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SECANT PILE
Introduction:
Over the last few decades construction of retaining walls in urban areas has grown significantly, as a result of territorial and economical development of the cities. In most cases, retaining walls are deep structures with the existence of groundwater level because urban areas are often at a few meters above sea level. Furthermore, construction of retaining walls in cities, introduce special problems as a result of being directly in touch with urban elements and citizens.
In situ pile retaining walls are very popular due to their availability and practicability. There are different types of pile walls contiguous (intermittent) bored pile construction, (spacing between the piles is greater that the diameter of piles)
In these conditions, for deep borings and with the existence of groundwater level, diaphragm walls and secant pile walls are the only feasible retaining walls in urban areas. Secant pile walls are usually used in hard ground, where it is not possible to install diaphragm walls, and in rocks, and it is technically possible to install secant pile walls in a extensive range of grounds.
Secant bored pile walls are formed by keeping spacing of piles less than diameter (S<D). It is a watertight wall and may be more economical compared to diaphragm wall in small to medium scale excavations due to cost of site operations and bentonite plant.
Construction:
The traditional construction techniques for cast-in-situ concrete retaining walls have included principally:
a) Contiguous bored pile walls
b) Secant bored pile walls
c) Diaphragm walls
In contrast to the open structure of a contiguous bored pile wall, in which individual piles are spaced at 1.1 to 2 pile diameters, the piles in a secant wall are spaced at 0.8 to 0.9 pile diameters. Primary piles are secanted by secondary piles thus providing a closed structure to act as a barrier in water bearing soils, and to prevent ingress of soil between the piles. In some soil conditions the gaps between the piles in contiguous bored pile walls are grouted after the piles are installed in an effort to achieve the same objective. [3]
While a diaphragm wall consists of a sequence of panels, typically 3m to 7m or more in length, which may be interlocked at panel joints. A secant pile wall consists of interlocking piles with diameters ranging from 410mm to 1500mm. Piles are usually of constant diameter but, occasionally, primary piles may be of a smaller diameter than secondary piles.
The secant pile method consists of boring and concreting primary piles at centres slightly less than twice the nominal pile diameter. Secondary piles are then bored at mid-distance between the primary piles, the boring equipment cutting a secant section from them. Secondary piles are bored through primary piles before the concrete has achieved its full strength should this operation be delayed, wear on the auger or the cutting edge to the casing is likely to be much increased. Concrete quality control is therefore important in this respect and deviations in maximum strength may be important as minimum strength deviation. In practice three types of secant piles are constructed:
• Hard-soft secant pile wall
• Hard-firm secant pile wall
• Hard-hard secant pile wall
“Hard-soft” secant pile walls are formed by a series of interlocking drilled shafts. Primary piles without reinforcement are cast first; these are constructed of a soft pile mix of cement and bentonite or cement, bentonite and sand. The mix has a weak characteristic strength of 1-3 N/mm2. The primary piles are used as water retaining structure rather than a load bearing column. Soft piles can retain up to 8m head of groundwater. The unreinforced soft pile is not usually used as a permanent wall material. As the bentonite and cement mix dries, it will shrink and crack, losing its water-resisting properties. Some soft piles have been designed to retain their water-resisting properties for the life of the structure; often this necessitates the mix remaining hydrated throughout the life of the building. The gaps left by primary piles are filled by secondary reinforced piles, which overlap the primary ones. [1]
“Hard-firm” secant pile walls are similar to “Hard-soft” secant piles,but here the characteristic strength of primary pile lies in d range of 10-20N/mm2 . During the construction, the strength of the pile is held low by adding a retarding agent to the concrete mix. Primary piles are usually designed to hit their target strength within 56days rather than the more typical 28 days. Obviously, such practice ensures that the construction of the secondary/hard pile is easier as the auger has to exert less force when secanting the soft pile. Piles usually overlap a minimum of 25mm.
“Hard-hard” secant piles, here both primary n secondary piles re cast with full strength concrete and both are fully reinforced. The female piles are cast first and a high torque cutting casing is used to drill through the primary pile. The reinforcement in the primary pile is positioned so that the rig does not cut through it when boring the secondary pile. The depth of overlap is usually about 25mm; considerable care is required to ensure that this is maintained along the full length of the pile. I-section beams can also be added to the pile to further increase the lateral strength of the wall.
The “hard-soft” concrete secant wall offers similar soil and water retention properties compared to the “hard-hard” concrete secant wall [3]. The “hard-soft” wall is cheaper than the “hard-hard” wall, primarily due to the lower strength of the primary piles and hence avoidance of secanting into structural concrete. Since “hard-soft” secant walls are weaker, they are used for shallow excavations or to where high bending stresses within the wall can be avoided.