5.10 Stabilising cut and fill slopes during construction

It is essential to remove all stumps prior to formationWell-constructed roads are safe, they minimise environmental impacts, and are cost effective to build. Implicitly, a major component to meet these objectives is constructing a stable road. Stabilised cuts and fills are therefore an essential part of a well-constructed road. This includes the ongoing stability of cut batter slopes, constructed fill slopes, and slopes adjacent to waterways, river margins, culverts and bridge abutments. All components of the road need to be stable – the cut banks, ditches, road carriageway, berms, and fill slope. A core component of road stability is water control. Dilute and disperse, especially away from fills, is a fundamental requirement of any road. Water control is discussed in detail throughout this Manual.

Poor practice can lead to significant sedimentation and high maintenance costs until cuts and fills self-stabilise. Fixing poor construction is often more expensive than doing it right, first time. Organic material, stumps and other vegetation should not be used as fill material, because it will not compact to a hard surface, and may eventually break down, leaving soft spots which could develop into holes and embankment failures.

Vegetative and non-vegetative methods to reducing erosion on cut and fill slopes will be discussed in Chapter 7, Erosion, Sediment and Slash Structures.

Engineered stabilisation structures will be briefly covered but these generally require technical engineering input so are beyond the scope of this Manual.

5.10.1 Methods available to stabilise earthworks

Ways to stabilise earthworks include:

• Compaction of fill
• Benching
• Correct cut and fill angles including benched cut slopes
• Appropriate water control
• Controlling erosion and sediment including vegetative and non-vegetative methods
• Engineered stabilisation structures like gabion baskets, geosynthetic matting and textiles (discussed later in this section).

Cut batter slopesFill batter slopesSelecting an appropriate cut and fill angle is critical to the stability of the earthworks. Soil has a natural angle of repose, and slopes that are steeper than this will have increased shear stress and potentially fail when they become saturated. The type of material will dictate the cut and fill slope batters. Cut slopes are challenging because opening up the in-situ material often leads to a certain amount of slumping, irrespective of how well they are planned and the mitigation measures that were in place. For example, slumping can occur, especially after prolonged rainfall, even if cut batters are at the correct angle for the material, and the bank has been oversown. The adjacent tables give ranges of values for these by material type.

The easiest way to determine what works in your area is to drive around existing roads, with similar material type, and see what angle provides the most stability.

Papa/mudstone can have steep batter slopes which also reduce the volume of material that needs to be movedLowering the batter angles leads to more earthworks and additional construction costs. However, maintenance and the potential for environmental effects are reduced. In some situations, it initially appears counter intuitive to have steep batter for some materials like pumice. Steep batters reduce the amount of rain intercept and rilling or erosion.

Once a stable angle of repose is reached, there is little benefit in further flattening, since this will only increase the area of exposed soil, and, therefore, the susceptibility to erosion. Changes in slope angles should be rounded to reduce erosion potential. Rounding the top of cut batter slopes reduces the tendency for material to erode from the edge of the batter.

Ensure the clearing limits or the road are sufficient so that stumps close to the edge of the cut banks will not be undermined, leading to maintenance issues.

5.10.2 Benched cut and fill slopes

Benched cut slopesBenching above the road levelThe stability of large cut slopes can be improved by removing some of the overburden material to reduce the shear stress. An example is the benching above the road level, see diagram on page 85. By doing this, the slope must only support approximately 2/3 of the original overburden material. The bench should be in-sloped and have a ditch installed at the toe. Failure to install appropriate drainage may cause the cut slope to become saturated – dramatically increasing the likelihood of slope failure.

Fill slope stability is greatly improved by constructing a bench below the road to contain the compacted fill. This is discussed in an earlier section on balanced cut and fill construction methods.

Where possible, revegetate as quickly as possible. An interim measure for fill slopes is to oversow, hydroseed, or have material like slash or hay placed over the fill to help intercept the rain and slow down its velocity. Maintain cut and fill batters, and water control structures like ditches, berms, culvert inlets, and flume outlets especially where they are prone to erosion. For example, if they are constructed from light, mobile material like ash and pumice, and are in areas that have high intensity rainfall.

5.10.3 Retaining wall structures

How retaining walls affect cut and fill slopesThe use of retaining walls reduces the gradient and the length of the cut and fill slopeWhen there is inherent cut bank or fill slope instability that standard forestry roading stabilisation techniques are unlikely to effectively control, additional specialist engineering methods may be required. Do not try constructing these if you are not a certified engineer. This section will give an overview of retaining wall techniques used overseas that are not used or have had limited use in forestry in New Zealand. Some of the structures are widely used in council and national roads here. For example, mechanically stabilised earth structures (MSES), timber counter-levered pile walls and gabion structures. It is likely they are not used in forestry because they require expert technical input, are expensive, and forest engineering contractors may not be skilled in their construction. Retaining walls can be used for both new construction and slope failures.

Retaining walls can be used in forest road design and construction to increase slope stability and reduce soil erosion. Wall design should consider the different site factors like geology, soil type, and groundwater and of the required wall attributes like type, length, and height. Their initial cost is offset through road whole life costs reduction, mainly due to reduced maintenance.

A significant advantage for new road construction is that retaining walls reduce the gradient and length of cut and fill slopes by:

• Reducing the total road width and excavated material
• Increasing productive area, as less area is within the roadway
• Increasing stability through the structure.

Constructing a successful retaining wall requires design and construction focused on wall stability. Many of the commercially available retaining wall options will come with design guides, but obviously for more substantive or complex structures geotechnical expertise is strongly advised. There are three separate stability aspects to consider – external, global and internal.

