, the greenish outlook of a trenchless drain merges with nature to beautify the environment and provides a healthy and sustainable scenario.

8.4 Flooding

Concrete drain is easily submerged in course of rainfall due to obstructions and low grade terrain. The water may remain stagnant for a good length of time, providing a good breeding ground for mosquitoes. This often results to exponential increase in malaria cases. Trenchless drain does not retain flood water for a period of over 1 hour after a torrential rainfall [12]. It therefore provides minimal opportunity for mosquito breeding. On heavily flooded roadway it is often difficult for road users to identify the limit of an open concrete drain and such persons may run the risk of falling into the drain. Trenchless drain provides, apparently, a continuous surface cover with no perceived danger to road users.

8.5 Erosion

Where an open concrete or pipe drain is functional its discharge can cause erosion at the outlet of the drain or its culvert, especially where the drain or culvert is not properly terminated. Over 240 well developed gully erosion sites in South East of Nigeria were observed to have been caused by wrong termination of drains and culverts [13]. The trenchless drain can hardly yield significant surface runoff that can initiate any form of erosion down slope.

9. Limitations

• *The trenchless drain is suitable for areas flatter than 2% grade. For slopes greater than 2% and less than 7% special design features may be introduced. It is not applicable for slopes greater than 7%.

• The system is not suitable in areas with very low permeability soils such as fat clay

• Where groundwater level is close to the surface the method is ineffective. If hard pan or rock outcrop is shallower than 1 m, the effectiveness of the drain is in doubt.

• The concept does not accommodate deluge of discharge from built – up environment with impermeable land cover. Such land covers may include interlocking stone, asphalt or concrete slab. For instance, a rainfall intensity of 100 mm/hr. on a plot of built up area (33 m by 20 m) can generate 66 cubic meters of water per hour. To handle flow of this magnitude special design consideration may be involved.

10. Further research

To further this work the following areas may be of interest for investigation

• Establishment of empirical factors T (time factor), k₁ (inflow loss coefficient), k₂ (infiltration loss coefficient) for varying climatic conditions.

• Evaluation of cases where the terrain slopes beyond 2% and up to 7%,

• Adaptation of the concept to accommodate slopes steeper than 7%,

• Evaluation of its applicability in low permeability soils.

• Determination of effective absorption surface in the trenchless drain.

11. Conclusion

Construction of road network in an urban or semi – urban settlement requires a good drainage system that can convey rainfall surface runoff from impermeable surfaces created by road surface and built up areas. The efficiency of such a drain depends upon the road/invert grade, construction standard and ethics adopted, and maintenance culture.

On flat areas open concrete and pipe drains are hardly efficient in the disposal of rainfall runoff, and are unsustainable especially in a developing economy. A sustainable drainage system in such an environment is the trenchless drain with inbuilt drainage facilities comprising water absorption unit and grass cover. Field and laboratory observations have shown that the system is low cost both in construction and maintenance. It disposes accumulated surface water soon after a rainfall and reduces the incidence of erosion downstream since potential concentrated runoff is eliminated. It also introduces aesthetics values to the environment, and an antidote to mosquito breeding which consequently reduces the incidence of malaria. It is not suitable in sloping area and in districts with impermeable soil layer just below.

Built up environments or housing units could be provided with articulate arrangement of trenchless drain, absorption units and green cells within the premises to minimize the volume of water discharge onto the roadway. This concept will reduce the quantum of water concentrating to form a deluge that may ultimately result to urban flooding, fluvial flooding and downstream erosion. The articulate arrangement of trenchless drain and comprehensive absorption units is an area of research and development interest.

Acknowledgments

The author wishes to acknowledge the assistance of TETFUND, a Federal Government of Nigeria sponsored funding agency, for providing the enabling grants for the research that buttressed this discussion. The effective participation of all academic and non- academic staff of the Institute of Erosion studies in the Federal University of Technology Owerri Nigeria, at the various segments of the research, is highly appreciated. My secretary, Ms. Rita Ugwu, who meticulously prepared this manuscript is highly esteemed.