Effects of Drilling for Tieback Anchors on Surrounding Ground: Results from Field Tests

It is well established that deep supported excavations in soft clay deposits can cause significant ground settlements in the areas surrounding the excavation, ranging from approximately 0.5% to 2% of the final excavation depth $H$ (e.g., Peck 1969; Mana and Clough 1981; Karlsrud and Andresen 2008). Recent experience, however, shows that ground settlements caused by initial and secondary effects from the installation of drilled tieback anchors and bored piles from inside an excavation can be significantly larger than 2% of the excavation depth (Langford et al. 2015). For excavations in urban areas, deformations of such magnitudes imply a large potential for causing damage to neighboring buildings and structures and for the associated large liability potentials. A recently completed research project in Norway focused on understanding and identifying the causes of excessive settlements associated with excavations and foundation works (Baardvik et al. 2016).

Although negative installation effects related to drilling for ground anchors and piles in varying soil conditions is recognized in some literature, e.g., Kempfert and Gebreselassie (1999), Kullingsjø (2007), Konstantakos et al. (2004), and Bredenberg et al. (2014), the problem has not been systematically addressed and studied. There is specifically a lack of knowledge related to the effects of drilling on the surrounding ground and the extent to which it may cause ground movements. Disregarding some general guidelines related to the design and implementation of drilled piles (Finnish Road Authorities 2003; FHWA 2005), the authors have not found specific guidelines for selecting appropriate drilling methods or installation procedures to reduce the risk of excessive ground movements.

Kempfert and Gebresellassie (1999) reported excessive settlements and damage on an adjacent building due to drilling and pulling of casings for tieback anchors as support for an up to 7.0-m-deep excavation in soft lacustrine clay. Konstantakos et al. (2004) reported a case study from an up to 23-m-deep excavation in Boston. The excavation was supported by a 0.9-m-thick diaphragm wall embedded in bedrock and four to six levels of poststressed tieback anchors into bedrock. A maximum of 65-mm ground surface settlements was recorded on the outside of the excavation. The excessive settlements were explained by local cavities and loss in soil volume around the anchors during drilling through sand and silt layers. The hypothesis was confirmed by finite-element analyses, corresponding to a loss in soil volume of approximately $0.36–0.50{\mathrm{m}}^3/\mathrm{linear}\mathrm{meter}$ of the supported diaphragm wall. Results from the analyses agreed well with monitoring results. Details regarding drilling method and execution were, however, not presented.

Kullingsjø (2007) presented monitoring data for a deep supported excavation in soft clay in Gothenburg, Sweden. Results from inclinometers on the sheet pile wall and extensometers installed in the ground behind the wall clearly indicate that drilling of casings for tieback anchors caused loss of soil volume (cavities) in a silty sand layer just above bedrock. The volume loss resulted in significant large ground settlements of up to 40 mm, approximately 0.4% of the excavation depth. Bredenberg et al. (2014) describes a case record from Stockholm, Sweden, where casings with an outer diameter of 168 mm ($\mathrm{OD}=168\mathrm{mm}$) were drilled for steel core piles through soft marine clay and into bedrock. Drilling was carried out with a so-called down-the-hole (DTH) air hammer and a concentric drill bit designed to reduce the risk of cavities due to high-pressure air flushing. Monitoring data showed that drilling with the new concentric drill bit caused from 10- to 15-mm settlements on nearby basement floors. That was approximately 70% less settlement compared to an adjacent construction project with similar ground conditions where a conventional eccentric drill bit was used. Rønning (2011) gives a brief description of monitoring results from a test installation for a bored steel pipe wall ($\mathrm{OD}=610\mathrm{mm}$) in quick clay in Trondheim, Norway. This field study showed that it is possible to drill through sensitive and quick clay causing only a limited mechanically remolded zone close to the pile wall. Total pressure sensors installed at the pile tip showed maximum excess pressure of approximately twice the effective overburden stress during drilling. Piezometers installed 0.5 m from the pile wall at depths of 6 and 13 m showed excess pore pressures of 23 and 25 kPa, respectively.

The main scope of the full-scale field test program described herein was to identify the main so-called installation effects and better understand the ground response to drilling for tieback anchors through a soft clay deposit and into bedrock. The study seeks to investigate the difference between five drilling systems, including with or without casing and with air-driven and water-driven hammers. The field trials and the primary results were briefly presented by Lande and Karlsrud (2015) but are assessed in more detail in this paper.

