What is the soft shell of a timber pile?

The soft shell (Dutch: zachte schil) is the decayed, weakened outer layer of a timber pile, usually a foundation pile beneath a building. This part of the wood has become soft and no longer contributes to the strength of the pile. Only the remaining sound core counts towards its load-bearing capacity. The term is common in the Netherlands, where many older buildings stand on timber piles.

Cause

The decay is caused by bacteria or fungi:

• Bacterial decay can occur even under water and progresses slowly from the outside inwards.
• Fungal decay (pile rot) mainly occurs when the pile head dries out, for example because the groundwater level has dropped. With oxygen present, fungi can break down the wood much faster.

How is the soft shell measured?

During a foundation survey a pile is excavated and the hardness of the wood is measured with a calibrated impact tester fitted with a steel pin (the Pilodyn is a well-known example). The pin penetrates deeper into soft, decayed wood than into sound wood. The penetration depth is read in millimetres. Measuring around the pile gives a picture of the thickness of the soft shell. Knots and other irregularities are skipped because they distort the result.

Application: what does the result mean?

The thickness of the soft shell is deducted from the pile diameter. Using the remaining sound cross-section, a structural engineer calculates whether the pile can still carry its load. Together with groundwater level, tilt and cracking in the building, this determines whether and when foundation repair is needed.

Tips for owners

1. Use a specialist firm that works to recognised guidelines for foundation surveys.
2. Ask for the readings per pile, not just a conclusion.
3. Monitor the groundwater: pile heads that dry out decay faster.

Related terms

• Pile rot
• Wood rot
• Foundation survey
• Foundation problems with timber piles
• Knot in wood

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