Rockfall Mitigation

Welcome to the March 2015 edition of the Maccaferri MacNews.

As we come out of the winter months there are still freeze-thaw conditions across most of the country. Rockfall Mitigation will be on the minds of many Transportation Department employees. This time of year can be very dangerous for so we decided to go into detail on our Rockfall work. 

ROCKFALL IN THE NEWS

The beauty and majesty of our mountainous areas is often overshadowed by tragedy. Avalanches and landslides usually are the first to come to mind but the simple rockfall can have the most devastating results. Whether it is a holiday traveler caught off guard by falling debris or hikers that trigger a collapse that causes damage to everyone and everything below.

Recently, the state of California was home to a rockslide that took the lives of two Caltrans (California Department of Transportation) workers and seriously injured a third. The workers were killed during a pair of rock slides which occurred while they were removing rocks from a hillside above Highway 96 near Happy Camp in Siskiyou County. That operation entails the men rappelling down a hillside to break away rocks, preventing an unexpected release onto traffic. Most maintenance workers volunteer for training for rock-scaling and workers are trained with yearly refresher courses.

To prevent any future deaths, Caltrans selected the Maccaferri Rock Mesh 3030 (RM 3030). RM 3030 is made of steel and acts as a ‘drapery’ to completely cover the rock face, not only holding the rocks in place but also limiting and guiding the fall if there is one. This product is a composite mesh and is particularly suitable for rockfall protection as a pinned or simple drapery system for surface or soil veneer slope stability. RM 3030 is a woven composite mesh system made of steel wire and steel wire ropes woven together during the manufacturing process of the hexagonal double twisted wire mesh.

The wire ropes are inserted longitudinally every 12 inches (300 mm) into the wire mesh in both directions. The transversal ropes are inserted within each double twisted mesh and are secured to the selvedge ropes during the manufacturing process. The Rock Mesh 3030 forms a biaxial 12 inches (300 mm) x 12 inches (300 mm) cable net grid within the mesh.

While cost can be a very common reason for the selection of a product, the RM 3030 also provides the following advantages compare to the traditional cable net system with double twisted mesh.

• Overlapping is not needed to cover the rock face reducing wasted materials
• Rolls are 12 ft x 150 ft which provides long panels of drapery laid on the rock area creating a safer work zone
• Connections are not needed between panels every 12 or 24 ft.
• One product installation – the mesh and cable net are combined at the manufacturing point and not laced on the site
• A continuous connection between the mesh and cable net prevent rocks from being squeezed between the mesh and the cable net which could create an extra hanging hazard
• The RM 3030 can be spliced at the opposite ends of the cable net panels
• The installation is very quick and the product is similar to a Double Twisted Mesh

The Maccaferri RM 3030, used prior to the repelling of the work crew could have prevented the slide and at the very least provided the Caltrans workers a few extra second to clear the danger area. The investigation into exactly what occurred and how to prevent a future disaster will be completed soon. The recommendation of the RM 3030 or a similar product should be an easy call.  

New Design Software for the Rockfall Simple Drapery Systems

The natural processes of weathering, increased by climate change, generate geological instabilities, which frequently expose populated areas and infrastructures to a wide range stability issues that can vary from erosion to rockfall. These shallow instabilities should not be underestimated because they frequently are the cause rockfall events. Due to the fact that they happen with a high frequency over large areas, the probability of rock strikes and accidents is elevated. In this situation, the design must consider the efficiency of a remedial solution in terms of performance and low maintenance costs. Maccaferri has developed a software design solution to assist in the development of a protection system.

A typical mitigation solution is divided into two different design approaches related to their means of stabilizing the slope area. The first is an Active system, the other a Passive system. Each is explained below:

An Active Protection Systems is applied directly on an unstable zone in order to prevent or control the movement of a shallow instability. The most common solutions inside this category are:

• Soil Nailing: Improves soil stability by inserting reinforcement bars in the soil in a regular pattern. The nails are then grouted and fixed soundly to the ground for their entire length (nailing). The frequency and the length of the nails can be calculated in accordance with FHWA, EN 1997 1 or BS 8006. The ground surface is reinforced with a structural facing which can be flexible (steel mesh) or rigid (shotcrete)
• Pre-stressed Soil Anchors (tie back anchors): Pre-stressed anchors are installed in a shallow instability to modify the internal stability since an external force is applied to tie the instability into the slope area.
• Secured Drapery System: Is composed of an anchor system spaced at regular intervals where the rocks are held in place by a surficial structural, flexible (steel mesh), or rigid (shotcrete) facing interconnected to ground anchors.

