dosen, bina darma UNIVERSITAS BINA DARMA - PASCA SARJANA - TEKNIK SIPIL - REKAYASA GEOTEKNIK LANJUT - PILE FONDATION. UNIVERSITAS BINA DARMA.
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2. Kuliah 2- Review of Foundation Engineering PASCA SARJANA 2019 - 2020 GENAP UBD.pdf Download (603kB) | Preview |
Abstract
PILE FOUNDATION DR. IR. NURLY GOFAR, MSCE 2 Pile Foundation Pile foundation is used in cases where the soil upon which a structure is to be built is of such poor quality that a shallow foundation would subject to bearing failure/excessive settlement They are differentiated from footing foundations in that the ratio of the depth of the foundation to the size of the pile is greater than four. It works by transferring load to greater depth where the firmer soil is conditions where pile is selected as foundation of a structure Piles are needed when designing foundation of transmission tower, offshore platforms or basement mats subjected to uplifting force. Pile should extend to stable soil layer when the foundation soil is susceptible to swelling or collapse. Piles are required to support bridge abutments to avoid scouring at the foundation base. Piles are used extensively to resist both vertical and lateral loads from retaining structures and tall buildings, as well as harbor and offshore structures. 4 Pile Foundation Pile may be categorized based on some characteristics such as: ◦material forming the pile, ◦transverse and longitudinal sections, ◦installation method ◦load transmission. 5 Types of piles By Material Type; Allowable Load & Length 6 Timber piles cannot withstand hard-driving stress, therefore; the pile capacity is usually limited. Timber pile is highly durable when embedded in saturated soil but deteriorate easily when subjected to change in moisture. Steel piles are selected when load is high, but they may be subjected to corrosion. Usually H or O section. Pipe piles are often filled with concrete after driving . Steel pile may withstand hard driving condition. Pre-cast concrete pile is made of reinforced concrete which may be pre�stressed to provide high capacity. High strength concrete is to be used for pre�stressed piles. Cast-in-situ concrete piles are created by filling a drilled hole with concrete. The hole can be cased or uncased. A bulb or expanded based can be formed by dropping a hammer on the fresh concrete to provided larger contact area at the base. Types of piles By Material forming the pile 7 Types of piles By transverse & longitudinal sections By Transverse and Longitudinal section 8 Types of Pile By Installation Method Driven/displacement pile ◦totally preformed piles driven into the ground (displacement piles) e.g. timber piles, pre-cast reinforced concrete, pre-cast pre�stressed concrete, and post-tension concrete piles. ◦ driven cast in-place (small displacement) piles. e.g. shell and steel H sections. Drilled/replacement piles or non-displacement e.g. bored piles, micro piles, and flight auger piles. 9 Types of Piles by load transmission 10 End bearing piles transfer the load directly to the pile base which rests on a relatively firm soil such as rock, very dense sand or gravel and the base of the pile bears the load of the structure. The load of the structure is transmitted through the pile into this firm soil. Examples of this type of pile are preformed timber pile and in-situ reinforced concrete pile. Friction piles transmit the load of the structure to the penetrable soil by means of skin friction or cohesion between the soil and the embedded surface of the pile. It is more likely to predominate in clays and silts. 