Question

Ada kesan tak dari segi bearing capacity pile cap dan friction capacity bored piles kepada 2 nos bored piles bila 1 bored pile 1 lagi ter'offset' di tapak sebanyak 380 mm ke dalam. Saiz pile 600 mm dia.

Answer 1

Code of practice for foundations 2004 (Hong Kong) ada menyatakan dalam perkara 5.1.3 "minimum spacing" seperti berikut:

5.1.3 MINIMUM PILE SPACING
The spacing of piles shall be determined with due regard to the nature of the ground, the method of construction, the group effects and shall be sufficient to prevent damage to the piles or any adjacent construction.

For driven piles and other piles, which derive their capacities mainly from frictional resistance, the minimum pile spacing shall be not less than the length of the perimeter of the pile or 1m, whichever is the greater, and the piles shall be placed at not less than half the length of the perimeter of the pile or 500 mm, whichever is the greater, from the site boundary.

For mini-piles, which derive their capacities mainly from bond strength between grout and rock, the minimum pile spacing should be 750mm or 2 times the outer diameter, whichever is the greater.

For bored piles and the like which derive their capacities mainly from end bearing, the minimum clear spacing between the surfaces of adjacent piles should be based on practical considerations of positional and verticality tolerances of piles. It is recommended to provide a nominal minimum clear horizontal spacing of 500 mm
between shaft surfaces or edge of bell-outs, as appropriate.

5.1.2 GROUP EFFECT
A pile group exists when piles are closely placed such that the load carrying capacity
and settlement behaviour of a pile may be affected by other piles.
In such case, a group reduction factor determined by recognized foundation engineering principles shall be applied to the total allowable load carrying capacities of any group of piles. A group reduction factor of 0.85 may be considered as generally acceptable for a group of 5 or more vertically loaded piles. Other values of group reduction factor may be used having considered the particular ground condition of the site and justified by recognized engineering principles.

Generally, group reduction factors need not be applied where :
(a) the centre-to-centre spacings are of more than 3 times the perimeter of the piles
or the circumscribed rectangles in the case of H-piles; or
(b) the load capacity of the piles is derived from end-bearing; or
(c) the piles are rock-socketed piles; or
(d) the piles are driven to refusal with the toes resting closely onto bedrock (see
Clause 5.4.11(2)(c)).

Comments

1. Kesan kepada bearing capacity pile cap

Normal practise, pile cap tidak diambil kira untuk mengira bearing capacity. Boleh juga rujuk kepada perkara 7.3.4.5 BS8004 (1986) foundations.

2. Kesan kepada friction bored piles -

Bored piles biasa diguna untuk end bearing atau secara rock-socketed. Bored piles boleh ambil horizontal loading (bending moment) lebih tinggi berbanding spun atau concrete piles. Pandangan saya jika end bearing dan spacing melebihi 500mm, kesan friction bored piles tidak timbul. Jika end bearing tapi spacing kurang 500mm, penelitian lanjut perlu dibuat kepada jenis bedrock.

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Terima kasih Encik Rosli

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Pada pendapat saya, tetopi cerucuk hanya perlu direkabentuk semula sekiranya perubahan jarak antara cerucuk (x-direction in 2D) yang bakal merubah kedudukan center of pile cap / moment of inertia yang mana akan memberi kesan kepada transfer load dari tiang

Answer 2

Pada pendapat saya,

1. jika berlaku anjakan melebihi 75mm (dari mana-mana arah, 'x' atau 'y' atau kombinasi 'x&y'), tetopi cerucuk (pilecap) perlu dianalisis dan direkabentuk semula. Ini kerana mungkin daya yang ditanggung oleh tetopi cerucuk tersebut berubah. Semak samada tetopi cerucuk direkabentuk secara 'truss system' atau 'beam system'.

2. Bagi kapasiti geoteknik cerucuk tersebut boleh disemak melalui analisis yang telah dilaksanakan samada adakah kapasiti geoteknik yang ditanggung melebihi/kurang daripada yang dibenarkan ('allowable geotechnical capacity' boleh merujuk daripada laporan syor asas). Sekiranya bilangan cerucuk melebihi dua, ianya boleh dianalisis secara 'pile group analysis'.

Sekiranya analisis dan rekabentuk adalah gagal, maka tetopi cerucuk peru direkabentuk semula (perkara 1 & 2) manakala tambahan cerucuk diperlukan (perkara 1).

Wallahualam...

Comments

Terima kasih Faiz

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Sama-sama tuan...

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Cerucuk dikategorikan sebagai gagal bila mencapai salah satu daripada kriteria berikut :-

1. Baki mendapan melebihi 6.50mm apabila ujian beban diturunkan
2. Jumlah mendapan melebihi 12.50mm dibawah beban rekabentuk
3. Jumlah mendapan melebihi 38.0mm dibawah 2 kali ganda beban rekabentuk atau 10% diameter/lebar ceruk, mana yang lebih rendah.

KIta boleh melakukan beberapa perkara seperti berikut :-
1. Mengkaji semula keupayaan galas cerucuk yang direkabentuk dengan mengurangkan beban tanggung cerucuk.
2. Menambah bilangan cerucuk pada setiap lokasi.
3. Menukar jenis dan saiz cerucuk
4. Menukar rekabentuk struktur dengan mengubah kedudukan tiang