A Passive Protection Systems is not implemented at the source area, but is placed to affect the trajectories of falling rocks, arresting or reducing the falling rock velocities. They are generally applied far from rockfall source areas. This category includes the following solutions:

• Simple Drapery System: This system consists of a steel mesh drape system, secured at the top of the slope with ground anchors and steel wire rope cables.
• Rockfall or Debris Flow protection Barriers: A structure composed of posts, cables, energy dissipaters and interception structures (steel or wire mesh) capable of arresting and containing falling rocks. The barrier is also composed of elements to anchor support cables, post foundations, and ground anchors.
• Hybrid Barriers: A combination structure composed of posts, cables, energy dissipaters, and a tail of mesh designed to reduce the energy and the velocity of falling rocks which are driven into the slope by a steel drape system reducing energy through ground collisions.
• Rockfall or Debris Embankments: This is a gravity or mechanically stabilized earthfill structure forming a steep berm that contains the falling rocks or debris, generally installed at the toe of a slope

SIMPLE DRAPERY SYSTEM:

A simple drapery system consists of a rockfall steel mesh installed along the face of the slope. The drapery is hung in the same manner as a curtain (figure 1) and suspended by longitudinal ropes and anchors at the crest (Rc). Anchors are positioned along the crest (AC) and toe (AT) of the slope and depending on the design and the prevailing rockfall conditions at the site. They are commonly placed in a line and are fitted with suitable connections to accept the crest rope (RC). Once the crest anchors and the upper longitudinal cables are installed, the mesh can be fixed to them and left free-hanging all along the slope.

(Above it a sketch of a simple drapery system application (left) and disposition of the anchors (right)) 

(A photo of a mesh installation at the crest )

(A debris accumulation at the toe of the simple drapery system)

(Reunion Island (Fra) – more than 40,000 sqm of drapery system were installed in a rocky slope higher than 150 m)

The steel mesh can be fixed as well at the bottom where run out space is limited. With this configuration, the falling debris can pile up into a pocket (figure 3). In order to reduce the stress on the mesh and reduce the costs as well, the mesh at the toe of the slope can be unsecured. This requires a catchment trench or a fence to collect the fallen debris. This type of system is usually installed on a large rocky slope (figure 4), where the secured drapery systems are not cost-effective, or where the rockfall barriers and rockfall embankments cannot be installed because the slope morphology is either too uneven or too steep.

Design: Preliminary Remarks

In order to design the most cost effective and suitable mesh system, the designers must first analyze the main factors affecting the effectiveness of the mesh.

First of all, the stress applied on the mesh and the performance of the simple drapery system largely depends on the slope morphology. For example, on a very uneven slope, the drapery system may only be in contact with the slope at the crest area and convexities where the debris can freely run down into the gullies and concavities. In this situation the drapery has a negligible capacity to control erosion, and the falling rocks can reach higher velocities. The installation of the drapery then requires particular care to maximize the contact between the ground and the steel mesh, or the slope must be preventively re-shaped and scaled.

Another important factor affecting the selection of the mesh is the existing rock slope instability.

If erosion is the main problem, typically on a gentle slope, the appropriate selection of drape system should have a small mesh opening and enough weight to maintain constant pressure on the ground surface. When there is contact between the mesh and the ground, the drapery is quite effective in erosion control and allows both the re-growth of the vegetation and the confinement of large boulders.

If the slope is vertical, the drapery must be stronger to absorb impacts and funnel falling debris to the toe of the slope. In cases of large blocks (i.e. in the basalt cliffs), a “dynamic” drapery, like cable panels or ring nets, should be considered, whereas in cases of small blocks (i.e. thin layered limestone cliff) lighter draperies, like steel composite Rock Mesh or double twist wire mesh could be suitable.

Other important design factors are the expected life span of the drapery and its maintenance costs. Concerning the life span, designers should consider exposure to atmospheric conditions (i.e. salt spray or wind), and abrasion due to movement of falling debris. If the drapery is applied for temporary protection, as in the mining industry, a light corrosion protection could be enough. If the application must be permanent or it is close to aggressive environments (i.e. seaside), a stronger corrosion protection is required. In the last case, the designer has to plan for maintenance suggesting the maximum size of the debris pocket acceptable for the mesh.

