Timber retaining walls are used to support a wide range of geotechnical features. The last few years has seen an increase in the number of issues relating to timber post retaining walls. This is mainly due to many retaining walls built pre-1980-1990 nearing the end of their expected service life. While many have a long life ahead of them, those in more severe environments that have not been inspected and maintained may be at risk of failure.
This blog will cover why there is an increase in issue, the importance of a good foundation, the key points for selecting an alternative foundation material, and how this will impact inspections.
Why is there an increase in issues with retaining walls?
Most of the issues with timber retaining walls are created due to treatment-resistant decay organisms like soft-rot, or a lack of proper treatment retention levels, both of which are exacerbated by certain backfill arrangements in use. Industry guidelines recommend that the best type of backfill is to use concrete and to slope the surface of the concrete backfill away from the pole. This method of backfill works really well in theory, or when it is done to perfection, but it often creates more problems than other methods.
When the concrete and sloping method is not completed to the highest standard, gaps form between the pole and the concrete, leading to water pouring in. The problem can be aggravated even further due to timber’s ability to shrink and swell. If there are any checks open near the interface between the concrete and the post, it is a sign that you have potential issues with the structural integrity of your retaining wall and that further investigation is required.
Why is a good foundation important?
Poorly designed or installed backfills can lead to long-term issues, including soft rot. Soft rot is a particular group of decay organisms that attack the cell walls and softens the fibres. They can attack both the lignin and the cellulose, but most importantly they live in areas of the cell walls that are hard to penetrate with common treatment chemicals. It will usually attack the timber faster in high moisture environments and is difficult to inspect when the timber is encased in concrete.
To work around the issues created by soft rot and fully concreted backfills, one method is to change the backfill for your retaining wall. By stopping the concrete backfill at a level around 400-500mm below the finished groundline, the foundation strength won’t be significantly altered. Well compacted site spoil, well graded aggregate or well compacted sand would suffice for the top section of backfill, allowing good access for inspections. The 500mm mark below groundline is the point where it becomes unlikely to get high oxygen levels and soft rot, making the use of concrete safer. The substitute backfill in the top part of the footing will help the post dry out and minimise the speed of advance of any decay.
To reiterate, as most of the foundation strength comes from the section of the pole that is still concreted, the potentially weaker backfill material in the final 500mm won’t make a considerable impact on the foundation strength. This can be taken into consideration when designing the footing.
What are some alternative backfill materials?
Foundation foams are a relatively new backfill material now available in the market (they have been used for power poles for many years). In particular, foundation foam has a lot of benefits over concrete.
While in some cases it is more expensive than concrete, foundation foam is easier to get into tight spaces and increases the embedment strength early on, reaching 75-85% of its final compressive strength within 30 minutes. The use of good quality foundation foam reduces the chances of decay in that lower region due to the foam expanding to fill all of the cracks in the timber pole and providing a barrier to the degradation organisms in the soil. Our destructive testing has shown that filling up half of the embedment depth with foundation foam results in less than half the rotation of a pole foundation when compared to well compacted backfill that has had over three months of soil consideration.
Another key advantage of foams is that reinforcing systems can be driven through them if this is to be considered as a repair option for the posts in the future.
Key points for selecting foundation foam
There are a number of foundation foams on the market, but they are not all equal. The high expansions ratio foams tend to shrink over time, are weaker, and are not normally closed cell. Closed cell is important because it prevents moisture and contaminants from migrating through the foam. These foams tend to come in small consumer packets and are sold at hardware stores. These are not recommended for structural applications.
The better performing foams will have unconfined compressive strength >700kPa (corresponding to weak rock), and they will not shrink over time. Generally, this will mean an expansion ratio less than 1:14. If the foams are to finish close to ground level in bushfire prone areas, we recommend looking for fire resistant ones, but anything that finishes more than 200mm below ground would not be impacted by fire.
Finally, foams tend to be adversely affected by water in the bottom of the holes. Water will make polyurethane foam expand more than desired as the chemicals will react with the water faster than they do with themselves and create a very weak, light foam. Most good foundation foams are water tolerant. This means that they will handle wet soils but standing water in the bottom of the hole should be avoided during pour, regardless of any product claims.
Failures in timber retaining walls are becoming more commonplace in Australia (and globally). Decay due to reaching life expectancies, aggressive environments, and/or poor treatment are being made worse by the use of concrete foundations/backfill.
To combat this, regular inspections and maintenance of existing structures are critical for existing structures (this is the case for all retaining wall materials as nothing lasts forever). However, future issues can be avoided, and longer life expectancies reached using newer materials and good construction detailing.