The external stability is based on three criteria:

• Overturning resistance – either through soil movement or water pressure. The most common failure of retaining walls is ‘overturning’, where the structure is pushed forward over time. Ensuring good drainage immediately behind the structure is the main design criteria to limit the soil pressure on the wall. A heavier or stronger wall with greater back leans will help resist the overturning moment, but you might need to anchor the retaining wall back into the soil. Overturning failures typically develop over time and initial movement will be readily visible.
• Sliding resistance – as above, soil movement will push the retaining wall forward. If the wall is not anchored adequately with the ground, the whole structure might push forward. Ensuring a strong wall to ground interface prevents the wall from sliding forward.
• Sufficient bearing capacity of the underlying soil – most retaining walls will have considerable weight and as such, the ground underneath should be strong. For any ground that is not rock, this normally means some compaction is required, whereby using geogrid materials can also work well.

Retaining wall failure

Retaining wall global failureWhere external stability analysis takes into consideration the wall and the terrain condition where it will be constructed, global stability considers the surrounding environment in terms of mass slope failure. This looks at the effect the retaining wall structure might have on the stability of the wider area. Unlike the external stability issues, global failure is likely to be more dramatic and less predictable. During maintenance the surrounding ground should be inspected for signs, such as cracks in the ground, that may indicate global movement.

Most retaining walls are designed using multiple elements. The design and construction should consider the internal stability in terms of bulging, internal sliding and topping. Most commercially available systems will come with recommendations on how the elements should be combined.

Retaining wall internal stability

Mechanically stabilised earth structures

MSES fill slope of a forest roadMechanically stabilised earth structures (MSES) are made by overlapping layers of soil reinforced with wire mesh and or geosynthetic materials (geotextile and geogrids) until reaching the needed wall height. Each layer is built using an appropriately shaped container, and is infilled with compacted soil.

The main advantages are the simple materials used, low cost and fast construction, minimal foundation preparation (typically), and that they can sustain large loads. The structure can be hydroseeded to look like a typical fill over time. The main disadvantages are the high amount of excavation required, the needs of well-skilled operators and the good quality of filling soil (compaction susceptibility).

Gabion structures

Gabion wall being constructed to support a fill slope

Gabion basketHistorically, boulders have been the most commonly used material to construct gravity walls. However, the introduction of the gabion basket system now makes them the preferred and proven gravity options through their flexibility and reliability.

Gabion structures are designed to support slopes, and to provide erosion protection. They are ideal for use as a gravity retaining wall to support steep cut and fill slopes where the optimum cut or fill slope angle would produce a large exposed surface, or where the fill slope would otherwise encroach into a waterway. The retaining wall can be stepped, sloped, or vertical, depending on the situation.

Wire mesh baskets are filled with stones, rock or rubble, and are laced together to form a continuous structure. There are two main types of gabion structures – gabion baskets and gabion (reno) mattresses. The former is designed to use their mass to support a toe of a slope, or to provide an effective retaining wall. The latter may be used to overlay a riverbed or other surface, to reduce the erosion effect of water flow. The high permeability of gabions provides free drainage through the structure, which reduces hydrostatic pressure. This is an advantage in areas where high seepage flows are expected. Gabion baskets and mattresses should be provided with a geosynthetic filter fabric placed between the backfill and the basket to prevent the loss of fines from the fill through the basket.

Both gabion baskets and mattresses are flexible because of the combination of mesh and rock fill. This flexibility allows them to be used in variable conditions, such as soft or unstable ground where movement is expected due to settlement or frost heave etc. They are ideal for river and waterway erosion control, bridge abutments and approaches, slope stabilisation and toe support. Different sizes of wire mesh baskets are available commercially – they typically range from 2 m in length, 1 m in width, 0.5 m in height to 4 m in length, 1 m in width, 1 m in height. The wall is constructed by step-stacked layers with the resulting structure able to sustain heavy loads. Gabions tend to be a very cost-effective solution. They use material that is usually obtainable on site – any solid, hard material such as rubble, broken rock or concrete. The main disadvantage of this solution is the visual impact.

Gabion baskets and mattresses can be used on their own, or in combination, to form effective bridge abutments. Gabions can also be used to provide erosion protection of existing abutments.

Reno (gabion) mattresses are used to reduce water velocity and eliminate scour of the waterway bed, especially downstream of a ford or battery culvert. Mattresses are laced together to form a continuous mat overlaying the waterway bed. Mattresses are generally manufactured 6 m in length, 2 m in width and in thicknesses of 240 mm and 300 mm. They are flexible and, therefore, can be folded to accommodate undulating ground conditions. If additional erosion control is required, the mattresses can be overlaid with concrete.

Timber cribs

Timber crib construction overview

Treated timber crib as retaining wall of forest road fill slope. Never use untreated radiata logs as they are not durable

Crib construction spike componentTimber cribs are a traditional retaining wall option in some parts of the world. The construction components are logs with high natural durability, stones and spikes. The space between the layers are filled with stones and, where necessary, geotextiles and drainage pipes may also be used. The logs are connected with spikes 20-30 cm in length and 10-12 mm in diameter. The result is a highly water-permeable and cost-effective structure. When compared with a gabion, it is aesthetically more pleasing, the construction materials are less expensive, but the construction is more laborious.

Timber cantilever pile wall

Timber cantilever retaining wallAnother common retaining wall design option is using vertical poles fixed in the ground that sustain horizontal logs or lumber containing the backfill soil. These are common outside forestry in New Zealand. The design should consider the bending resistance of the poles used and pole depth into the soil. A rule of thumb is that the depth of the poles should be as the wall is high. Cantilever pile walls are a low impacting solution with the major advantage is a low amount of excavation. However, on ground with bedrock or soils with larger rock components it may be difficult to drive piles to the necessary depth and other options need to be considered. Use either ground durable or treated wood for structures.