Deptt. of Irrigation and Drainage Engg. Land grading Shri M. U. Kale Assistant Professor Deptt. of Irrigation and Drainage Engg.

Land grading and field layout Introduction : Land grading is the reshaping of the field surface to a planned grade. it is necessary in making a suitable field surface to control the flow of water to check soil erosion and provide for surface drainage. Land grading is beneficial in unirrigated areas to conserve moisture, reduce soil erosion and provide for surface drainage.

Land leveling operations may be grouped in to three phases:- Rough grading :It is removal of abrupt irregularities (mounds, dunes and ringes) and fill pits, depression and gullies. Land levelling – It reshapes land surface to planned grade by moving large soil over distances. It sometime leave irregular surface due to dumping. Land smoothing – It makes plane surface.

Criteria for land levelling Land levelling is influenced by characteristic of soil profile, prevailing land slope, rainfall characteristic, cropping pattern, and methods of irrigation. In land levelling excessive cuts are necessary to eliminate cross slope. In bench levelling, the amount of earthwork is governed by the magnitude of the diagonal slope at right angles to the direction of irrigation. Border strips and check basins don’t permit any cross slope within the strip.  

Furrow and corrugation methods permit a certain degree of cross slope within a field. The rainfall characteristic of an area is important in determining the maximum and minimum grades allowable for a field. Grubbing is the process of removal of stumps, sunken stones and boulders and all roots over a minimum size and up to a specific depth   

Land levelling design methods Plane method Profile method Planinspection method Contour- adjustment method

Plane method Most commonly used Restricted to those field where it is feasible to grade the field to a true plane. Following is the procedure Determine the centroid of the field . Determine the average elevation of the field. Compute the slope of the plane of best fit. Compute the formation levels, cuts and fills . Determine the cut - fill ratio.

Determine the centroid of the field For Rectangular field:- centroid is located at the point of intersection of its diagonals. Triangular field :- centroid is located at the intersection of the lines drawn from its corner to the mid points of the opposite sides. To determine the centroid of the irregular fields, the area is divided in to rectangles and right angle triangles. The centroid is located by computing moments about two reference lines at right angles to each other. The distance of the centroid of the field from any line of reference is equal to the sum of the products obtained by multiplying the area of each part times the distance from the line of reference to its centroid, divided by the area of the entired field.

Determine the average elevation of the field. This is obtained by adding the elevations of all the grid points in the field and dividing the sum by the number of points. With the elevation of the centroid known and the downfield grade and cross slope selected, the elevation required at each grid point can be calculated The desired cut or fill may be computed from a comparison of the original and the proposed elevations.

Compute the slope of the plane of best fit The slope of any line in X or Y direction on the plane which fits the natural ground surface can be determined by least square method. In rectangular area, this can be represented by S = where, S = Slope of line in a plane, dimensionless D = Distance from reference line, m H = Elevation of grid point, m n = Number of grid points

Determination of cut-fill ratio Cut-fill ratio should be greater than one. It usually varies from 1.2 to 1.6. In extreme cases of heavy or light textured soils and deep or shallow excavation, the ratio may be as low as 1.1 and as much as 2.0. In case the soil is composed principally of organic material such as peat or muck, the ratio must be about 2.0.

Cut fill ratio depend on – Type of soil, quantity of earthwork involved and type of equipment. A settlement allowance (or amount of lowering of elevation) to be applied in plane method may range from 0.3 to 1 cm for compact soils and 1.5 to 4.5 cm for loose soils. A small change in elevation will cause a considerable change in the cut-fill ratio.

Profile method It consists of plotting the profiles of the grid lines and then laying the desired grade on the profiles. It is adapted to levelling design for very flat lands or land with undulating topography on which it is desired to develop a fairly uniform surface relief. It is trial and error method.

Plan inspection method It is adapted for moderate to flat land slopes. In this method grid point elevations are noted on the plan and design grade elevations are determined by inspection after a careful study of topography. It is trial and error procedure. It is rapid method, as it doesn’t assume minimum cuts and fills or shortest haul distance.

Contour adjustment method It is trial and error adjustment of contour lines on a plan map. It is adapted to the smoothening of steep lands that are to be irrigated. It is sometime useful on moderate slopes. A contour map is drawn and proposed ground surface is shown on the same map.

Determination of earthwork quantities Earthwork quantities are determined with end area method, prismoidal formula and four point method. End area method Volume between adjacent profiles or grid lines is computed by where, V = Vol. of cut or fill, m3 L = Distance between profiles or line, m2 A1 and A2 = Area of cut or fill in the profiles, m2

Prismoidal formula Exact method of computing the volume of earthwork in land levelling. V = (A1 + 4 Am + A2) where, L - Perpendicular distance between end planes, m A1 - Area of first end plane, m2 A2 - Area of second end plane, m2 Am - Area of middle section parallel to end planes, m2 It is laborious method

Four point method It is sufficiently accurate for land levelling. where, Vc = Volume of cut, m3 Vf = Volume of fill, m3 C = Sum of cuts on four corners of a grid square, m F = Sum of fill on four corners of a grid square, m L = Grid spacing, m

Equipment for land grading and field layout Land development machinery For land grading - Tractor drawn or bullock drawn machineries are used. For rough grading, bulldozer, tractor-drawn carrier-type scrapers, small scrapers drawn by tractor and animal drawn buck scrapes could be used. Land smoothing is usually carried out by Tractor drawn land planes or bullock drawn wooden floats. Crawler tractor with dozer blade is known as bulldozer. Bull dozer is more capable. Speed of bulldozer is limited to 8 km/hr. Rough grading when haul distance not exceed 25 m.  

Tractor drawn scrapers Range in size from terracer blade to heavy carrier-type scrapers. Carrier-type scrapers are used for large scale land grading Capacity of carrier-type scrapers range between 1 to 25 m3. Wheeled scrappers and leveler blade are frequently used for medium and small scale levelling jobs. It is also called ‘bottomless scrappers’. It requires loose soil.

Land plane It is bottomless scraper with long frame mounted on four wheels. The frame of land plane is at least 15 m long. Levelers It is two wheeled automatic type leveler usually used for fine grading of small and medium sized fields. It is operated by medium size-wheel tractor.

Animal-drawn scrappers It is most efficient for land grading when animal power is used in job. It is used to move soil loosened by ploughing or other tillage operations. It is operated satisfactorily when haul distance not exceed 50 m. Animal drawn buck scraper

Wooden float It is long sled-like drag with bullock power. Wooden float used for land smoothing Wooden wood leveller It is long sled-like drag with bullock power. Wooden U-leveller is useful for smoothening small fields for check basin method of irrigation.

Equipment for making ridges and ditches Equipments for ridge making bullock drawn bund former, A-frame ridger Tractor drawn disc ridger – generally used for border ridges. Bullock drawn bund former is suitable for making small ridges for check basins. Wooden frame ridger – for border strips and check basins. Wooden A-frame ridger

Tractor drawn disc ridger - Four discs in two gangs. Bullock drawn lister or middle breaker or double mould board plough is for making furrows. V-ditcher for construction or cleaning of field channels for irrigation and drainage. Dragline excavators equipped with V-shaped cutter blades and tractor mounted digger loaders are frequently employed for constructing large size irrigation and drainage canals.   Isometric view of wooden V-ditcher Plan of V-ditcher