In Scandinavia the use of tieback anchors and piles (both micropiles and large-diameter steel pipe piles) that are drilled through soils and into bedrock has increased significantly during the last decades. There are several reasons for this. First, typical ground conditions with soft clay overlying solid bedrock favor anchors and piles to bedrock due to the considerably larger capacity compared to soil anchors and friction piles. Second, contractors often prefer tieback anchors instead of internal struts for deep excavations due to more efficient excavation and construction processes. Third, installation of piles by drilling can be performed efficiently using relatively small, lightweight drill rigs. Finally, piles installed by drilling and grouting into bedrock can resist both axial compression and tensile forces.

Many drilling methods and systems are available for drilling tieback anchors and piles in diverse ground conditions. The drilling method has traditionally been selected on the basis of efficiency and cost of construction, and often with less focus on minimizing soil disturbance and damage to the surroundings. According to the Federal Highway Administration (FHWA 2005), drilling can be divided into two main categories: open hole drilling, i.e., without casing, or with a continuous casing supporting the borehole. The latter is referred to as overburden drilling in this paper. Drilling in soft and sensitive soil often requires the use of a casing to support the borehole. Fig. 1 illustrates three systems for overburden drilling using the rotary percussive duplex drilling method (FHWA 2005), where the drill bit is both percussed and rotated. Fig. 1(a) shows a hydraulic powered top drive (top hammer) with an eccentric drill bit where both rotation and percussion are applied at the top of the drill rod by the drill head of the rig. Figs. 1(b and c) show examples of DTH hammers where a percussion hammer is located just above the drill bit, and the drill rod is rotated by the drill head. DTH hammers are driven by compressed air or water with high pressure. Both top-hammer and DTH drilling methods use continuous flushing with compressed air or water to remove soil cuttings from the front of the drill bit and transport them up to the ground surface through the annulus between the casing and the drill rod. Fig. 1(c) illustrates a reversed circulation (RC) drilling system with a double-tubed drill rod (dual wall) where the cuttings and flushing returns along the inner tube.

Many different drill bits are in use for different ground conditions and applications, most of them available for both top hammers and DTH drilling. The traditional eccentric drill bits are the most commonly used for overburden drilling. The system consists of a concentric pilot bit in the front followed by an eccentric reamer with slightly larger diameter than the casing, illustrated in Figs. 1(a and b). During penetration, a guide device on the drill bit acts on a casing shoe that is welded to the bottom of the casing, pulling down the casing. A disadvantage with the eccentric system is that the reamer may cause a gap between the casing and the borehole wall. This gap increases the risk of compressed flushing air escaping up through the gap, resulting in excessive erosion and disturbance of the surrounding soil.

To mitigate some of the shortcomings of eccentric systems, concentric systems have been developed. The concentric system consists of a pilot bit in the center, a casing shoe that is welded to the casing, and a symmetrical ring bit that is locked onto the pilot bit, illustrated in Fig. 1(c). The ring bit drills a borehole slightly larger than the outer diameter of the casing, allowing the casing to advance. The face of the pilot bit is placed almost in line with the ring bit, which facilitates keeping the borehole in its desired alignment and also in entering an inclined bedrock surface. During the past 10 years or so, several manufacturers have developed new drill bits to minimize overcoring effects. The main concept with these concentric drill bits is to redirect the air flow at the front of the bit, to limit compressed air from evacuating into the ground, and creating unwanted cavities, thereby reducing the risk of settlements.

The previously presented monitoring data clearly show that Drilling Methods 2 (Area B) and 5 (Area E) resulted in significantly larger excess pore pressures in the surrounding clay than any of the other methods. The main reason for this difference is likely due to the higher penetration rate with Methods 2 and 5 ($12\mathrm{m}/\min$) compared to Method 3 ($1–2\mathrm{m}/\min$). Measurements in Areas B and E showed that the amount of drill cuttings from single anchors was 50%–80% less than the volume of casings installed, thereby indicating significant soil displacement. With Methods 1 (Area A) and 3 (Area C) the drill cuttings tended to be larger than the volume of the drill rod/casing, indicating a net volume loss or so-called overcoring, which may explain the smaller excess pore pressures measured in these areas (Fig. 8).

