Deep Foundation Elements (DFEs) .1 General

The term Deep Foundation Element (DFE) shall include all foundation elements designed to transfer loads by shaft friction and/or end bearing. These elements shall include but not be limited to piles, pile groups, diaphragm walls, barrettes, secant and contiguous bored pile walls and all other similar load bearing structures. In the event of any uncertainty as to whether or not a particular foundation type or element is covered by this definition written clarification shall be sought from the Engineer. The Engineer’s decision shall be final and binding.

Deep foundation elements shall be used where other types of foundations are not suitable.

When choosing the type of foundation consideration shall be given to the impact of noise and vibration during DFE installation and current legislation on the use of piling and other construction equipment.

For cast in situ DFEs the minimum grade of concrete used shall be G30.

Timber piles shall be permitted only for lightly loaded structures to the acceptance on the Engineer.

In designing precast piles and pile joints, stresses arising from impact and shock from piling hammers shall be considered.

6.2.3.2 Settlement

The maximum allowable foundation settlement shall be in accordance with Section 6.3, Table 6.1

Settlement calculations shall take in to account both short term and long term settlements and shall include inter alia immediate (non-recoverable), elastic, consolidation and creep settlements both at the DFE/soil interface and within the DFE itself.

6.2.3.3 Negative Skin Friction

Negative skin friction shall be considered in the design in marine soils, alluvial soils, estuarine soils, made ground and any other material that is prone to consolidation or is to be consolidated.

Computation of negative skin friction shall be by effective stress analysis.

6.2.3.4. Working Loads

The working loads of the DFE shall be the greater of:

(1) Applied dead load + Negative skin friction and non-transient live load.

(2) Applied dead load + total live load.

6.2.3.5 Lateral Loads

Where lateral loads are anticipated, the DFE shall satisfy the requirements given in Table 6.1. The analysis shall use “p-y” curve, finite element or finite difference. The use of such a method shall be subject to prior acceptance by the Engineer.

6.2.3.6. Ultimate Bearing Capacity

The size of the DFE shall be demonstrated to be sufficient to provide the required bearing capacity and meet the settlement criteria. Total stress analysis and effective stress analysis shall be carried out with the more critical of the two adopted.

The overall factor of safety on working loads shall be not less than 2.5, unless otherwise accepted by the Engineer. In addition the factor of safety shall be not less than 1.5 in shaft friction alone, except in the following cases:

Case a. When the DFE has been installed by driving

Case b. Where there is safe man access to the base of the DFE, any loose or remoulded material is removed from the base, and the base inspected and confirmed dry before casting the concrete

Case c. If the DFE is base grouted, using a proven method of base grouting to the satisfaction of the Engineer.

Case d. Where the end bearing is provided by S1 or G1 rock, and toe coring is carried out to confirm the pile/rock contact for every DFE.

If the design is based on any of these four cases, the appropriate requirements for driving, access and inspection, base grouting or toe coring must be included on the loading plan drawings. The designer may specify which of the cases is to be used, or may allow the contractor the option of selecting from two or more of these cases.

The DFE design shall be verified by Preliminary Load Testing.

6.2.3.7. DFE Interaction

The failure of a DFE group shall be checked in settlement and in bearing capacity. The interaction between DFEs shall be assessed and considered in the calculation of the capacity and settlement based on Poulos and Davis ⁽¹⁾ or other method accepted by the Engineer.

6.2.3.8 DFEs Acting in Tension

The factor of safety against failure shall be demonstrated to be not less than 3.5 if the skin friction is derived from preliminary DFE tests carried out in compression. In addition, for driven piles the ultimate skin friction under tension shall be taken as no more than 75% of the ultimate friction measured in compression. No reduction factor is required for bored piles.

Where the skin friction is derived from preliminary DFE tests with the DFE loaded in tension, then a factor of safety of 2.5 shall be used.

DFE groups under tension loading shall be checked for:

a) The sum of the uplift resistance of the individual DFEs, allowing for interaction effects.

b) The sum of the shear resistance along the perimeter of the group and the effective weight of the soil and DFEs within the perimeter.

All structural connections shall be designed for the design tensile force with appropriate factors of safety.

6.2.3.9 Effect of Future Developments

In addition to the calculated vertical and lateral loads, the designer shall allow for the effects of future developments; to do this, he shall demonstrate that the DFEs are designed to allow for movements of 15mm in any plane at the junction of the DFE and the structure.

6.2.3.10 The structural design of DFEs shall meet the following requirements:

a) Pure Compressive Axial Load

Concrete/reinforced concrete DFEs subject to axial load only shall be designed such that average compressive stresses across the whole section of the DFE at serviceability limit state shall not exceed 0.25 times the characteristic cube strength of concrete at any point along the DFE. The designer may propose to increase the allowable compressive stress for piles that are provided with a permanent casing, to the acceptance of the

b) Axial Load with Coexistent Lateral Load

Concrete/reinforced concrete DFEs shall be checked at both the ultimate and serviceability limit states. In addition, concrete bored piles subjected to both vertical and lateral loads shall be designed such that maximum combined bending and compressive stresses at the extreme fibre, for the whole length of the pile under working load condition shall not exceed 0.3 times the characteristic cube strength of the concrete. The average combined bending and compressive stresses across the whole section shall not exceed 0.25 times the characteristic cube strength of the concrete.

c) Pure Tensile Axial Load

The safe tensile axial working load of the DFE shall be determined by multiplying the cross-sectional area of steel reinforcement with the permissible tensile stress in high yield steel reinforcement. The permissible tensile stress under working load condition (serviceability limit states) shall be 130N/mm² for deformed, type 2, grade 460 bars. The DFE shall be reinforced to the depth necessary to mobilise the required ultimate skin friction capacity in tension. Laps shall be avoided wherever possible. Where laps are necessary they shall be full strength laps assuming the reinforcement is working at the ultimate limit state at full design strength (i.e. characteristic strength of reinforcement divided by the partial safety factor for reinforcement).

d) The durability assessment shall demonstrate how the durability of DFEs will be achieved over the design life of the structure.

The minimum condition of exposure per SS CP 65 Table 3.2 shall be taken as ‘severe’, except where more onerous conditions are required elsewhere (such as in Chapter 8).

Particular consideration shall be given to:

• Where the DFE is in tension in any area that is in direct contact with fill or made ground

• Where the DFE is subject to wetting and drying cycles due to fluctuating water levels

• The chemistry of the ground and water around the DFEs.

Where the durability assessment demonstrates that durability of the DFE is of concern, then suitable measures shall be taken to improve the durability. Measures to be considered would include one or more of: sacrificial concrete, sacrificial outer casing, protection to the reinforcement, cathodic protection, or other suitable measures