1. Bridge Engineering (3) Foundations – Pile Foundations (1) 1. *Types of foundations 1.1 Shallow foundations (buried less than 5m deep) 1.1.1 Spread (isolated) footings: supporting a concentrated load, such as a loading delivered by a column; 1.1.2 Strip (wall) footings: supporting a line load, such as a loading delivered by a bearing wall; 1.1.3 Grade beams: supporting a repetitive series of concentrated loads, such as a loading delivered by a line of several columns; 1.1.4 Slabs, rafts, and mats. 1.2 *Deep foundations (buried 5m deep or over) A deep foundation is a foundation unit that provides support for a structure by toe resistance in a competent soil or rock at some depth below the structure, and/or by shaft resistance in the soil or rock in which it is placed. 1 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

2. Bridge Engineering (3) Foundations – Pile Foundations (1) 1.2.1 Pile foundations 1.2.2 Caisson foundations 2. Pile foundations * 2.1 Types of pile foundations 2.1.1 According to installation: driven, bored, cast- in-situ; jetted, excavated, and augered; 2.1.2 According to displacement: displacement, low-displacement, and non-displacement; 2.1.3 According to material: concrete, steel, and wood; 2.1.4 According to function: shaft bearing, and toe bearing; 2.1.5 According to capacity: high, moderate, and low; 2 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

3. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.1.6 According to shape: square, round, hexagonal, octagonal, and H-section; 2.1.7 According to environment: land, marine, and off-shore; 2.1.8 According to inclination: vertical and battered; 2.1.9 According to length: long and short; 2.1.10 According to structure: bridges, buildings, platforms, towers, machinery, etc. 2.2 Geotechnical design of pile foundations 2.2.1 The design method depends on the soil in which it lies (cohesive—clay or cohesionless—sand; or soil or rock). 3 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

4. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.2.2 Each pile design should be based on consideration of both stability (load capacity) and serviceability (expected deformations or settlements). 2.2.3 Consideration for pile design is given to the axial capacity and settlement of piles in soil and onto rock, and the lateral capacity and lateral movement of piles in soil and onto rock. 2.2.4 Piles derive their load-carrying from both toe and shaft resistance. The relative contribution of each to the total capacity of the pile depends, essentially, on the density and shear strength of the soil and rock and on the characteristics of the pile or pile groups. 4 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

5. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.2.5 The capacity of a single pile can be estimated by summing the shear stresses along the shaft, and adding the bearing capacity of the pile base. 2.2.6 The allowable loads on bored piles are determined from a combination of shaft resistance and toe resistance, after the application of appropriate factors of safety on the calculated resistance values. 2.2.7 A closely spaced pile group can act as a “block” whereby the soil between adjacent piles is dragged down between them, shaft resistance develops around the perimeter of the group only, and end-resistance develops under the whole of the pile-soil block. 5 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

6. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.2.8 The objective with the dynamic methods of pile design is to relate the dynamic pile behavior (acceleration or driving resistance) to the ultimate static pile resistance. 2.3 Structural Design of pile foundations 2.3.1 The structural strength of a deep foundation unit represents the load which the unit can support as a structural member. 2.3.2 In most cases, the bearing capacity of a deep foundation unit is governed by geotechnical considerations, rather than by the structural strength of the unit. 6 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

7. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.3.3 In determining the structural capacity, it is important that permissible deviations in alignment and location of the unit be established and considered in design. 2.3.4 The geotechnical capacity of a driven pile is a function of the dynamic response of the pile, i.e., the dynamic impedance EA/c, where E is the modulus of elasticity, A is the cross-sectional area of the pile, and c is the speed of the strain wave in the pile. 2.3.5 The strength of the pile material has no influence beyond a minimum value. The geotechnical capacity of a driven pile differs from the structural capacity. 7 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

8. Bridge Engineering (3) Foundations – Pile Foundations (1) * 2.4 Precast and prestressed concrete piles 2.4.1 Use of precast and prestressed concrete piles; Because of their structural strength and wide choice of possible dimensions, precast and prestressed piles have a wide range of loading as follows: 2.4.1.1 as shaft bearing piles when driven in sand, gravel, or clays; 2.4.1.2 as toe bearing piles; 2.4.1.3 suitable for resisting uplift forces, when designed for it; 2.4.1.4 suitable for driving in soils containing boulders, when correctly designed; 8 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

9. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.4.1.5 for depths up to 15 m for precast concrete piles and up to 40 m for prestressed concrete piles without splicing devices, and to greater depths with splicing devices; 2.4.1.6 with typical square, hexagonal, and octagonal, or cylindrical cross sections (The larger diameter cylinders are usu. hollow and prestressed); 2.4.2 Pile splices 2.4.2.1 The length limit of concrete piles due to handling conditions calls for special mechanical splices to allow the construction of very long precast concrete piles. 9 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

10. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.4.2.2 General requirements for splices are as follows: * The strength of the splice must be comparable to that of the pile in compression, tension, and bending; * The splice must be designed and positioned so as to ensure and maintain the alignment of the joined pile segments; * The splice must be designed so that the tolerance play (slack) between two joined pile segments is less than 0.5 mm in either compression or tension (values in excess of this amount will impair the drivability of the pile). 10 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

11. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.4.3 Installation 2.4.3.1 Required quality for piles * structural integrity before driving: careful inspection before driving and rejection of those severely fissured, spalled, or otherwise damaged; * pile head: perpendicular to the pile axis in order to avoid uneven distribution of impact forces; usu. protected by a steel plate of 13 mm thick anchored in pile by means of separate reinforcing bars; encased with a steel collar connected to the head plate; * pile toe: chamfered at the edges and corners for common piles; attached with a special steel shoe for toe bearing piles under hard driving conditions; 11 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

12. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.4.3.1 Required quality for piles * central tubes for inspection: for piles longer than 25 m 2.4.3.2 Driving hammers * Types of hammers: drop and diesel hammers, single acting and differential acting air/steam hammers. (vibratory hammers not recommended). * Height of fall and impact velocity: during early driving, reduction of ram velocity or drop height to avoid the formation of tension cracks. * Dimensions: fit loose to avoid the transmission of torsion or bending forces but not as loose as to prevent the proper alignment of hammer and pile; * Cushion (usu. of plywood): to avoid damage to the head of piles and to assist in controlling tensile stresses 12 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

13. Bridge Engineering (3) Foundations – Pile Foundations (1) * 2.5 Steel pipe piles 2.5.1 Use of steel pipe piles (driven with the lower end of the pipe open or closed, or left open or filled with concrete, or used as shaft-bearing, toe-bearing or socketed piles): * variable length because of easy cuts and splicing; * diameters up to 600 mm; and * loads up to 1800 kN 2.5.2 Materials * steel: the application of coatings to protect the steel, such as coal tar epoxy before driving, encasement by cast-in-place concrete jackets, cathodic protection, inclusion of copper content in the steel; * concrete 13 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

14. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.5.3 Installation * piles driven with closed toe: a float plate of 10- to 20- mm thickness with a diameter 20 mm larger than that of the pile under normal driving conditions; a special shoe of cast iron for weathered rock or through bouldery soil. * driven with open toe: a special driving shoe needed for dense gravel; and * driving equipment: a wave-equation analysis used in selection of the appropriate hammer. 2.5.4 Common installation problems * high porewater pressure may develop in fine-grained soils due to pile driving, causing thin-wall piles to buckle locally. 14 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

15. Bridge Engineering (3) Foundations – Pile Foundations (1) * high porewater pressure may develop in fine-grained soils due to pile driving, causing thin-wall piles to buckle locally; * the impedance of thin-wall pipes may not be sufficient to allow the development of sufficient force in the pile to achieve adequate pile capacity; *2.6 Bored piles 2.6.1 Use of bored piles [among different shapes and dimensions, cylindrical piles are most common with elements of diaphragm walls (I, H, X-shaped)]: * best for toe-bearing, high-capacity piles to rock or dense till; 15 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

16. Bridge Engineering (3) Foundations – Pile Foundations (1) 2.6.1 Use of bored piles * in stiff clay; and * for variable lengths (up to 100 m, diameters in excess of 1 m and up to 3 m) 2.6.2 Materials * structural steel casings; * tremie or pumped concrete 2.6.3 Installation * excavation: __ by using a large auger or bucket drill to remove the soil above the founding level; 16 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

17. Bridge Engineering (3) Foundations – Pile Foundations (1) * excavation: __ by driving, vibrating, or pushing down a heavy casing to the proposed founding level, and by removing the soil from the casing either continuously as driving proceeds or in one sequence after the casing has reached the founding level; __ by using a clamshell mounted on a Kelly bar to remove soil, and by keeping the excavation open by use of a bentonite slurry; __ by drilling, coring, or chopping when penetration into rock is specified (blasting could adversely affect the properties of surrounding rock and soil). 17 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015

18. Bridge Engineering (3) Foundations – Pile Foundations (1) * placing concrete __ placed during one continuous operation; __ accurately placing and adequately supporting steel reinforcement, steel stubs, or core sections; __ placed by buckets, funnels, or chutes to prevent segregation when excavation dry. __ best done by pumping, and tremie used with adequate safeguards 18 Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015Saturday, October 24, 2015