Queries about sheet piling arises from time to time, varies from project to project, differ from experience to experience. Here’s to cite few of them.
1. What is the proper vinyl sheet pile driving method for different soil types?
“Pitch and drive” method is the most practical way of pile driving in loose soils especially when using short piles. Also called “set and drive”, this method installs piles one by one. However, with only one interlock in control, sheet piles have high tendencies of rotating from the axis and leaning forward or backward.
Panel driving is recommended for the dense deposit of sands and compact cohesive soils such as heavy clay or in presence of obstructions. In this method, a number of piles are interlocked together before driving. The panel of piles is supported by guide frames and driven gradually in sequence, sometimes in a staggered manner. The method can achieve installations of longer piles in more difficult ground conditions with better results in sheet pile alignment and verticality.
In cases of difficult ground conditions such as hard strata and the presence of obstructions, driving assistance is employed along with a proper selection of driving method and equipment. Jetting and drilling help by loosening or eliminating obstructions by creating pressure right under the tip of the sheet pile being driven. Long and slender PVC sheet piles require steel mandrels as supports when driven in such conditions. Mandrel, which shape is similar to sheet pile profiles, increases the rigidity of Plastic Sheet Pile and supports the weight of the driving equipment such as a vibro-hammer.
2. What is the proper machine to drive piles in different conditions?
Impact driving is suitable for all soil types but appears to create the highest degree of stress to sheet piles. It is more appropriate to cohesive soils and soft soils such as silts and peats, loose medium to coarse sand, and gravels free of rock inclusions. Drop hammers such as the hydraulic hammer are the common types of impact-driving machinery.
Driving using vibratory hammers is ideal for non-cohesive or granular soils such as soft soils and round-grained sand or gravel. It has also shown satisfactory performance in sandy, silty, or softer clay soils. It is less effective in angular-grained materials and soils with a firm consistency. With the introduction of vibration, however, the sub-soil can be further compacted, increasing penetration resistance and ultimately-refusal. If this is the case, driving aids like jetting and drilling might be necessary.
The panel driving rigs and pressing rigs such as the “Japanese Silent Piler” are suitable in heavy clays and exemplary in soft cohesive soils.
More complex conditions may require a combination of light impact driving and pressing. These can be cohesive clays, dense fines, medium and coarse sand and gravels, and soft to medium rock strata. Multi-layered soil profile might also need combinations of different equipment and methods practically as possible.
3. What are the pvc sheet pile selection criteria for soil condition of different hardness?
General rule of the thumb: “the harder the soil, the stronger and stiffer sheet pile section should be used”.
Driveability of a pile section is a consequence of its cross-sectional properties (thickness, depth, and width), designed shape, length, grade, and the applied load. Hardness of soil strata characterized by SPT or CPT values is used to predict soil resistance which is essential in the calculation of driving force to be applied. Knowing the range of force from the selected driving machine, corresponding sheet pile sections can be picked according to their yield strength and elastic modulus.
The larger the surface area, the greater driving force is required to drive a sheet pile to a given depth. To avoid unnecessary stress and deformation, the sheet pile section should be compatible with the prevailing site conditions.
Higher material grades can withstand higher stresses.
Selection of a suitable plastic sheet pile section for hard and dense strata is a complex process. The factor that plays a valuable role in this aspect is experience.
4. How should I choose a sheet pile section for soils with, say, SPT Value, N=30? Can I still consider using vinyl or shift to steel?
Every sheet pile material has a design purpose, strengths, and areas of improvement. The availability of different materials denotes the need for a special material for a specific application. Plastic or synthetic sheet piles for example are known for being light and corrosion-resistant; Steel sheet piles are recognized for their extraordinary strength and versatility; and so on.
SPT value, N=30 generally denotes medium-dense to dense non-cohesive soils; and very stiff to hard cohesive soil profiles. This figure can be an implication of a need for higher force or higher amplitude of driving, thus, more superior sheet pile sections should be selected.
If SPT values >50 in cohesive soils, water-jetting should be considered. If there is a high content of fine particles (<0.1mm), filling of the leading interlocks with foam or grease is recommended.
If CPT values>1.0MPa in non-cohesive soils, pre-drilling and strengthening of the pile toe is essential.