11 Selection & Design Criteria Selection of pile type should be based on some consideration e.g.: • Topography: surface and drainage conditions • Soil condition at site • Type of structure and applied load • Equipment and technical difficulties such as obstructions etc. • Environmental condition such as adjacent structures, chemical conditions etc. 12 Pile Installation DRIVEN PILES Pile driving system 13 Construction of driven piles • Construction of pile foundation consists of driving the piles & installing pile caps • Most piles are driven by pile hammer, by alternately raising & dropping • Several types of pile hammers are available • Selection of a pile hammer for a specific job depends on a number of factors such as soil condition and pile material. 14 Pile Installation DRILLED PILES 15 Construction of drilled shaft Casing or slurry may be required when there is a potential of cave-in or if ground water table presents. The base of bored pile can be enlarged to provide greater end bearing capacity of suitable strata and resistance to uplifting. Construction of bored piles in deposits of dense sand and gravel is easier than driven piles, but this pile is also effective on soft ground or in situation where subsoil condition consists of different soil layers. Bored pile is versatile in which the depth and diameter of pile can be easily varied. Drilling equipment is relatively light and easy to use. Drilling process does not cause excessive noise and ground vibrations BEARING CAPACITY 17 Bearing Capacity of Piles Capacity of pile depends on structural strength & supporting strength of soil Soil strength ◦Bearing Capacity ◦Settlement Structural strength ◦size & shape ◦type of material 18 Qu = Qb + Qs Qu = qb Ab + fs As = + Df o u b b s p dh Q q A f Bearing Capacity of Piles 19 End Bearing Capacity End bearing capacity of piles can be calculated based on Terzaghi BC equation: qb = c Nc + q Nq + ½ γ B Nγ The Ng term can be neglected because the pile dimension B is usually very small compared to the depth of pile embedment. Since the shape of pile is usually square or circular, and the pile is placed at a substantial depth, the Nc and Nq should be adjusted to shape and depth factors. Furthermore, adjustment for pile weight should be made. qb = c Nc* + s ’ vo (Nq* - 1) 20 End Bearing Capacity Meyerhoff Terzaghi, General BC Berezantzev 21 Friction Bearing Capacity Ultimate skin resistance is produced at small values of relative slip between the pile and the soil. The slip is progressing down the shaft with increasing load. The amount of slip required to produce maximum skin resistance is on the order of 5 to 10mm. This is independent of pile diameter and embedment length, but solely depends of the soil properties. On the other hand, the mobilization of the base resistance requires a settlement of the order 10 to 30% of the pile diameter. There are three methods available for obtaining unit frictional resistance of pile. The a and l methods are normally used for piles embedded in clay, while b method is commonly used for pile in sand. 22 Friction Bearing Capacity the a method f s = a c + svo’ K tand a = empirical adhesion factor, c = is average cohesion for soil stratum of interest, svo’ = the effective vertical stress at depth of interest, K is the coefficient of lateral earth pressure, and d is the friction angle between the soil and the pile. 23 Friction Bearing Capacity the b method For cohesionless soil f s = b svo’ = K tand svo’ svo ’ = the effective vertical stress at depth of interest, K = the coefficient of lateral earth pressure d= the friction angle between the soil and the pile. Pile material K tand ( o ) Loose sand Dense sand Steel (corrugated) 