Piling & Foundation Contracting
By virtue of the geographic & geologic setting of the UAE and Gulf region at large; in terms of; prevalent eolian dune environment, proximity to the marine environment (in plan and elevation) and recent tectonic history of the region; the predominance of; loose dune sands, soft tidal sabkha deposits, soft fractured rock deposits & their residual soil derivatives as well as their soluble (evaporate) content has availed weak and compressible material in association with high groundwater levels near the ground surface. This is in addition to the excessive hydraulic conductivity of near-surface deposits and that has made it incumbent on constructors in the vicinity to deal with foundation piles, shoring systems and dewatering as a matter of routine practice.
Piles are reinforced concrete (RC), steel and composite slender members of high strength and thus; in a very broad sense, designed to carry loads axially (in compression and tension) as well as laterally in bending. The exact mechanisms of load transfer between a pile and ground are quite complex and statically in determinant but generally are a combination of vertical end-bearing (downwards & upwards) and shaft (skin) friction (upward and downward shear resistance) as well as lateral bearing (compression) in the case of laterally loaded and slender compression piles.
The strength, stiffness, dimensions, distribution, surface characteristics and orientation of piles with respect to the applied loads in collaboration with the prevailing ground conditions, determine the safe load capacity of piles and pile groups (axially and laterally).Types of foundation piles SGFCs offer are:
- - Drilled - Cast In-situ Concrete Piles
- - Micro-Mini Piles
These are RC concrete piles normally drilled within a common diameter of 450-1500mm. The weak, collapsible soil section is normally cased (temporarily or permanently) while the rock is not; as it is self–supporting. The pile bottom (end bearing surface) is usually cleaned up properly prior to casting, the steel cage is lowered in a centralized manner and placed in position (with the designed concrete cover allowance) and the concrete is either gravity-casted using a shoot or hopper connected to a gravity tremie pipe or pressure-tremied using a concrete pump and hose. The concrete is first placed at the bottom and allowed to displace any contaminating soil cuttings, collapse debris, groundwater and drilling fluid upwards until fresh, uncontaminated concrete emerges at the surface.
This system involves drilling a large number of small diameter (100-250mm) holes to depths ranging between 3m and 15m and installing central reinforcement (rebar, steel rods or other rolled steel sections), adequately centered to acquire the designated cover. The hole is subsequently grouted or else micro-concreted using a suitable tremie hose displacing water, drilling circulation media, cuttings and sediment from the bottom upwards and out of the hole and until uncontaminated grout or micro-concrete exits the hole.
In particular cases, the central steel reinforcement is the tremie pipe itself and in other cases the central reinforcement is the drilling rod, the tremie and sacrificial bits may also be used too. In yet other cases the hole is first grouted or filled with flowable micro-concrete and the reinforcing element subsequently dropped in position in a centralized manner and pushed down statically or using vibratory methods. In some cases, casings are used to reinforce the section with or without central reinforcement.
Holes may be cased permanently or temporarily using steel or polymer-based casing pipes. Of particular interest in temporary steel casing is Symmetrix / Robit type of drilling in which the casing and drilling string with the drilling bit proceed simultaneously and once the hole is partially or fully drilled and cased, the string and bit retracted, the hole cleaned and reinforced, concreted (or grouted) followed by retraction of the casing. Hollow stem augers of different inner and outer diameters may also be used for similar applications.
Micro-piles and mini-piles are considered highly slender deep foundation members and therefore buckling, P-δ effects and lateral forces are very important to be considered in their design. Group design is more important to reckon with for such slender pile systems.
Pile Boring Systems
Bored piles involve boring a cylindrical hole into the ground (cased if so needed), installing designated steel reinforcement in a centralized manner at the designated level and filling the cylindrical hole with concrete to form a RC columnar element.
Boring is usually carried out to the required depth and diameter by means of purpose-built, crawler-mounted, hydraulic drilling machines of different makes, models and capacities to suite the ground conditions and pile dimensions (diameter & depth) of concern. Bored pile foundations are suitable for all types of ground (soil and rock) and compared with their conventional driven counterparts, bored piles are more productive, less tedious and less disruptive to the surrounding environment in terms of noise and vibrations.
