PLICA Nº 1.- AUTENTICO SABOR DE HOGAR S.L

7.4 STEEL H-PILE

Steel H-Pile sections are rolled by steel mills in South Africa and are thus available for installation as piles. The cost of steel is relatively high, however, so it is not a widely used pile type due to the fact it is not normally an economical solution. It has advantages in that the steel sections can be driven through soil profiles which have minor obstructions in the form of cobbles and small boulders as well as fill materials containing builders rubble.

Positive features

• The pile has good penetrating ability.

• The pile can be installed to considerable depth- • H-sections are ideal for use as soldier piles.

Negative features

• Steel H-sections are relatively expensive. • Noise levels are high if the pile is driven.

• The end bearing capacity is limited unless driven onto rock.

PILE DETAILS

The H-section is rolled in two forms known as Universal Columns and Universal Bearing Piles. Details of these sections are given in Figure 7.4.1.

The working load shaft stress can be up to 125 MPa for Grade 43 steel and up to 165 MPa for Grade 50B steel. Using the higher stress the range of working loads varies between 500 to 3300 kN for the sections available.

INSTALLATION TECHNIQUE

The first section of shaft is lifted up by the piling rig and located into the piling helmet which is then released from the hammer. The first shaft section together with the helmet is then lowered down onto the pile position and the hammer is lowered so as to rest on the helmet. The leader of the piling rig is adjusted for verticality or rake.

The pile can be driven with either a vibratory, drop, diesel or hydraulic hammer. Once the first shaft section has been driven the second one, which has been prepared for welding, is lifted up and then lowered so as to line up with the first section. Guiding lugs tack welded to the first section assist in this operation. A butt weld is then carried out.

The pile is driven to a set which has been calculated to provide adequate bearing capacity. The set should be checked after 24 hours so as to determine whether there has been any relaxation. If there has the pile should be driven until the required set has been achieved and again checked after 24 hours.

The head of the pile is finally trimmed to the correct level. Shear transfer cleats or lugs may need to be welded to the head of the pile so as to ensure transfer of load from the pile cap into the pile.

VARIATIONS IN INSTALLATION TECHNIQUE

Predrilling and placing

In the case of soldier piles the alternative to driving is to auger a hole and drop the steel H- Pile sections in. This is common practice in all soil profiles which are cohesive and not saturated thus allowing the augering of the hole to take place without collapse.

One of the main advantages of placing the H-Pile sections in predrilled holes is the fact that position and verticality tolerances attainable can be very stringent whereas with driving this is not the case. Noise reduction is also an important factor.

Having placed and fixed the H-Pile section in position the annulus around it is filled with concrete or grout. Below the excavation level this concrete is full strength but above the excavation level a weak concrete or grout is used. The reason for this change is to facilitate the removal of the weak material from in front of the H-pile section on the excavated face thereby achieving a plane face to the excavation.

Predrilling and Jetting

To obtain relief from hard driving and to assist in reducing noise levels predrilling or jetting can be resorted to.

POTENTIAL PROBLEM AREAS

Noise Pollution

The driving impact of steel on steel is unfortunately one of the noisiest operations in piling. Even with a cushioning helmet on top of the H-Pile section there is still a high noise level. Certain measures have been used in Europe to reduce the noise levels and these involve

Pile Heave

Because there is limited displacement when driving steel H-Piles there is a very low risk of pile heave. If heave is measured then the piles should be re-driven.

Vibration

The vibration caused by driving can be problem but it is very difficult to forecast beforehand how severe the vibration will be. Because of the limited displacement associated with steel H-Piles the risk is even lower than with a precast pile. The general approach from a contracting point of view is to take the risk which is relatively low and sort the problem out in the unlikely event of one arising. Due cognizance should be taken, however, of the condition of surrounding buildings and whether these buildings are occupied by businesses whose operations are susceptible to vibration such as photographic studios and lens grinding.

Bent Pile Shafts

Because of the elastic properties of steel it has been observed that the shafts of steel H-Piles can be deflected and bent during driving particularly if there are obstructions in the ground. Unfortunately there is no easy way in which to monitor whether bending has taken place and if so to what degree other than if the toe of the pile emerges from the ground surface! A pile load test programme will indicate whether the performance of the piles is acceptable but this is an expensive exercise. A study of the founding levels and driving records may highlight discrepancies and suggest a limited number of piles for load testing.

7.5 TIMBER PILES

This pile type is not often used in South Africa but it has been included in this text for the sake of completeness. Permanent timber piles have been used in harbour construction in Southern Africa but because of severe attack by borers their life was limited even with very hard woods. The use of timber piles in permanent works is presently limited to very light structures such as marina jetties. They are also used for temporary staging over water and stitching earth embankments. The timber used is generally one of the gums such as saligna.

Positive features

• Timber piles are relatively cheap.

• They are relatively light to transport and handle.

Negative features

• The available sizes and lengths are limited. • Load capacity is very limited.

• Timber is su.bject to attack by various insects and organisms. • There is risk of the timber pile splitting during driving.

PILE DETAILS

The sizes of readily available timber piles is dictated by what one can purchase from the Department of Forestry or from one of the commercial suppliers. Smaller sizes are readily available but the larger sizes have to be ordered and delays can be experienced. Timber piles used in temporary staging are typically 12 metres in length with a tip diameter of 250 mm. Larger sizes are available locally but are more difficult to source. Special hard woods can be procured from some West African countries but these are expensive.

Timber piles driven as a temporary staging are normally friction piles with a capacity normally limited to 200 kN. With the larger sizes of timber pile and dense soil conditions this could be increased.