Note Building Construction

Pile may be classified as either end bearing and friction piles, according to the manner in which the pile loads are resisted.

END BEARING PILE

The shaft of the piles act as columns carrying the load through the overlaying weak sub-soils to firm strata or a layer of the pile toe has penetrated. This can be a rock strata or layer of firm sand or gravel which has been compacted by displacement and vibration encountered during driving.

[pic 1]

FRICTION PILE[pic 2]

Any foundation imposes on the ground a pressure which spreads out to form a pressure bulb. If a suitable load bearing strata cannot be found at an acceptable level, particularly in stiff clay soil. It is possible to use a pile to carry this pressure bulb to a lower level where a higher bearing capacity is found. The friction or floating pile is mainly supported by adhesion or friction action of the soil around the perimeter of the shaft.

[pic 3]

METHOD DRIVEN FOR 250X250 DRIVEN PILE

1. Set out the position of each pile and establish the temporary benchmarks (TMB) on site to determination of cut-off levels of piles.
2. Check the verticality of the piling ring using a plumb or spirit level.
3. Provide markings along the pile section to enhance recording of penetration and to serve as rough guide to estimate the set during driving.
4. Install mild steel helmet. protect pile head/joint plate with packing or cushioning within e.g. a 25 mm thick plywood between the pile head and helmet.
5. Hoist up and place the pile in position
6. Check on verticality regularly
7. Proceed with the hammering. Monitor pile penetration according to the markings on pile. When the rate of penetration is low, monitor pile penetration over 10 blows. Hold one end of a pencil supported firmly on a timber board not touching the pile. The other end of the pencil marks the pile displacement on a graph paper adhered on the pile over 10 blows.

1. Lengthening of pile can be done by means of a mild steel splice sleeve and a dowel inserted in and drilled through the centre of the pile.
2. The connection is sealed by welding the pile head/joint plate which is pre-attached to both ends of a pile in manufacturing process.

METHOD CONSTRUCTION BASEMENT

OPEN CUT

• Open cut technique is the simplest method of providing an excavation to the required depth.
• The sides of the excavations are sloped in some cases with possible slope protection to ensure the stability of the solid mass.
• Upon excavating to the required level, the basement is then constructed from bottom upwards, in the conventional way.
• After the completion of the basement, the remaining excavation areas i.e. between the basement and the side slopes are subsequently backfilled.

[pic 4]

CUT & COVER

• The cut and cover technique is used when the site is tight, e.g. located within a heavily built-up urban area, where ground movement to the adjacent surrounding must be kept to a minimum.
• Soil retaining structures are firstly erected to prevent the surrounding earth from sliding out whereas stuttering and bracing are also provided according to design so that excavation works can be proceed safely and at reasonable pace until reaching the deepest basement level.
• The basement is then constructed in the conventional way, bottom upwards in sequence with removal of the temporary struts.

[pic 5]

TOP DOWN

• The top down technique is an extension of the cut and cover technique.
• In order to optimise the cut and cover technique, permanent basement structural walls (i.e. Diaphragm walling, contiguous bored pile wall) are employed to support the excavation face with the basement floor slabs acting as struts for external walls.  
• The slabs depend on the prefounded columns as supports for the vertical loads on the floor.
• These columns are known as ‘profounded’ because they are constructed prior to bulk excavation. In this technique, the basement floor slabs are cast before excavation to the next level and construction proceeds from the top downwards for the basement structure
• The unique characteristic of the prefounded column is that it can be installed either for temporary use (for construction of basement only) or it can be designed as part of the permanent load carrying member for the completed structures.
• As obvious advantage of this form of construction is that the superstructure can proceed upwards from ground level simultaneously with the excavation downwards.

• The overall development may be completed months earlier than the conventional bottom-up approach in view of this overlapping of substructure and superstructure construction time schedule.

[pic 6]

COMPOSITE

• After completing the basement 1 slab, the middle portion of storey 1 slab is then constructed in a bottom-up upwards scheme.
• Since this form of construction utilises both bottom-up and top down technique
• The superstructure can then proceed upwards while simultaneously, the substructure proceeds downwards.
• Access openings are provided in the floor slabs for the removal of excavated material in the lower basements.
• The obvious merit of this technique is that excavation is facilitated with no headroom limitation except for the portion underneath the cast slab.
• This scheme will obviously speed the construction process considerably.    

[pic 7][pic 8]

DEWATERING SYSTEM

Dewatering systems are divided into two, which are:-

1. Sump pumping
2. Well point system

SUMP PUMPING

• A sump is constructed to collect the ground water, providing a suitable depth for pump to work in.
• The depth of the sump may exceed 7.5m.
• The water pumped out must be directed away from site.

[pic 9]

WELL POINT SYSTEM

• A more effective method of ensuring that water is removed from area of an excavation during the period of the work is de-watering by the well point system.
• This system requires a small device called a well-point to be forced into the ground using water jet. This is called jetting.
• The well-points are jetted into the ground around the proposed excavation down to a depth of five  to six meters, which is the maximum for single well point system.
• The well-points are placed about every 1.5m and are link together by a ring main connected to a pump.
• As the well-point is jetted in, lengths of pipe are attached to it until the required depth is reached.
• When the suction pump is started, water is drawn through the well-point, which also screens out fine material.
• The pump must kept running continuously to keep water out of the excavation throughout the work.
• To increase the depth of the de-watering, a two stage installation can be employed.

[pic 10]