Gabion Retaining Wall: Do It Yourself!
By: Paul Collar
Paul is a civil engineer and geologist and the publisher of this newspaper.
You may reach him at firstname.lastname@example.org
With the many steep slopes of the Osa, the often saturated soils, and the high clay content and frequency of earthquakes, retaining walls are common backyard civil engineering features. Whether at the residential scale to shore up a steep bank or to terrace a slope for gardens or to contain the cutting edge of a creek or river, or at the civil works scale of building roads and bridges, dams, and other structures, the gabion is often the basic building block used for retaining walls.
The first recorded use of the gabion was by Leonardo da Vinci, and the word itself is derived from the Italian gabbia, or cage. The gabion cage—or basket—was patented in Italy in the late 1800’s. In contemporary times, the gabion wall is often preferred to poured and anchored concrete structures due to lower cost. But for the do-it-yourself Osa finca owner, gabions are a great option not just for low cost but because they don’t require permits, special tools or know-how, and are easy to engineer. Here’s what you need to know to get started.
Gabions work because of their weight. They are in effect, piles of rock, albeit neatly ordered inside galvanized iron baskets. Gabion walls do not require anchoring structures and like gravity dams achieve their function on the basis of weight alone. They last as long as the wire baskets hold up to corrosion and have a life expectancy of 50 years, surely less on the Osa. The sheer mass of the wall of rock creates resistance to being moved, hence to overturning. The design principle is that the overturning resistance force exceed the forces of soil creep and failure pushing against the wall. A rule of thumb is the steeper and higher the slope, the greater mass required. But for walls of less than 30 feet or so on non-vertical slopes, a single row of gabions is more than enough.
Those seeking to go through the entire design process can find this elsewhere. For the DIY gabion enthusiast, there is an engineering credo not unlike the medical profession’s “First, do no harm.” It goes, “when in doubt, build it stout.”
For best results, walls should be battered, that is, angled into the slope being retained. A slope of 6 degrees is typically adequate. Gabion baskets, or mallas, have dimensions of 1m x 1m x 2m and are the building blocks of the gabion wall. For each gabion, two cubic meters of cobble rock are required. A full dump truck provides the rock, therefore, for six gabions.
- The first step is to lay out the design, taking into account the point where the wall must end on the upper slope and the height of the wall to its logical base. You will likely need to excavate one meter or so for the foundation mattress, which will measure two meters in width, 50 cm in height, and have a length equal to that of the wall being planned. You can estimate this before breaking ground and then polish the design layout once you have completed the excavation. You will also need to decide at this point whether your gabion wall is to have a smooth face, or to have steps in its rise. In the example, I used steps of 10 cm width for each gabion row.
- Excavate the base to the point of undisturbed soils or bedrock if it is near the surface. The face will be unstable—or else you wouldn’t need a wall in the first place—so take care for wall caves. Don’t worry about them, as you will backfill and compact as your wall rises, but the first rule is to work safely. The exposed footprint must angle 6 degrees (or whatever your design is if you have reason to increase the batter angle). Since the foundation will be 2 meters wide, the outside floor surface must be 8 cm higher than the inside floor surface, the entire length smooth and even.
- Cut enough gabions in half to situate them for a foundation. You will need half as many more to cut away the tops for the mattress baskets. Tie in tensor cables on 25 cm centers across the width of the mattress. For each open gabion, you will need to add six cables from front to back at the mid-point of the mattress rise. Fill with rocks, then sew the tops closed with galvanized wire.
- The mattress is twice the width of the gabion baskets to follow, and the centroid of the structure should ideally be situated slightly inward from the center of the mattress. Re-do all your numbers, taking into account any steps that you have in order to decide how to position your first row of baskets on top of the foundation.
- The internal tensor wires are critical to contain the rock without deforming the cages. In each gabion, you will tie in 18 tensor wires as shown in the drawing on 25 cm centers. But you will need to place the rock by hand, so you’ll want to add the tensors as you work so that you have room to work.
- Hand place the rock so that it is well settled and when full sew the cage tops closed with galvanized wire.
- Backfill and compact the space behind the first row of gabions. It will be tempting to use the excavated soil. Forego this temptation and haul in river sand for this compaction. You can mix this with excavated soil, but you need sand and gravel for best results.
- Repeat steps 6 and 7 consecutively with each new gabion level until you reach the top.
|Backfill||Build it till it’s finished||Groom|
- Groom disturbed areas and plant verdiver or other deep-rooted plants in disturbed soils on the sides and manicillo or plants of your choice on shoulder.