What is Ground Anchor?
Anchored walls have become popular in braced excavations because of a) the substantial progress in the technology and availability of high-capacity anchor systems, and b) the absence of interior obstructions that permit uninterrupted earth moving and thus improve the construction conditions of the underground portion of a building (Xanthakos, 1994). Figures 1 and 2 show a photo and a schematic of tied-back slurry wall excavations. In some projects tiebacks have been used in combination with rakers and soil berms and/or corner braces (Gnaedinger et al., 1975). Tieback anchors comprise a barrel anchorage located either in a bearing layer which is tensioned at the front face of the wall. The part of the anchor that transfers the force to the surrounding soil is frequently called the "fixed length", while the "free length" transmits forces from the fixed length through the anchor head to the slurry wall.
In order to minimize wall movement and ground settlement, tieback anchors are designed to achieve the highest stiffness possible within economical considerations. In urban cities like Boston, Chicago, New York, and Washington where land is precious such deep excavations are more common. Tieback capacity depends on the vertical and horizontal spacing of anchors and on surcharge conditions. Prestress levels typically range from 40 to 250 kips when the grouted portion of tiebacks is within soil, higher loads are used when the ties are located in bedrock. Typical tieback spacing ranges from 7ft to 13ft in the vertical, and from 5ft to 15ft in the horizontal direction (from the current database). Tieback capacity is reduced if the spacing is too close due to interference between adjacent grouted zones.
Often the tiebacks are used only for temporary excavation support, while the basement floors provide permanent lateral earth support. In such projects the tiebacks are detensioned when the basement floors have gained sufficient strength. The basement floors should be designed to resist permanent lateral earth pressures, since stress transfer from the tiebacks to the floor system will take place when the ties are detensioned. This stress transfer has reportedly caused long-term cracking of many the basement floors.
Tieback installation follows a predetermined sequence as to minimize soil movements and speed the excavation construction (Fig. 3). The excavation is carried a couple of feet below the tieback to enable access for the drill rig. Further excavation occurs only after prestressing and proof-testing of the anchors. As Figure 4 illustrates, the process can be repeated for additional levels of tiebacks. Building codes require that all tiebacks are proof-tested to an excess percentage of their final lock-off load, which usually ranges from 120 to 150% of the final lock-off load. Regroutable tiebacks are most commonly used because their capacity can be increased by regrouting (to meet test requirements) without having to drill a new anchor hole.
A tieback is made by first drilling a hole with an auger and then placing a bar (tendon) in the hole, concrete is then poured in the hole and the connection with wall is made (Figure 3). Different types of augers are used to drill the tieback holes. The choice of the drilling method depends on the soil/rock conditions on the site.
Drilling should be done carefully since inadequate procedures can cause significant soil losses. The biproduct of drilling is removed by flushing the hole with either air, water, or slurry. Air is most efficient in dry ground, but it requires special attention because it can become entrapped during drilling, building up zones of high pressure in the soil that can eject material for several feet and at high speeds (potentially injuring workers). Water flushing is best used in sticky clayey soil, and it also cleans the sides of the hole by its sweeping action, providing a stronger bond at the grout-anchor interface. Bentonite slurry flushing works the best since it keeps particles in suspension, while the sealing action keeps the hole from collapsing.
Significant soil losses through the tiebacks cause significant settlements even if the retaining walls do not move towards the excavation. In granular soils the drilled hole must be cased to avoid collapse.
Some tieback creep can be expected especially if the ties are very short and the fixed length of the tie is within soft ground. For stability reasons, the fixed anchor should be located beyond the active zone of movements. As a result, tieback anchors may not be an option at sites congested where there are adjacent underground utilities or when adjacent owners do not grant permission to drill them under their properties.
Special attention should be given to the waterproofing details at the anchor heads and at the tieback holes. Significant leakage can be caused by inadequate waterstopping details at these locations.
Figure 1: Picture from a tieback slurry wall excavation (World Bank Project Washington)
Figure 2: Tieback slurry wall excavation (Dana Farber Tower, Boston).
Figure 3: Tieback configuration, free and fixed lengths (Adapted from Schnabel, 1982)
Figure 4: Steps in making a tieback: (a) hole drilled; (b) bar placed in hole; (c) concrete poured for anchor; (d) wall connection made (Adapted from Schnabel, 1982).
Figure 5: Steps in making a multilevel tieback excavation, (A) first level of tiebacks installed and second level of tiebacks drilled, (B) second level of tiebacks installed.