Soil Nailing & Anchorage
Building Soil Nailing is a method of stabilizing excavation walls by reinforcing the surrounding soil. Steel reinforcing bars, known as nails, are installed into the soil and grouted with cement grout. As excavation progresses, the nails stabilize the wall step by step. In a city such as Tehran, where land is expensive and working space is limited, building soil nailing is considered one of the fastest and most economical methods for ensuring the safety of excavations and adjacent buildings.
What Are Soil Nailing and Anchoring?
Soil nailing and anchoring are complementary methods and among the principal tools for excavation and slope stabilization. Passive soil nails reinforce the soil mass and are installed progressively with each stage of excavation. Prestressed anchors can carry higher loads and provide active deformation control along sensitive excavation faces. Combining these two systems provides a flexible and economical solution for stabilizing deep excavations and unstable slopes.
How Does Soil Nailing Work?
Nails are reinforcing bars installed in drilled holes within the soil and grouted with cement grout to mobilize the shear resistance at the soil-grout interface. Since nails are passive elements, their resistance is mobilized only after a certain degree of deformation begins within the soil mass. For this reason, each row of nails is installed as excavation progresses to the next level, ensuring that the underlying soil is never left unsupported. The excavation face is typically covered with reinforced shotcrete to maintain the stability of the soil between the rows of nails. How Does Anchoring Work?
Unlike a nail, an anchor is an active element. After drilling and grouting the bonded length, the anchor is prestressed using a hydraulic jack and then locked off. From that moment, it applies an active force to the wall or slope. The anchor’s free length extends beyond the potential failure wedge, while the bonded length is installed within the stable soil behind it. This prestressing enables anchors, unlike nails, to control deformation from the outset rather than only after deformation has occurred.
Why Combine Nails and Anchors?
* Economical coverage: Nails provide a cost-effective solution for general stabilization of excavation faces. * Targeted control: Anchors are used only in rows or locations where higher load capacity or active deformation control is required. * Property constraints: Unlike methods such as top-down construction, which do not require extending anchors into the adjacent property, tension anchors typically need to cross the excavation boundary and be bonded within the ground behind it. Therefore, where sufficient space or the adjacent owner’s consent is available, building soil nailing combined with anchoring is generally faster and more economical than methods such as top-down construction.
Numerical Design and Analysis
The design of nails and anchors is performed numerically in PLAXIS using staged construction analysis. This allows stress distribution, deformation, and the factor of safety to be evaluated separately at each stage of excavation and installation of each support row.
Field Testing and Monitoring
Following installation, the actual capacity of each anchor is verified in the field through pull-out testing. At the same time, instrumentation monitoring, including inclinometers, load cells installed on critical anchors to monitor prestress losses, and settlement monitoring points, continues at each excavation level. This ensures that the actual behavior of the retaining system remains consistent with the design predictions. Where Are Soil Nailing and Anchoring Used?
This system is widely used for temporary and permanent excavation support at sites where sufficient space is available behind the excavation boundary, for stabilizing natural and man-made slopes, and for strengthening existing retaining walls or slopes.
Technical Challenges and Considerations
* Corrosion protection for permanent anchors and nails, particularly in projects with long service lives * Verification of the soil-grout bond capacity in fine-grained soils or weak soil layers * Drainage behind the shotcrete facing to prevent the accumulation of pore water pressure * Coordination with the owner of the adjacent property when the bonded length of an anchor must extend beneath neighboring land
Soil nailing progresses through staged excavation. First, approximately one to two meters of soil is excavated, followed by the following sequence:
* Drilling the nail hole at a slightly downward angle * Installing the reinforcing bar and injecting cement grout * Installing the steel reinforcement mesh and applying shotcrete * Installing the bearing plate and nail head nut after the grout has set * Continuing excavation to the next level
This sequence ensures that the soil is never left unsupported and that building soil nailing proceeds under controlled conditions from the very first stage. Soil nailing performs well in stiff cohesive soils and dense granular soils. However, in soft clays, loose sands, and areas with a high groundwater table, drainage, ground improvement, or a combination with anchoring is typically required.
Building Soil Nailing Design: 7 Key Considerations
Building soil nailing cannot be properly designed without a detailed understanding of the soil conditions. The most important considerations include:
1. Geotechnical investigations: Cohesion, friction angle, unit weight, and groundwater level form the basis of all calculations. 2. Excavation geometry: Excavation depth, wall inclination, and loads from adjacent buildings and roads. 3. Nail layout: Nail length, horizontal and vertical spacing, and installation angle. 4. Soil-grout interaction strength: Verified through pull-out testing. 5. Internal, external, and overall stability: Assessment of the excavation wall throughout all excavation stages. 6. Staged numerical analysis: Used to estimate deformation adjacent to neighboring buildings. 7. Design of shotcrete, bearing plates, drainage, and corrosion protection.
In building soil nailing design, achieving an adequate factor of safety is not the only consideration. Wall deformation must also remain within acceptable limits for surrounding structures.
Building Soil Nailing Adjacent to Existing Structures
When building soil nailing is carried out next to existing or older structures, drilling-induced vibrations, soil settlement, and water leakage become more critical. Therefore, wall displacement is monitored throughout construction using instrumentation and compared with the design predictions. Where necessary, the excavation depth per stage or the sequence of excavation stages is modified. You can view examples of Ziggurat’s excavation and stabilization projects at the Iran Mall excavation, the Mobasher project , and the Khovardin project. Consultation for Building Soil Nailing Design and Construction in Tehran
If you want to determine whether building soil nailing is suitable for your excavation, or whether another method such as top-down construction or anchoring would be more appropriate, the Zigurat team can evaluate the available geotechnical report and adjacent-property conditions with you to identify the more technically and economically suitable solution. Frequently Asked Questions
What Is the Main Difference Between a Nail and an Anchor?
A nail is a passive element whose resistance is mobilized as soil deformation begins. An anchor is an active, prestressed element that applies force from the moment it is locked off and provides deformation control in advance.
Are Soil Nailing and Anchoring Suitable for Every Excavation?
They are suitable for sites where sufficient space or the necessary consent is available to extend the bonded length behind the excavation boundary. In excavations that are completely confined between adjacent buildings, methods such as top-down construction may be more appropriate.
At Paydar Peykaran Zigurat, we numerically evaluate the design of nails and anchors in PLAXIS using staged construction analysis. We then verify the consistency between the actual excavation behavior and the design predictions in the field through pull-out testing and instrumentation monitoring at each excavation level.