- The range of concrete pile diameters available are:
- - Bored cast in situ piles: 450mm to 1500mm.
- - Micro-Piles /Mini-piles: 100 mm to 250 mm.
Piles Shoring Wall System
Pile shoring systems are walls that are formed in progressive manner within the ground by constructing a sequence of piles along the outline of the site and used to laterally retain ground including soil, rock and groundwater. The design of the shoring varies with depth of excavation and prevailing nature of the ground and potential surcharge loads imposed outside the shoring, the allowable deflections and whether the shoring system is permanent or temporary in purpose.
- Bored Contiguous Concrete Pile Walls
A contiguous bored pile shoring wall is an earth retaining system formed by installing closely spaced (nearly touching), bored concrete piles and thus not suitable when a water-tight wall structure is needed. To improve water tightness of contiguous pile shoring walls, a combination of temporary dewatering, shotcreting, surface mortar seal applications and at times; local grouting may be needed. Piles are constructed alternately to avoid disturbance to freshly casted neighboring piles.
Either all piles are reinforced or else alternately reinforced depending on strength and stiffness requirements of the system. Contiguous pile walls are either designed as a cantilever or else supported by one or more level of supports (braces, props and/or tie-backs). The wall at its top is normally fully encapsulated with a continuous RC capping beam and the piles bay be joined at several levels with steel or RC waler beams (continuously or intermittently as needed).
- Bored Secant Concrete Pile Walls:
A secant pile wall is very similar to its contiguous pile wall counterpart except that adjacent piles are constructed overlapping in plan along the full depth of adjacent piles. And depending on the extent of overlap in plan, the wall is rendered water-tight to different extents. Due to the overlap, normally female (primary) piles are not reinforced and are constructed using modest, strength and well retarded concrete (to be easily over-cut during constructing the male piles), Male (secondary) piles are on the other hand are usually heavily reinforced and constructed using high strength concrete.
Nonetheless, with changing the outline geometry of reinforcement in alternating piles, all piles could be reinforced if strength and stiffness requirements deem so. Water-tightness could also be improved by a combination of mortar repairs, local grouting and shotcreting. Normally secant pile walls are needed for exceptionally deep excavations and high groundwater levels.
- Berlin Walls:
A sequence of I / H steel soldier beams of selected section moduli are normally vibrated (or dropped into pre-drilled holes and gravel backfilled) to the designated depth below excavation level and in a manner that the respective flanges of adjacent soldiers are oriented to line up with each other. The soil is thin gradually excavated as timber lagging or precast concrete panels are sled down the cannels formed between the flanges and webs of mutually adjacent soldiers and the process continues in stages until full excavation depth is reached. Berlin walls may be cantilevered or constructed with supports (braces and/or tie-backs) depending on; the prevailing ground conditions, depth of excavation supported and preferred design/construction details
Depending on the prevailing ground conditions, normally 5.0-7.5m excavation depths seem to be the upper limit in the vicinity supported using this system. Berlin walls are naturally not water tight and therefore can’t be used without extensive external and internal dewatering systems provisioned. Length, section and spacing of the soldiers are determined in the design to ensure strength, stiffness and overall stability of the wall which could be analyzed and determined. Likewise, strength and section properties of the lagging and/or concrete panels must be designed as well, so should the supports, if any (including bearing plates and waler beam sections).
In the case of temporary Berlin walls, and once the permanent construction is completed and the excavation backfilled, the beams are normally extracted by conventional vibratory pile extracting methods.
