A ground anchor consists of a steel bar or a tendon that is installed into a drill hole, typically of six-inches in diameter or less. A mixture of water and cement, which is also called grout, will then be pumped into the bottom of the drill hole until it is filled to the top. In some cases, ground anchors can also be constructed using mechanical anchor systems and helical pile systems. The right type of ground anchor system will depend on the size and complexity of the project, as well as how deep the system needs to be in the earth or stone.
There are a variety of names used for ground anchors, including rock anchors, tieback anchors, earth anchors, micropiles, tiedown anchors, soil anchors, and foundation anchors. No matter which of the terms is in use, a ground anchor is responsible for transferring load from a structure into the ground. While these terms are often used interchangeably, the correct term depends on both the context and the type of ground conditions.
Ground anchors are the most versatile anchoring option with versatile applications and the ability to choose between a temporary or permanent installation. Many of them are easy to transport due to the capability of stacking together, and they can be used for all sorts of different projects.
A tiedown anchor is a tool that is used to resist uplift from buoyant forces on structures that are submerged below the groundwater table. Swimming pools, buried storage tanks, buildings with basements, and pipelines all need to be designed in a way that resist buoyant forces in case of a situation where the groundwater table submerges so much of the structure that the buoyant force exceeds the structure’s weight.
A tiedown anchor can also be used in dams to increase the level of resistance to sliding or overturning failure. These anchors can use a bar or cable strand design. Cable strand anchors become more cost-effective as the anchor is placed deeper in the earth since the strands can be created to a continuous length. They tend to be less rigid than a bar anchor and require a unique grouting technique.
The drilling method used also correlates to whether bars or cables are chosen for the project. The most common options for drilling the hole for the anchor are rock drilling and core drilling, each of which has its own advantages and disadvantages.
A tieback anchor, often simply called an anchor, is used in earth retention systems. It can also be implemented into landslide mitigation systems. Soil screws, known as helical tiebacks, are a foundation system that uses helical bearing plates that are welded onto a central steel shaft. The load is transferred from the shaft into the soil using the bearing plates.
Tieback anchors are installed through rotating anchors into the ground using rotary drilling equipment. Once the correct depth is reached, capacity is determined by reading the installation torque and correlating it to the holding capacity ratio.
Steel rod anchors are a threaded, hollow rod that extends to any length needed to meet the project requirements of various soil and rock conditions. The installation involves a combination of placing, drilling, and grouting in a single operation. This makes the system useful for working in a small space.
A grouted tieback is a structural item that can offer permanent or temporary resistance to a structure through pre-loading and locking in the tension load. The anchors are drilled into stone, and resistance loads are reached through friction between the grout interface and the rock.
Also known as an earth anchor, a soil anchor refers to a type of anchor traditionally used in soil nail and shotcrete earth retention systems. However, these anchors can also be used in any area founded by soil that achieves its capacity through the soil. These anchors can either be run in spirally or impact-driven into the ground. The choice will depend upon the project design and the force-resistance characteristics that are needed.
There are several types of earth anchors, including polyester resin anchors, cement grout bonded anchors, mechanical rock anchors, and mechanical soil anchors. The first option is used to create an anchor between the rock and an anchor bar. These anchors tend to be an economical option that is installed quickly for temporary support. Grout bonded anchors bond together with the anchor and the soil or rock and can come in a variety of steel grades.
A mechanical rock anchor has a head assembly with a cone and expansion shell that is used to create a friction lock between earth and rock and the head assembly. For the mechanical anchors, a pivoting plate is used instead. It’s driven down to a certain depth and then rotated 90 degrees to anchor itself into the soil.
Micropiles are a type of anchor that can be used in foundation application. Micropiles can be compression or tension, but the use of them implies that there is at least a compression load. A hydraulic hollow ram places the anchor in tension before they are locked off at a prescribed load.
A post-tensioned anchor helps reduce the deflection of the engineered system once it is under service load. Post-tensioning provides several benefits, including:
Micropiles are intended for high capacity projects and used in deep foundation elements. Typically, they are between five and 12 inches in diameter and can reach depths of up to 200 feet with working loads of more than 200 tons. Micropiles are made out of a combination of rebar, steel casing, and grout.
The micropiles transfer the load through subpar soil layers to better foundation soils. The load then gets transferred from the foundation to the grout and steel of the micropiles and sheds to the nearby soil or rock through high values of friction. Most of the time, micropiles are used in difficult ground conditions such as areas with obstructions, places with low headroom, and locations with karstic geology.
Ground anchors are typically post-tensioned. Those that are not post-tensioned are called passive anchors, instead. Grouted ground anchors are typically made of several components, including the anchorage, the bond length, and the free stressing length. The anchorage is a system that includes an anchor head, a trumpet, and a bearing plate.
Design and successful construction of ground anchor systems require knowledge and understanding of the specific application and project location as well as understanding the ground conditions and site access constraints, including overhead and underground utilities. Selection of the appropriate equipment and drilling means and methods integral to success. The applications available for a ground anchor include retaining wall stabilization, uplift slab stabilization, slope stabilization, and concrete dam stabilization.
Numerous types of ground anchors can be used. Underreamed anchors use grouted boreholes, including a set of underrears or enlargement bells. The anchor is useful for harm and firm deposits. Post-grouted anchors use multiple delayed grout injections that enlarge the grout body of straight shafted gravity grouted ground anchors.
The straight shaft pressure-grouted ground anchor works well for weak fissured rock and coarse soil. It also can be used for fine-grained soil without cohesion. The last type is the straight shaft gravity-grouted anchor, which is usually installed into rock or extremely dense soil. A hollow stem auger method or a rotary drilling process is used. With gravity displacement options, the anchor is grouted into a borehole with a straight shaft.
Geo Craft Builders has the expertise, developed over 20 years, and includes projects of all sizes and access constraints in both urbanized areas and the most remote mountain summits. If you would like more information about ground anchor systems or have any questions regarding feasibility, constructability, or pricing, reach out to us at Geocraft Builders today. We look forward to working with you.