0.5 1.0 Use tan of sand Steel (rough, rusted) 0.5 1.0 0.4 Steel (smooth) 0.5 1.0 0.2 Timber 1.5 3.0 0.4 Concrete 1.0 2.0 0.45 24 BC of Piles in Cohesive soil Qu = Qb +Qs Qb = Nc* cu Ab = 9 cu Ab where cu is the average cohesion in the vicinity of the pile base Qs = f s As = f s p L = a cu p L where p is the perimeter of the pile, L is the incremental pile length, and f s = a cu and cu is the average cohesion along the incremental length of pile 25 Notes on Piles driven in clay Soft clay adjacent to piles may lose a large portion of their strength as a result of being disturbed by pile driving. The original clay’s full strength is usually regained within a month after pile driving stops In cases where the pile has to be loaded immediately after driving, the effect of decreased strength must be taken into account Slender piles driven in soft clay have a tendency to buckle when loaded Heavy steel, timber & concrete piles do not tend to buckle if embedded in the soil for their entire length The ultimate structural load can be computed by: where l is a ratio between 8 and 10, c is cohesion, E and I are modulus and moment of inertia of the pile Q cEI u = l 26 Example 1 12 m Clay, g b = 16.5 kN/m3 gsat = 17.6kN/m3 cu ave = 105 kPa Pile diameter = 300 mm cu base = 170 kPa A pile of 0.6 m in diameter is driven into clay layer as shown in Figure. If the adhesion coefficient a is 0.45. Calculate the bearing capacity of the pile if it is embedded between depths of 1 m and 13 m 27 = = ( ) = 2 2 0.6 4 d 4 π π Ab 3 Qu Ultimate bearing capacity of pile Qu = qb Ab + f sAs qb = cu Nc * = 9 × 170 = 1530 embedded length of pile L = 13 – 1 = 12 m f s = a cu = 0.45 ×105 = 47.25 A s = d L = × 0.6 × 12 = 22.62 m2 Qu = qb Ab + f s As = (1530 × 0.2827) + (47.25 × 22.62) = 432 + 1068 Qu = 1500 kN For FS = 3 Qall = = 500 kN 0.2827 m2 28 BC of Piles in Cohesionless soil Qu = Qb +Qs the end bearing capacity of pile in cohesionless soil is: qb = Nq* svo ' The shaft friction can be estimated using b method Qs = f s As = f s p L = Ksvo' tan d p L where p is the perimeter of the pile, L is the incremental pile length, and svo' is the effective overburden pressure 29 Dense sand: Dc = 20d Loose sand: Dc = 10d d = diameter or the least dimension of pile 30 Example 2 Q design? Medium dense to dense sand g = 20 kN/m3 f = 36o K = 0.95 7.5 m Pile 300 mm diameter Dc = 6.0 m g × Dc = 20 kN/m3 × 6m = 120 kN/m2 A concrete pile is to be driven into sand to a depth of 7.5 m as shown in Figure. No groundwater was encountered during site investigation. Estimate the pile axial capacity if K = 0.95. Use FS = 2 31 The ultimate bearing capacity of the pile Qu = Ab qb + As f s For dense sand Dc = 20 pile diameter = 20 × 0.3 m = 6 m At depth of 6 m, svo' = 20 × 6 = 120 kN/m2 = = ( ) = 2 2 0.3 4 4 π d π Ab 2 766 Base resistance: Qb = qb Ab From Figure 4.10 f = 36o , Nq * = 60 qb = Nq * svo' = 60 × 120 = 7200 kN/m2 Qb = 0.073 × 7200 = 525 kN Friction Resistannce: Qs = fs As = K svo' tand p L Area of pressure diagram svo' L = ½ ×120×6+120×(7.5 – 6) = 540 kN/m p = d = × 0.3 = 0.942 m2 For concrete pile tand = 0.45 Qs = K svo' tand p L= 0.95×540×0.45×0.942 = 241 kPa Qu = 525 + 241 = 766 kN For FS = 2 → Qdesign = = 383 kN 0.073 m2 Pile in clay and sand layer Soils are not homogeneous in nature. There are cases where pile has to penetrate different layers of soil, some time of different types. The solution is to treat individual layer for friction bearing AND to consider the end bearing of soil at pile tip 32 Example 3 33 Clay Loose sand Medium gravelly sand gb = 18 kN/m3 cu = 80 kPa a = 0.70 gb = 17 kN/m3 f ‘= 23o gb = 19 kN/m3 f' = 34o 7 m 3 m 1 m GWT Pre-cast