- Sheet Pile Walls:
Sheet pile shoring wall systems are flexible, hot rolled structural steel retaining wall sections; coming in different shapes, dimensions, strength and stiffness. The sections are driven by conventional vibrating or hammering systems into the soil or else pre-bored and driven into rock. The common-most of sheet pile types used in the area are Larsen, Z, U, straight, angled and other standard sections. Normally, sheet piles are rolled in mild steel version or in their high tensile counterparts. They are also rolled in a range of alloying elements to suite the environment in which they serve. Traditionally, sheet piles come in 12m long sections and may be either cut to shorten or spliced to lengthen as needed. Sheet pile sections come with strong interlocking clutches to join adjacent sections of piles together to compose a wall that is rather water tight. Clutches could be mortar filled or grouted with different materials to be made fully water-tight. Sheet pile walls may be combined with other structural steel sections or concrete piles for added strength, stiffness and attain the outline shape required as needed.
Sheet piles walls are constructed by establishing a steel guide frame ensuring the wall is controlled in alignment and verticality and that the sections do not declutch during driving. Driving is normally achieved by standard hydraulic vibrating hammers systems fixed to the booms of a rig or suspended from heavy crawler cranes. Sheet piles when well clutched and penetrate sufficiently below excavation levels form water tight shoring wall systems. Sheet pile walls may be designed as cantilevers or may be tied-back, propped and waled. And all these elements need to be designed for strength and stiffness and dimensioned carefully.
- The range of concrete pile diameters available are:
Bored piles involve boring a cylindrical hole into the ground (cased if so needed), installing designated steel reinforcement in a centralized manner at the designated level and filling the cylindrical hole with concrete to form a RC columnar element. Boring is usually carried out to the required depth and diameter by means of purpose-built, crawler-mounted, hydraulic drilling machines of different makes, models and capacities to suite the ground conditions and pile dimensions (diameter & depth) of concern. Bored pile foundations are suitable for all types of ground (soil and rock) and compared with their conventional driven counterparts, bored piles are more productive, less tedious and less disruptive to the surrounding environment in terms of noise and vibrations.
The anchorage is formed of a dial plate resting on a bearing plate fixed at the wall surface and strands transfer load to the dial plate and hence to the bearing plate through steel wedges. Strands are normally pre-stressed to different designated degrees. Anchors are either supported on waler beam systems or in capping beams of shoring walls or in cases directly bearing onto the pile concrete surface.
- Soil Nails & Rock Bolts
These are a form of tie-backs involving a central reinforcing element centrally positioned in a borehole grouted in place using a high strength grout material along its full length and externally tightly bearing on an anchorage plate through a form of stressing element (nut or other). Design, detailing and material selection depends on the service purpose, time and environment.
- Propped Systems:
Due to non-accessibility of adjacent space, anchorage may not be permitted for supporting shoring walls and then internal strut (prop) systems are to be provided. A prop is a horizontal (or inclined) strut that supports in between two shoring wall or at an inclined on the ground (or a lower bearing surface) and using a form of load distributing element like a waler beams etc. The design, detailing and material selection of propped support systems depend on purpose and spatial constraints.
Shoring is a term used to describe a system that functions to retain earth, water, and adjacent existing assets when an excavation is required. Shoring design can be a very complicated matter. The designer has to reckon with many unknowns and factors that influence the behavior of the shored excavation and surroundings. Typically, there are two systems in excavation shoring that must be designed:
- - The earth retention System that restrains the earth i.e. the shoring wall and;
- - The supports (i.e. the internal or external bracing such as tie-backs, struts, waler beams…etc) and that brace up the earth retention wall system.
Performing detailed analysis & design calculations for both systems can be a time consuming process; especially when governing parameters have to be changed. Also many current software packages do not offer an integrated platform for geotechnical and structural analyses required to properly design shored excavations. As a result, the designer is forced to use numerous software programs to analyze the system separately. With the exception of the finite element analyses method, there are limited theoretical solutions for analyzing shoring systems in an integral manner. As a result, the whole process can become unnecessarily complicated and time consuming. Shoring can be designed with both traditional linear and non-linear methods of analyses. For while it is realized that traditional methods of analysis have obvious limitations in accurately predicting shoring behavior, they are important for framing the problem and providing a check on the more rigorous numeric modeling methods.
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