Timber Retaining Walls
Preservative (pressure) treated wood is among the most common materials used in the construction of retaining walls. Ground-contact rated is standard for that use, but a GC label doesn’t tell the entire story. The important factor is preservative retention – the amount of chemical that remains in the wood after treatment, usually expressed as pounds per cubic foot or PCF. Optimum levels vary with the chemicals used and some products considered suitable for ground contact have retention levels that are less than desirable for retaining walls.
The American Wood Protection Association (AWPA) lists three categories for ground contact use, General Use (UC4A), Heavy Duty (UC4B) and Extreme Duty (UC4C). Heavy Duty protection would be suitable for most circumstances. Extreme Duty would be appropriate in such locations as freshwater lake or pond side walls. AWPA has separate categories for use in marine (saltwater) environments.
The timbers most used in retaining walls measure six by six or eight by eight inches in cross section and full preservative saturation is difficult to achieve in material that thick. Consequently, the central portions are less protected and decay (rot) can occur deep within the wood before there’s any visible evidence of distress. So-called Ground-Contact-rated, forty-year timbers can experience advanced deterioration within as few as fifteen years, while appearing sound.
In order to truly evaluate timber retaining walls it’s often necessary to sample the material by extracting full-depth cores with a specialty bit. Usually, a simple visual examination is sufficient to determine the level of degradation. Sounding (with a hammer) can reveal advanced decay. Rot reduces wood to dust, which will produce a hollow sound if sufficient wood has been compromised.