concrete pile 305 x 305 mm 34 Ultimate bearing capacity of pile Qu = qb Ab + fs As For pile embedded in medium gravelly sand qb = Nq * svo ' Take Dc = 20 pile diameter = 20 × 0.305 m = 6.1 m At depth of 6.1 m, svo ' = 18 × 6.1 = 109.8 kN/m2 For f = 34, use Figure 4.10 Nq * = 40 qb = Nq * svo' = 40 × 109.8 = 4392 kN/m2 Ab = 0.305 × 0.305 = 0.093 m2 Qb = qb Ab = 4392 × 0.093 = 408.5 kN 35 Friction resistance: Qs = (fs1 L1 + f s2 L2 + f s3 L3 ) p Clay layer: f s1 L1 = a cu L1 = 0.7 × 80 × 7m = 392 kN/m2 Loose sand: svo ' = 18 ×6.1 = 109.8 kN/m2 (uniform) K = 1 – sin 23o = 0.61 tand = tan (0.6 × 23) = 0.25 f s2 L2 = K svo' tand L2 = 0.61×109.8×0.25×1m = 16.75 kN/m2 Dense sand: svo' = 109.8 kN/m2 (uniform) K = 1 – sin 34o = 0.441 tand = tan (0.6× 34) = 0.372 f s3 L3 = K svo' tand L2 = 0.441×109.8×0.372×3 m = 54 kN/m2 Qs = (fs1 L1 + f s2 L2 + f s3 L3 ) p p=4B=4×0.305=1.22 m2 Qs = (392 + 16.75 + 54) × 1.22 = 564.5 kN Qu = 408.5 + 564.5 = 973 kN FS = 3, Qall = 3 973 = FS Qu = 32 kN 36 Empirical Bearing Capacity Pile capacity based on SPT values (Meyerhoff, 1976) The end bearing capacity qb = 40 N’ (Df /B)≤ 400 N (kPa) driven pile qb = (40/3)N’(Df /B) ≤ 400 N (kPa) drilled pile N = the corrected SPT N value near the pile base or within the range of 1 B above the tip and 2 B below the tip, Df = embedded length of pile, and B is the smallest dimension of the pile. Most piles have greater ratios, thus the upper limit nearly always control. 37 Empirical Bearing Capacity Pile capacity based on SPT values The friction bearing of the pile: f s = 2 N (kPa) large displacement piles f s = N (kPa) small displacement pile N is the average SPT value along the embedded length of pile. Note that these equations are applicable for piles embedded in cohesionless soils because the standard penetration test does not give reliable estimation of pile capacity in cohesive soil. An HP 310 steel pile is driven into medium dense sand at depth of 22 m. The smallest dimension of the pile cross-section is 308 mm. The corrected N value near the pile base is 45. Assume that friction resistance of the pile is to be neglected, calculate bearing capacity of the pile based on SPT value. qb = 40 N 400 N qb = 40 × 45 = 125871 kN/m2 38 B Df 0.308 22 3 Qb 3 1719 Limiting value qb = 400 × 45 = 18000 kN/m2 Since qb> qb limiting, then use qb limiting Qb = Ab qb = 0.0955 × 18000 = 1719 kN Friction resistance is to be neglected, then Qa = = = = 573 kN Example 4 Daya dukung tiang berdasarkan hasil sondir (Beggemann, 1965) Untuk Tip resistance qb berdasarkan Briaud and Milan (1991) qb = qc x kc dimana qc adalah tahanan konus rata2 dari 1.5B di atas tip sampai 1.5 B dibawah tip; sedangkan kc untuk pondasi tiang di tanah kohesif adalah 0.6 sedangkan untuk tiang bore 0.375. Nilai koreksi ini dimasukkan dalam FOS dimana untuk tiang pancang FOS = 3 sedang untuk tiang bore FOS = 5 Untuk friction resistance berdasarkan Nottingham & Schmertmann (1975) dimana untuk tanah kohesif f s = a x Tf dimana nilai a tergantung tipe tiang dan tipe sondir. Nilai a ini dimasukkan dalam FOS = 5. Empirical Bearing Capacity Pile capacity based on CPT values Berdasarkan referensi dari Beggemenn di atas maka Untuk Tiang pancang
| Item Type: | Other |
|---|---|
| Subjects: | T Technology > TA Engineering (General). Civil engineering (General) |
| Divisions: | Faculty of Engineering, Science and Mathematics > School of Civil Engineering and the Environment |
| Depositing User: | Mr Edi Surya Negara |
| Date Deposited: | 17 Feb 2022 05:10 |
| Last Modified: | 17 Feb 2022 05:10 |
| URI: | http://eprints.binadarma.ac.id/id/eprint/5777 |
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