MACRO 2: The Calculation Approach

The design of simple drapery depends on different variables related to the geometry of the slope, the type of the mesh, and the assumed debris accumulation at the base of the system. One of the available references to give as a design guideline for these applications was prepared by the Washington State Department of Transportation (Muhunthan et al. 2005).

Using this study and the results obtained from several laboratory and field tests, Maccaferri has developed new software (MacRO 2) able to perform stability analysis for the selected mesh, the diameter of the crest wire rope cable and the steel and geometric (diameter and length) characteristics of the crest anchors. If time and money are not a problem, a complex numerical analysis with very precise data from the field could be completed, but this is not practical for every project, especially if the system has a modest size and has to be done in a short period of time (emergency protection). MacRO 2 allows designers to have a quick and reliable solution for design. The design procedure that is the basis of the software is simple, but it gives reliable results considering the low level of accuracy generally available from the input data.

Mesh Design

The simple drapery system is a passive system capable of controlling rockfall and containing the debris at the bottom of the slope. It is designed taking into account all the different components able to transmit loads on the mesh per linear of slope section. These are the key areas of consideration:

1) The proper weight of the selected mesh
2) The weight of the debris accumulated at the toe of the slope
3) External weight like the snow or ice accumulation on the drapery

The software has been designed to compile all the data entered and produce a design capable of mitigating the hazardous situation being faced. A short article about the system is great for learning more about the product but an in-person or even remote demonstration of the system is the better way of learning firsthand about the abilities that this software possesses. Please feel free to contact us for a free, no commitment demo.

CASE HISTORY: PEERLESS PARK, MO

A Simple Drapery Systems is commonly used to protect workers, buildings and infrastructure from rock fall hazards. A typical application of this mitigation measure was used in Peerless Park, Missouri. The solution was designed to control rockfall with a system that would be easy to install, economic and low maintenance, and of course reliable. The design was determined considering two main aspects: The visual impact needed to be kept at a minimum; and the system had to have low maintenance requirements.

(A general overview of the protected slope)

The height of the slope was around 20 m (65 ft) and the falling block size was supposed to be from 15 to 30 cm (6 to 12 inches). A simple drapery system was the best solution with the toe of the upper section secured in this case because:

• The very limited room at the toe of the slope was not suitable for a rockfall barrier; moreover from a numerical simulation the trajectories were too high to be intercepted by a standard fence due to the limited catchment area;
• No room for a rockfall embankment;
• The cost of a secured drapery was too high in relation to the saving of the maintenance costs.

More than 1,500 m2 (16, 2000 ft2) of Rock Mesh M4000 was installed to protect the car wash workers and their patrons. The bluff is separated into two tiers and measured approximately 55 m (180 ft) in length and a total height of 27 m (90 ft). The installation of the mesh was separated into an upper and lower section to blend better with the surroundings, regardless of its expense, rather than running the mesh from top to bottom. The rock mesh drapery systems were installed on each tier. The rock anchors were drilled 6’ in depth at 24’ centers, the length of the bluff, and grouted in place.

Rock Mesh is a woven composite product made of steel ropes and a double twist wire mesh woven together during the manufacturing process. The metallic cables are used in place of the conventional selvedge wire to increase the connection strength and to transmit the load of the debris directly to the top-anchors in order to reduce the stress and the deformations of the mesh. The hexagonal double twist mesh provides high resistance to the impacts of rocks, avoiding the unraveling in the event of wire breakage.

(Detail of the cable of the Rock Mesh and the secured toe of the upper section)

For the upper and lower section, two different design approaches were used. For the upper section, the toe of the drapery was secured to contain the rocks within the mesh and to avoid rocks falling over the lower section where the lower section the mesh was not secured at the toe. According to MacRO 2 calculations, the upper section had an extra load on the mesh due to the rock containment compare to the lower section that was not secured and free to fall into a catchment area.

As you can see, a Simple Drapery approach is an effective rockfall protection system for rock slopes. This type of solution is economical, easy to install, and has a low level of maintenance. It is recommended in areas where other mitigation systems (i.e. pinned drapery or rockfall barriers) cannot be applied because their cost and/or the morphology of the site are not suitable. Above all, the safety of the installation crew and later the maintenance crews will be dramatically improved.