Technology & Experiment of High Steep Rock Slope Soil Spray Sowing

With the rapid development of urban construction and transportation network, a lot of bare slopes have been formed all over the country. These bare slopes not only cause soil erosion which leads to slope collapse, but also destroy the ecological environment of the city and the traffic section. Although the traditional slope protection measures can ensure the stability of slop effectively, it needs to spend a lot of manpower and material resources and has poor coordination with the surrounding landscape. Thus, traditional slope protection can not meet the requirements of modern urban landscape and people's pursuit of comfortable living environment. Burgeoning thick-layer backing ecological slope protection technology is that mix the foundation soil with humus, cement, water additives and many other materials which are suitable for plant growth. Being fully stirred by machines, it is sprayed onto the slope by using a spraying machine to form a suitable soil layer for plant growth. When the seeds germinate and grow on the ground, it can effectively protect the slope, so as to achieve the purpose of rapid restoration of ecological system and slope protection.

Workers are spraying the slop surface which is covered with zinc coated chain link mesh.
ESS-01: When spraying, the mouth of the spray gun is about 2 meters away from the slope surface.
Chain link fence is covered on the slope surface, and grass is growing up.
ESS-02: Chain link fence can make the soil form a lasting and integral plate on the rock surface.

The test of raw materials and equipment of external-soil spray seeding for high steep rock slope

  • Main raw materials and their function
    • Soil. Adopt planting soil and filter it with 2.5-millimeter-diameter sieve after drying. It is the framework of the substrate and the growth space of plant roots.
    • Water. Adopt tap water which makes the substrate has a certain degree of mobility.
    • Cement. Adopts 42.5 R ordinary Portland cement which is the main adhesive material for planting substrate.
    • Mixture of humus. Adopt peat which can improve the growth environment, the porosity and the germination rate.
    • Thickening agent. Adopt PAM which enables the formation of granular structure of soil.
    • Cellulose. Adopt carboxy methyl cellulose sodium (CMC) which can increase the stability of planting base, including improving crack resistance, erosion resistance and so forth.
    • Water retaining agent. Adopt vermiculite which can improve the water holding capacity of base materials.
    • PH buffer agent. Adopt ferrous sulfate which can improve the basic environment of the plant to adapt to the growth of plants.
    • Galvanized chain link wire mesh. It can stabilize soil and improve the attachment ability of the substrate to the slope and the structural stability of the substrate.
  • Main test equipment
    Soil spray sowing machine.

On-Site construction process of external-soil spray seeding for high steep rock slope

  • Slope finishing. In this experiment, the prefabricated slab is used to simulate the rock slope whose gradient is 70°. Before the experiment, remove debris and loose block on the working surface and fill gaps between precast slabs with clay, in order to facilitate the construction of soil spray sowing.
  • Lay chain link fence. Adopt 6 cm × 6 cm heavy galvanized diamond chain link mesh (wire diameter is 2 mm). Fix chain link fences from top to bottom and two adjacent rolls of wire are connected and fixed with tie wire respectively. It is required that there should be a overlap, which is at least 10 cm, at the junction of two fences. Maintain a certain uniform clearance between chain link fence and slope. Fill the place where chain link fence is concave towards slope with rock block and twine with straw rope, to increase the adhesion and make sure the thickness of soil. Chain link fence can make the soil form a lasting and integral plate on the rock surface.
  • Seed treatment: Shrub seeds and grass seeds should be soaked to promote their germination. Before spraying, soak them with 80 °C hot water (containing seed soaking agent) for 1–2 hours.
  • Mix planting base material: Strictly according to the design mix proportion, planting base material is allocated and fully stirred manually or mechanically. The workload of this process is large, so you need to prepare the material in advance. At the same time, the material is required to be fully dry, so as to ensure that the base material can be used normally when spraying solid.
  • Spraying: Before the base material is sprayed, the slope surface should be wetted. Spray the fully mixing base material on the rock slope with spraying machine. This process is divided into 2 layers: The first layer is sprayed about 7 cm thick. After about 10–15 min, when the substrate is stable, spray the second layer which is 3 cm thick. The seeds are placed in the second layer. Due to the loss of matrix moisture will cause inadequate thickness of the substrate, it is commonly required that the spraying thickness is 125% of the design thickness. When spraying, the mouth of the spray gun is about 2 meters away from the slope, and the suitable amount of water is to keep the substrate flow.
  • Cover non-woven fabric: After the completion of spraying, lay the 20 g/m2 non-woven fabric quickly from the top of the slope. Make the fabric Laying down be close to the soil and it can be fixed by bamboo nails. Some uneven parts can be reinforced. It can effectively prevent the scouring of the rain water on the slope and the seed, prevent the surface soil and water loss and can also play the role of keeping water and moisture.
  • Maintenance and management: After spraying the soil should be kept moist. Herbs begin to sprout after 7 days, shrubs begin to germinate after 15 to 20 days and they will cover the slope after 2 months. Due to the rapid growth rate of grass, it should be maintained carefully on the early stage. timely remove of excess grass, providing favorable conditions for the growth of shrubs. From budding to seedling, plant seeds must be watered to keep the soil moist. With the growth of plants, watering times can be gradually reduced and adjusted according to precipitation. At the same time, it should be timely fertilized and pest controlled.

The ecological planting base material used in this experiment has good fluidity, bond strength and ecological restoration effect. Thus, it can be applied to the ecological restoration of high and steep rock slope. Tall fescue, bermuda grass and magnolia multiflora are the best choices.