Prolock Sigma is a hybrid sheet piling system. The plastic panels hold the ground, but the wooden posts give the build its strength. The right post choice is therefore necessary for a wall that stays stationary for generations.
In this article we cover which posts are suitable, how far completely they should be positioned, how deep they should penetrate, and which choice fits your soil type and wall height.
The role of posts in the Prolock system
With Prolock Sigma, the plastic panels are slid between the posts. The posts act as vertical supports that take the ground pressure. Without posts, the panels cannot hold the sideways pressure of soil and water.
The posts must:
- Sit deep enough in the ground so they do not fall over.
- Sit at the right distance so the panels do not bow.
- Be thick enough to take the ground pressure.
- Last a long time, because the plastic panels endure 50-plus years.
Post types compared
There are three common post types for Prolock Sigma walls.
1. Round milled spruce posts, the most utilised type
- Material: spruce wood, machine-milled round.
- Diameter: 10cm as standard.
- Lengths: 2,00 m, 2,50 m, 3,00 m and 4,00 m.
- Life: 15 to 20 years if untreated.
- Price: 9 euro 50 to 19 euro, depending on length.
- Suitable for conventional residential walls up to 150cm.
Advantages: light and easy to drive, most economical option, generally obtainable.
Disadvantages: reduced life than the Prolock panels (15 to 20 years), not appropriate for very hard ground, replacement may be necessary after 15 to 20 years.
Tip: Choose preserved spruce (class 4, autoclave treated) if you want to extend the lifespan to 25 to 30 years.
2. Hardwood posts in Azobe or Okan
- Material: tropical hardwood, Azobe or Okan, durability class 1.
- Diameter: 10 to 12cm.
- Lengths: 2,00 m to 5,00 m.
- Life: 25 to 40 years.
- Price: 25 to 60 euro, depending on length and kind of wood.
- Suitable for more substantial builds, higher walls and hard ground.
Advantages: life preferable aligned with the Prolock panels, stronger and stiffer than spruce, better in hard soil types, responsibly sourced available.
Disadvantages: heavier and harder to drive manually, more expensive than spruce, may release tannin that browns the water for a while.
3. Steel tube posts
- Material: galvanized steel or corten steel.
- Diameter: 60 to 100 mm.
- Lengths: manufactured to size.
- Life: 30 to 50 years if galvanized.
- Price: 40 to 80 euro per post.
- Suitable for very hard ground such as sand or gravel, high walls, and trade undertakings.
Advantages: extremely sturdy and thin, less visible, preferable where wood breaks in hard soil, long life with a proper coating.
Disadvantages: the most costly alternative, frequently requires mechanical installation, can rust if the coating is damaged.
Which post for which situation?
- Pond edge, up to 50cm: spruce post 10cm, 2,00 m. Lightweight project, low ground pressure.
- Ditch wall, 80 to 100cm: spruce post 10cm, 2,00 to 2,50 m. Typical circumstance.
- Bank wall, 100 to 150cm: hardwood post 10cm, 2,50 to 3,00 m. Higher pressure, extended lifespan.
- Heavy bank, 150 to 200cm: hardwood post 12cm, 3,00 to 4,00 m. Substantial loading.
- Hard ground, sand or gravel: steel tube post 76 mm. Wood breaks when driven in hard ground.
- Trade or public work: steel tube post or hardwood 12cm. Higher requirements on rigidity and life.
Post spacing: how far apart?
The maximum distance between posts depends upon the wall height and the ground pressure.
- 50cm panel: max 1,50 m between posts. 7 posts per 10 metres.
- 80cm panel: max 1,50 m between posts. 7 posts per 10 metres.
- 100cm panel: max 1,25 m between posts. 9 posts per 10 metres.
- 120cm panel: max 1,25 m between posts. 9 posts per 10 metres.
- 150cm panel: max 1,00 m between posts. 11 posts per 10 metres.
- 200cm panel: max 1,00 m between posts. 11 posts per 10 metres.
Rule of thumb: For walls up to 80cm, 1,50 m spacing is enough. For 100cm and up, reduce to 1,25 m or 1,00 m. Always have a strength check prepared for higher walls.
