Engineering techniques are used in constructing and reconstructing the rice terraces. These methods were developed in observance of water behavior and soil consistency. Part 1 of this article available here.
Site selection – Pondfields are usually constructed where the natural incline is least and where sufficient water supply and other essential materials are available. In U-shaped valleys, terrace building begins at the lower elevations near the main channels of the drainage basin.
In V-shaped valleys, terrace construction takes place in areas with gradually inclined slopes (Conklin:1980:16).
Hydraulicking – Hydraulicking refers to the method of sluicing (bulubul or budubud) fill materials such as soil, gravel, stones, and rocks from an upper elevation to a pond field below with the use of strong water force.
The soil is used by the receiving pond field either as fill material or topsoil. Apart from terrace formation, hydraulicking is done to widen ricefields and to clear erosion or landslides which has fallen on the pond field.
Sluicing earth and rock as a method of backfilling the stonewalls ensures that the constructed wall is bonded together by the finest material (Guimbatan:2003).
Process entails the creation of a pond at a higher level where water is impounded. Then, temporary channels from the site are constructed, sometimes passing across other pondfields, leading to the pondfield below so that the materials are guided to where it is needed when water is released.
Usually, hydraulicking is done during off season, from July to September, at the height of the rainy season. If rainfall is insufficient, irrigation and drainage water may be diverted to have enough water for sluicing.
Retaining walls – There are two types of terrace walling practiced by Ifugao farmers, the stone-walled (tuping) and mud-walled such as the clay type (mim-i) or those following the natural slopes of the hill or mountain.
Where the slopes are steep, farmers build stone walls to protect and strengthen the terrace walls. Conversely, areas with gradual slopes and rich in clayey soil make use of earthen walls.
While these are easily washed out during the rainy season, they are easily restored through a method of soil tamping that sometimes reshape the terrace ponds.
In Mayoyao, all rice fields are stonewalled whereas, in Kiangan, Hingyon, Banaue, and Hungduan, both mud and stone walls are utilized.
Stonewalls are best constructed during dry season. It has been observed that stonewalls constructed during rainy season collapse easily because the soil is loose.
Random Bonding – Preferred stones have different shapes with regular size. Only big ones are used for foundation or placed at the boundary to serve as a marker. Small stones, wedges, and chinking stones are used with stones of different sizes thereby ensuring a tight bond.
Except for the two foundation stones, random bonding is employed in stone walling. As fill is sluiced in and leveled behind the gradually rising masonry, each stone is carefully laid in its best tilted position to avoid unbounded alignments.
Larger rocks are placed lower in the wall than smaller ones. Where angular and rounded stones must be used, the angular elements must be used first.
Angular and irregularly shaped stones allow for more contacts between elements and ensure greater stability under conditions of weathering, seepage, and ground movement due to earthquakes (Conklin: 1980:19).
Subrounded and oblongish stones are laid so that they tilt back into the terrace body with the heavier and somewhat higher end facing out.
In contrast, flat stones are set up on their sides so that their longer cross-sectional dimension is placed more or less vertically. They are also used as sills for spillways. For individual stones with a longer dimension, they are laid more often as headers than as stretchers.
This helps to develop transverse strength through the wall (Conklin: 1980:19).
Buttressing – In buttressing, the stonewall is supported or reinforced by building a counterfort or a projection from the original wall. As observed by Conklin (1980) in his study areas in Banaue, the average height of main sections of stonewalls runs about two meters to six meters if there are no buttresses.
Conklin clarified that the batter of terraced stone walls depends on a number of features, the most important of which is the size of building stones. With very few exceptions, walls tilt at angles from 65 to 90 degrees.
In addition, the shape or profile of the terrace wall is also important. A straight incline or a slightly concave batter is desirable. Conversely, deviations from this type of profile are signs of a badly filled or poorly constructed embankment.
The purpose of sloped stonewalls and buttressing is to counteract the overturning moment made by active soil pressure. Bigger stone foundations are set to resist slipping at the base and a building technique is employed to bond materials together for structural strength (Guimbatan: 2003).
Jutting header stones are sometimes built into steep walls to allow for climbing from one level to the next. Stones or logs may also be set in high walls for women to stand on when weeding.
Water sealing – Water seepage into the foundation line of the slope may lead to a landslide thereby causing serious damage to the rice pond. To prevent this, moistness and plasticity of the pond bed must be maintained the whole year round.
During dry season when water is scarce and cracks (okak) appear at the top soil layer, water is sometimes gently poured over the cracks thereby making the soil expand and sealing the pond bed.
A crawling grass with furry leaves (il-ilit or il-li il-li) may also be spread over the dike and wall surface to protect the areas from direct sunlight (Guimbatan: 2003).
Irrigation and Drainage System – A carefully laid-out irrigation and drainage system is required to ensure that water is available all year round for the paddies and the rice crops. The entire system depends on gravity.
As Conklin (1957), Guimbatan (2003), Buyuccan (2009) illustrated, water from springs (otbol, ofob), creeks (wa’el), river (wangwang) are generally diverted and dammed in an impounding area (putut or paluk) before distribution to the different ricefields by means of artificial irrigation channels (alak) and water conveyor troughs ( tulalok/huyung).
There are three general principles being observed by farmers in water management: heavy flow of water must be diverted from the terraces; entry of water to the pond fields must be gradual; and inundation must be maintained at all times (Guimbatan:2003).
To protect the slopes from eroding, the hydraulic system in the rice terraces is designed in a manner whereby water channels are carefully laid out in recognition of the natural courses of water runoff and its velocity regulated by the angle of its conveyors.
In irrigating the terraces, the principle is to reduce the water force as it enters the pond field. The entry of water into the rice paddies is regulated by attenuation and this is achieved through an almost horizontal layouting.
This way, water gradually moves along irrigation canals that are either carved out of the mountain sides or through makeshift conveyors.
Flooding in the pond fields is balanced by constant maintenance of the spillways (guhing). Excess water from a higher ricefield is drained to the lower paddy or to a nearby creek through the spillways.
During rainy season, exit of water from the pondfield is done by expanding the spillway. On the other hand, the spillways are closed especially during dry season to prevent water loss.
To divert the flow of an otherwise destructive heavy volume of water, excess water from the irrigation source, natural drainage, and spillways, is directed to a channel (gohang, liglig).
The walls of the channel are self-standing and designed to withstand water velocity. In contrast to the principle of irrigation, the water flow of the drain channel travels the shortest distance possible from the top to the river below.