Construction technology entered a new phase with the industrial revolution around 1760s, when iron was first mass-produced from puddled steel in a process similar to what we use today. With the steam engine invented by James Watt a few decades later, a major increase in construction productivity occurred due to better transportation systems and increased access to raw materials.
Halfway through the 20th century around 1950s, we saw a radical change in construction when prefabrication was introduced. This has led to the prefabricated, statically stable building systems used everywhere today.
Use of Anchor Bolts
In building, anchor bolts are used to join concrete parts such as structural and non-structural components. The connection is made by joining several elements including anchor bolts, steel plates, and stiffeners. Tension forces and shear forces may be transferred via anchor bolts. A steel column joined to a reinforced concrete foundation is the equivalent of a structural connection between structural elements. Non-structural components connected to structural ones are known as “stirrups.”
Retaining walls, dams, wharves, bridge abutments and foundations for buildings all require immovable anchor points. Anchors are stressed (active anchorage) to prevent structural movement and usually transfer their weight over a fixed length. These are generally referred to as tension anchors and are ideal for rocky soil conditions. Load distributing compressive (and tension) anchors are used when anchors are constructed in soil or weak rock since they rely on the succession of little successive bond lengths rather than a single longer bond length.
Types of ground anchors used in construction:
- Straight shaft gravity-grouted with low pressure; these are most often utilized in rock, very stiff or hard cohesive soils, and sandy or gravelly soils utilizing rotary drilling or hollow-stem auger techniques.
- Pressure grouting involves pushing grout down a shaft at high pressure. These are typically used in coarse granular soils and weak fissured rock, as well as fine-grained cohesion less soils. Grout is injected into the bond zone with a force of more than 150 psi. Drilling and grouting procedures vary considerably.
- When you use single-under-reamed or multi-under-reamed, the holes are generally bigger and uncased. The grout is put in place without any pressure, as it is with cohesive soils.
- Post-grouted; these are formed by delaying multiple grout injections and are utilized to expand the grout body of the anchors listed above’ surface area to improve load transfer capacity.
Steel anchors are also known as wireline and line equipment and may be found in a variety of sizes and capacities, ranging from 70 to 3,000 KN. They’re strong yet lightweight corrosion-resistant ground anchors that may be put in place manually or with portable equipment depending on their size and soil conditions. When they’re loaded, they apply force against a cone around their length, offering formidable resistance to displacement. They’re a popular choice for securing a variety of constructions in place because, as they generate little soil disturbance during installation and can be stressed to a precise holding capacity, they’re used to anchor a wide range of projects.
Grouted ground anchors
Grouted ground anchors are built in grout filled drill holes and include graded ground anchors, also known as “tiebacks.” Grouted ground anchors are sometimes referred to as “tiebacks.” The following are the common components of a grouted ground anchor:
- Free stressing (unbounded) length
- Bond length
The anchor is a combination of the anchor head, bearing plate, and trumpet that may transfer the restressing force from the restressing steel (bar or strand) to the earth surface or supported structure. The components for bar and strand tendons. A bond breaker is a smooth plastic sleeve that is placed over the tendon in the unbounded length to prevent the prestressing steel from bonding to the surrounding grout.
It allows the pre-restressing steel in the unbounded length to stretch without being obstructed during testing and stressing, and it leaves the pre-stressing steel unbounded after lock-off. The length of the pre-stressing steel that is joined to the grout and can transmit the tensile strain into the ground is known as the tendon bond length. The anchor bond length should be positioned behind the critical failure surface.
The tendon is the portion of the complete ground anchor assembly that connects to the concrete. The pre-stressing steel component (strands or bars), corrosion protection, sheaths (also known as sheathings), centralizers, and spacers are all included in the tendon, but not grout. The sleeving is a smooth or corrugated pipe or tube that protects the pre-stressing steel in the unbounded length from corrosion by keeping moisture out.
The center of the tendon is positioned in the drill hole such that the minimum grout cover required around the tendon is achieved. To keep each strand or bar of a multibillion-pound tendon separate, separators are used to separate the strands or bars of the tendons and ensure that each element is adequately bonded to the anchor grout. The grout used in this application is a Portland cement compound designed to transfer stress from the tendon to the ground while also protecting against corrosion.
Design and installation of ground anchors
The life expectancy of an anchor is determined by the soil’s corrosivity and the materials used. The main component of the anchor, often known as a “tendon,” may be constructed from a variety of materials:
- A steel bar or wire strand
- Aluminum alloy – 30 years+
- Hard anodized aluminum alloy – 40 years+
Permanent anchors, on the other hand, may have corrosion resistance. Temporary anchors can be removed after they’ve served their purpose. The installation process will vary depending on the situation; drive rods, spiral sockets, and impact hammers are frequently used to push or screw the anchor into the ground, as well as simple hand tools. It may be necessary to bore a hole first before installing the anchor in some cases, and it may also be required to use a casing to support the hole before the anchor is installed.
After installation, the hole may be filled either before or after grouting. The anchor is often tensioned and secured against a head plate afterwards. During installation, extreme care must be taken to avoid harming any services or other objects below ground level.