Installation and driving assistance
D.1 Driving method
In the 'pitch and drive' method a single or double sheet pile, is driven to full depth before pitching the next one. This simple procedure has the advantage that the top of the sheet pile has only to be lifted a distance equal to the length of the pile above the ground surface. Moreover it easily can be guided manually into the interlock of the sheet pile which has already been driven.
In the case of dense sands, stiff cohesive soils and in soils containing obstructions, the 'pitch and drive" method can lead to de-clutching problems in the free leading interlock and occasionally to rather large deviations from the required position.
"Panel driving" and "staggered panel driving", enables better control of the position of the sheet piles along the wall length. At the same time the danger of declutching is minimised.
As a whole panel is pitched it is not necessary to drive all the sheet piles to full depth in order to maintain sheet-piling operations. If obstructions are encountered, individual sheet piles can be left high without disruption to the installation process.
"Staggered driving" is a particular form of "panel driving" which may be applied when difficult soil conditions are encountered. The sheet piles in the panel are driven in a sequence indicated in figure D.1.
The disadvantage of the "panel driving" method is that interlocking the sheet piles requires individual piles to be lifted to twice their length.
Driving the primary elements of a combined wall causes local compaction of the soil. This can cause problems when driving adjacent primary elements and it should therefore be taken into account when selecting the driving sequence.
To reduce the likelihood of de-clutching, the primary elements of a combined wall are driven in sequence to the design depth of the secondary elements. Having completed this succesfully, the secondary elements can be pitched and driven. Thereafter the primary elements are driven to the full depth.
|a direction of sheet pile installation
c driving direction (4, 2)
|b driving direction (1, 3, 5)
d upper guide
e lower guide
D.2 Driving assistance
It is often necessary to loosen very dense sand layers.
Normally applied methods are :
a) low pressure jetting with low water volumes :
- pressure : 1,5 MPa to 2,0 MPa;
- discharge : 2 l/s to 4 l/s per tube;
- diameter of pipes : approx. 25 mm;
- number of pipes : 1 to 2 per sheet pile.
The pipes are welded to the sheet piles and left in situ.
b) high pressure jetting :
- pressure : 25 MPa to 50 MPa (at pump outlet);
- discharge : 1 l/s to 2 l/s;
- pipe diameter: 20 mm to 30 mm;
- nozzle diameter: 1,5 mm to 3,0 mm.
c) predrilling, with or without cement bentonite.
d) blasting in special cases.
Low pressure jetting is mainly used in dense non-cohesive soils.
Low pressure jetting with low water volumes, in combination with a vibrator, enables sheet piles to penetrate very dense soils. In general the soil characteristics are only slightly modified and there is practically no settlement, although special care has to be taken when the sheet piles have to carry vertical loads.
This method, especially in combination with high frequency vibration is recommended.
In addition, low pressure jetting is sometimes used for pre-treatment of the soil prior to pile driving.
Low pressure jetting with high water volumes is rather crude, especially when the tubes are not fixed to the sheet piles, and is therefore not recommended.
High pressure jetting or fluidisation can be very effective in very dense soil layers.
Limited amounts of jetting fluid, water or sometimes cement-bentonite, are introduced into the ground through nozzles fixed to the sheet pile at a short distance above its tip. As a result of the limited water consumption this method permits effective control of the pile. The soil properties are only adversely effected in a limited area around the sheet piles. The overall performance will not be significantly influenced.
Pre-drilling is sometimes carried out prior to the sheet pile driving. The soil is locally loosened by this process. Normally flight auger drills are used. Pre-drilling is often carried out at the positions of the fixed interlock of a double sheet pile, but it can be more effective to pre-drill at the free interlock positions. If extreme difficulties with driving are expected or when there are special requirements regarding the Watertightness of the sheet pile wall, it is convenient to use the flight auger to exchange a column of soil at the interlock positions and replace it by cement-bentonite.
Fracturing by blasting is normally carried out if the sheet piles have to pass hard obstructions in the soil or if they must penetrate bedrock.