FURTHER TECHNICAL INFORMATION IN RELATION TO THE OPERATIONS OF OUR GROUP

Transcription

1 A. TECHNICAL INFORMATION OF OUR FOUNDATION WORKS The major types of our foundation works (with illustrative diagram, where applicable) are set out below: 1. Bored piles 0.80m 3.5m pile shaft By Grabbing and Reverse Circulation Drilling Method Existing ground Concrete cut off level Socket length Concrete Rockhead level 60 o Steel cage Bellout Founding level bellout diameter shall not exceed 1.5 times the diameter of pile shaft The bored piling works undertaken by us are mainly large diameter bored piles with diameters ranging from 0.80m to 3.5m. They are usually formed and installed by machine boring and grabbing to the required level and subsequently filling the bored hole with reinforced concrete. Normally a steel casing will be used to provide temporary support to the ground during boring operation. Bored piles are a type of end bearing pile that reach the underground bedrock layer from which bored piles obtain support to bear the load of the superstructure above. It is common to construct a bell-out at the base of a large diameter bored pile to increase its load bearing capacity. V-1

2 2. Driven H-piles By Percussive Method Existing ground level Pile cut off level Steel I-beam Termination level Rock Driven H-piling (which is a type of percussive pilings) works generally involve the driving of steel H-piles to the required depth by direct or indirect hammering or other percussive means, including by the use of a drop hammer, normally prohibited, double acting hammer, single acting hammer, internal drop hammer, pneumatic hammer, steam hammer or other percussive device. Driven H-piles have been widely used in Hong Kong due to the ease of handling and driving. Driven H-piles are a type of friction pile that derive the load bearing capacity from the friction between the pile and earth. Due to the percussive nature, this type of piling works creates more noise and vibration and its operation is generally restricted to 3 hours per day in urban areas in Hong Kong. As the size of machinery involved takes up relatively less space, driven H-piles are suitable for construction sites with limited space. V-2

3 3. Socketed H-piles 610mm By Drilling and Down-the-hole Method Existing ground level Pile cut off level Cement grout Steel I-beam Socket length Rockhead level Founding level Rotary boring machine is used to drill holes on the ground for piling and the drilling continues until the hole has extended to sufficient depth (socketing) and into a sufficiently bedrock. Both the diameter and the depth of the borehole are highly specific to the ground conditions, loading conditions and nature of the construction project. Socketed H-piles (also known as pre-bored H-piles) are then installed by inserting prefabricated steel H-piles into the boreholes sunk into bedrock, and subsequently grouting the holes with cementitious materials. V-3

4 4. Mini-piles 190mm 323mm By Drilling and Down-the-hole Method Existing ground Pile cut off level Cement grout Bundle steel bar Socket length Rockhead level Coupler Founding level A mini-pile usually consists of one or more steel bars encased by grout inside a borehole not exceeding 400mm in diameter. They are normally designed to be socketed into rock and are mainly used to resist compression or tension loads on sites with difficult access. Steel casing are to be provided to support the borehole within the soil and/or fractured rock during drilling operation. V-4

5 5. Footings Building structure By Excavation Method Starter steel bar Existing ground Footing top level Concrete Steel reinforcement Founding level Rock Footing foundation is a type of shallow foundation which transfers building loads to a shallow bedrock layer which is near the ground surface, usually less than two meters below ground. Most of the footings are formed by concrete poured into a trench and constrained by some kind of forms. Functionally, footings are similar to pile caps. However, footings transfer the load directly to the bedrock while pile caps transfer the load onto piles. V-5

6 6. Pile caps Building structure By Excavation Method Starter steel bar Existing ground Pile cap top level Concrete Steel reinforcement Constructed piles Pile caps are concrete structures built on the head of a pile or a group of piles for transmission of loads from the structure above to the pile or group of piles. V-6

