Thursday 3 June 2010

PILING, DIAPHRAGMS AND RETAINING WALL SYSTEMS

PILING, DIAPHRAGMS AND RETAINING WALL SYSTEMS

VIBRO-REPLACEMENT AND VIBRO-COMPACTION

Introduction

Where structures cannot be safely founded on loose soils or fill material, piling may be considered as a means of transferring the loads to suitable levels. However, piled foundations are not the only means of achieving satisfactory foundations in such situations - the Engineer may wish to consider geotechnical processes or vibratory processes.

The most economical is likely to be cement injection. However, vibratory processes can also be used to consolidate and strengthen ground conditions at a comparatively low cost; this is achieved by stabilising the soil so that greater loads can be carried without risk of settlement. It also allows simple, shallow foundations to be used on otherwise poor sites. Vibratory processes can also be used on recently filled sites which contain brick rubble, soil, concrete or other miscellaneous material. This means that sites which were considered totally unsuitable for construction operations can now be considered.

Methods and materials

Two principal methods are employed for the compaction of the ground, namely vibro-­replacement and vibro-compaction.

Vibro-replacement

In the vibro-replacement method stone columns are constructed through weak soils to improve their load-bearing and settlement capacities. There are three processes, the dry process, the bottom feed process, and the wet process. In the dry process a heavy vibratory unit is allowed to penetrate the weak soil to the designed depth and the cavity is filled with stone, the stone is compacted in stages. The vibrator is up to 4 metres long and weighs about 2 tonnes. It is either suspended from a crane or held in crane leaders. In the bottom feed process the vibrator is mounted on a specially designed track ­mounted machine, which employs leaders to guide the vibrator. A heavy duty tube on the outside of the vibrator carries stone to the tip of the vibrator. Stone is fed into the tube via a stone reservoir, which in turn is fed by a skip travelling up and down the leaders. The vibrator is powered by a diesel generator mounted on the machine. The vibrator remains in the ground during the construction of the stone columns - the stone being supplied to the tip of the vibrator. Stone columns up to 15 metres deep can be formed in this way.

In the wet process the vibrator is suspended from a crane and the weak soils are removed by water jetting. Stone backfill then replaces the weak soil and is compacted into the surrounding ground. This process requires a water supply of 10,000 to 12,000 litres per rig hour. The stone columns formed by these processes can carry loads of 10 to 40 tonnes, but a safe bearing factor of 3.0 is applied to the calculated ultimate loading.

Vibro-compaction

This process is based on the fact that non-cohesive soils such as sand, can be compacted into a denser state by vibration. The action of the vibrator, which is often accompanied by water jetting, reduces the forces between the granular material allowing it to consolidate to the optimum density. This form of compaction is permanent and can be used in loose soils up to 29 metres in depth. The sequence of operations is shown in the sketch. The high relative density leads to high safe bearing capacities and permit the use of shallow foundations designed to bear pressures between 250 kN/m2 and 500 kN/m". The process is not suited to cohesive soils simply because such soils do not respond to vibration.

In addition to the techniques described above, vibrated concrete columns may be employed instead of stone and gravels.

Economic considerations

These processes may be an economic alternative to piling and grouting methods used to improve bearing capacity. However, the site must be large enough to justify the use of the special equipment involved in the process. Since the depth compaction using these methods is approximately 12 metres, it can be used satisfactorily only on sites which will provide suitable resistance at these depths. Vibro techniques are capable of achieving safe bearing pressures of up to 500 kN/m2.

While these pressures are suitable for most spread foundations, they may prove unsuitable for concentrated loads such as those found in framed buildings, and the formation of extensive capping beams may be uneconomic. In both cases large quantities of fill material have to be used. Where stone is used for forming columns in very soft soil, the quantity of material used may produce cost figures which are only marginally cheaper than conventional piles, and the latter will give much higher safe bearing capacities. The vibro-compaction process is particularly valuable for consolidating loose sands prior to the formation of raft foundations and may be used in conjunction with raft construction more economically than piling.

Please refer to ‘Introduction to Civil Engineering Construction’ by Roy Holmes for a complete explanation.

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