How deep should the posts go?
Driving depth is crucial for stability. The post must have enough ground resistance so it does not tip under pressure.
Minimum driving depth
- 50cm panel: at minimum 60cm in the ground. Post length 1,50 to 2,00 m.
- 80cm panel: a minimum of 80cm in the ground. Post length 2,00 m.
- 100cm panel: at least 100cm in the ground. Post length 2,00 to 2,50 m.
- 120cm panel: at least 120cm in the ground. Post length 2,50 m.
- 150cm panel: at least 150cm in the ground. Post length 3,00 m.
- 200cm panel: at least 180 to 200cm in the ground. Post length 3,00 to 4,00 m.
Rule of thumb: Driving depth must be at least equivalent to the wall height. On soft soil such as peat or soft clay, add 25 to 50 percent extra.
Correction per soil type
- Firm sand: factor 1,0. For a 100cm wall, drive 100cm.
- Firm clay: factor 1,0. For a 100cm wall, drive 100cm.
- Soft clay: factor 1,25. For a 100cm wall, drive 125cm.
- Peat: factor 1,5. For a 100cm wall, drive 150cm.
Driving methods
- By hand with a sledgehammer and pile driver: soft to intermediate soil, posts up to 2,50 m. You require a 5 kg sledgehammer and a wooden pile driver.
- Hand vibrating hammer: average soil, posts up to 3,00 m. You can rent this from a hire shop.
- Water injection: granular soil. You need a garden hose or a water pump and an injection lance.
- Machine with a mini pile driver: hard ground, long posts, trade work. Hire a piling machine with an operator.
Tips for driving
- Always utilise a pile driver on top of the post. Driving without a block fractures the head.
- Pre-drill in hard clay. Drill to fifty percent of the driving depth, then drive the rest.
- Prefer wet driving. Moist soil offers less resistance than arid soil.
- Check after each post that it is upright and on the line.
- Start with corner posts. They set the remainder of the construction.
Extend post life
Spruce posts last 15 to 20 years. Prolock panels last 50-plus years. There are strategies to extend post life.
Option 1: Choose hardwood
The simplest alternative. Hardwood posts last 25 to 40 years. That is closer to the life of the panels.
Option 2: Preserved spruce
Autoclave-treated spruce posts in class 4 last 25 to 30 years instead of 15 to 20. A bit more expensive than untreated, but a good intermediate option.
Option 3: Plan post replacement
If you choose untreated spruce, plan to replace the posts after 15 to 20 years. The Prolock panels can be pushed up. Then you remove the old post and drive a new one. You reutilise the existing panels.
Cost compare of posts per 10 metres of wall
Based on a 100cm wall and 1,25 m spacing, which is 9 posts:
- Spruce post 10cm, 2,00 m: 9 euro 50 each, 85 euro 50 altogether, life 15 to 20 years, approximately 4 euro 50 per year.
- Preserved spruce 10cm, 2,00 m: 14 euro each, 126 euro in total, life 25 to 30 years, about 4 euro 60 per year.
- Hardwood post 10cm, 2,00 m: 28 euro each, 252 euro in total, life 30 to 40 years, about 7 euro 20 per year.
- Steel tube post 76mm, 2,00 m: 55 euro each, 495 euro in total, life 40 to 50 years, about 11 euro per year.
Untreated spruce is most economical per year, but needs replacement. Hardwood provides the finest equilibrium of cost and life for most undertakings.
Conclusion: the right post for your project
- Untreated spruce when the budget is limited, the wall is up to 100cm, and the ground is yielding.
- Preserved spruce when you want a reasonable price and a longer life on a typical undertaking.
- Hardwood post when the wall is over 100cm, you want longer life, or the soil is stronger.
- Steel tube post when the ground is hard, the job is professional, or you want maximum lifespan.
In uncertainty? Houthandel Jan Sok in Meppel is happy to advise. We have all post types available and we make a complimentary strength check. That indicates which post, diameter and spacing fit your site.
Call 0522-851551 or email info@houthandeljansok.nl. Site: houthandeljansok.nl.
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