7 B. FEATURES OF OUR CONSTRUCTION WORKS We set out below the features of various works by our Group: Type of Works Foundation Works Advantages Disadvantages Applications Limitations Cost Features Bored piles 1. Non percussive pile: 1. Needs relatively wide access for 1. High-rise buildings Low noise mobilisation of Low plant vibration 2. Relatively high Less plant costs disturbance to neighbours 3. Needs rest on designed grade of 2. High load bearing capacity bedrock which can be very steep 3. Can be installed at great depths and may lead to higher costs 4. Excavated material requires disposal, the cost of which will be high if it is contaminated 1. Not suitable for small site which is difficult for manoeuvring of bored piling plant 1. Relatively high fixed plant cost Driven-H piles 1. Relatively low cost as it can be rest upon stiff soil irrespective of depth and grade of bedrock 2. Low plant costs 1. Percussive piles: 1. High-rise buildings High noise and podium High vibration More disturbance to neighbours 2. operation is restricted to 3 working hours per day in urban areas in Hong Kong 3. Pile section may become damaged during driving 1. Not suitable for 1. Relatively high site next to material cost sensitive structures or utility installations V-7

8 Type of Works Foundation Works Advantages Disadvantages Applications Limitations Cost Features Socketed H-piles 1. Non percussive pile: Low noise Low vibration Less disturbance to neighbours 2. Lower plant costs as compared with bored piles Mini-piles 1. Non percussive pile: Low noise Low vibration Less disturbance to neighbours 2. Small size of plant which can be used in small site. Only bamboo platform is required to support plant for pile construction on slopes 1. Needs support in designated grade of rock which can by very steep and deep which in turn increases costs 2. Higher plant costs as compared with percussive piles 3. Risk of loosening soils during pile excavation and causing ground loss and hence settlement 1. Low load bearing capacity 2. Needs support in designated grade of rock which can by very steep and deep which in turn increases costs 1. Smaller sized high-rise buildings and podium structures Footbridge 1. Small structures e.g. footbridge and temporary working platform on slopes Noise Barriers 1. Not cost effective as compared with large diameter bored piles which can support heavier loads 1. Not suitable for large structures as its load bearing capacity is relatively low 1. Lower plant cost as compared to bored piles but higher plant cost as compared to percussive piles 2. Higher material cost as compared to percussive piles 1. Relatively low plant and material costs Footings 1. Very low cost as piles are not required 1. Noise generated by drilling and breaking of rock causes disturbance to neighbours 1. Buildings at a site where the load bearing bed rock is at a high level 1. Low plant and labour costs as it requires no special skill Pile caps n/a n/a 1. Transferring the superstructure loadings to underground piles n/a 1. Low plant cost, mainly labour and material costs V-8

9 Type of Works Foundation Works Advantages Disadvantages Applications Limitations Cost Features Civil engineering Site formation n/a n/a Excavation and filling Forming formation and safe slopes for foundation and building works Low plant cost, mainly labour and material costs Landslip preventive and remedial works to slopes and retaining walls Roads and drainage works n/a n/a 1. Ground investigation 2. nailing and spraying concrete 3. Mini-piling and soil anchors 4. Installation of flexible barriers 5. Reinforced concrete ( R.C. ) construction 6. Landscaping working 7. Drainage construction n/a n/a 1. Excavation and filling works for road formation 2. R.C. work for structures 3. Concrete paving or flexible paving work 4. Drainage construction 5. Traffic diversion and associated electrical and mechanical ( E&M ) installations 6. Installation of noise barrier Provision of Low plant cost, protection to slope mainly labour and or upgrading the material costs protection of existing slopes against landslip hazards Forming roads and connection network and upgrading/forming drainage system for future developments Low plant cost, mainly labour and material costs V-9

10 Type of Works Foundation Works Advantages Disadvantages Applications Limitations Cost Features General Building Works General building n/a n/a 1. R.C. construction 2. E & M installations 3. Plumbing and drainage work 4. Cladding and window installations 5. Fitting out works 6. Landscaping works Infrastructure construction for residential developments Low plant cost, mainly labour and material costs Renovation work n/a n/a 1. Renovation of the Renovation and Low plant cost, building upgrade of existing mainly labour and 2. E&M installations building structures material costs 3. Plumbing and Drainage V-10