The effects of soil displacement when installing a casing through clay may be comparable to the installation of displacement piles in clay if the displacement ratio is similar. The effects of pile installation on displacements has been reported by, among others, Randolph and Wroth (1979), Baligh (1985), Lehane and Jardine (1994), Edstam and Küllingsjö (2010), and Karlsrud (2012). However, the results presented in Fig. 8 show that the excess pore pressure and influence zone were much larger than expected based on past experience with driven closed-ended piles. This indicates that flushing with pressurized water [500–2,000 kPa (5–20 bar)] may have increased the soil displacement.

The soil displacements in Areas B and E should in theory have caused some minor ground heave, but this was not observed in the settlement measurements since they only captured the accumulated total effects after drilling of one or more anchors. It is therefore reasonable to assume that the high penetration rate and soil displacements may have reduced the ground settlements.

The immediate ground settlements occurring over the entire test site right after drilling with Methods 2 (Area B) and 4 (Area D) are likely explained by a combination of two main effects: local erosion and loss of soil volume and temporary pore pressure reduction above the bedrock:

Both monitoring data and observations substantiate the hypothesis that drilling by air flushing may cause air-lift pumping. Flushing with air will reduce the density of the soil-air-water mixture inside the casing, thereby creating lower pressure inside the casing than in the surrounding ground. The difference in pressure induces a flow of groundwater toward the drill bit that also may cause a substantial amount of fine-grained soils, such as silt and sand, to be transported (“sucked”) into the borehole. This effect was also observed and reported by Ahlund and Ögren (2016), and it could explain the ground settlements reported in case records by Konstantakos et.al (2004), Kullingsjø (2007), and Bredenberg et al. (2014).

The ground settlements in Area B continued to increase between 2 and 6 mm over a period of approximately 5 months after drilling was completed. This indicates that approximately 40%–50% of the resulting settlements is associated with the dissipation of excess pore pressures and reconsolidation (i.e., change in the void ratio) of possibly remolded/disturbed clay around the casings. The other four methods had similar, but smaller, impacts on pore pressures and settlements, and the settlements stopped shortly after drilling in these areas. Apart from some variations in the depth to bedrock, the marine clay deposit across the site is considered homogeneous. Variability in soil conditions is therefore not likely to explain the difference in settlements observed for the different drilling methods.

The total volume loss ($\mathrm{\Delta }{V}_{1+2}$) caused by reconsolidation of disturbed clay around the anchors has been calculated and is compared with the measured ground settlements (Table 4). The method is specific for tieback anchor installation, but some of the inputs are based on results from field tests with driven closed-ended piles in clay (Karlsrud and Haugen 1984) and model testing (Ni et al. 2009), which provide information on displacements and volumetric strains in the clay surrounding a closed-ended pile after complete reconsolidation. On the basis of these experiments, a potential volume reduction due to the reconsolidation of disturbed or highly strained clay around the drill string is estimated as follows: a volume reduction, ${\epsilon }_{v,1}$, of 15% within a 20-mm-thick layer of assumed completely remolded clay and a 10% volume reduction (${\epsilon }_{v,2}$) within partly remolded clay assumed to extend to a radial distance of twice the radius of the casing.

Table 4 presents the different parameters used to calculate the estimated settlements from reconsolidation. Fig. 11 illustrates the total area assumed to be affected by the reconsolidation of clay around the anchors in each specific test area. The ground surface area $A_{\mathrm{cons}}$ is assumed to be limited horizontally by an inclination of $2:1$ from the depth of the bedrock to the ground surface. The ground surface area $A_{\mathrm{cons}}$ is multiplied by the measured mean ground settlements from reconsolidation ${\delta }_{V,\mathrm{cons}}$ to find the total volume loss for all anchors $\mathrm{\Delta }{V}_{\mathrm{cons}}$. The measured volume loss can then be compared with the estimated volume loss $\mathrm{\Delta }{V}_{1+2}$. In Test Area B, a total area of approximately $680{\mathrm{m}}^2$ experienced a mean ground settlement of 2–3 mm due to reconsolidation. This equals a total volume loss for all anchors, $\mathrm{\Delta }{V}_{\mathrm{cons}}$ between 1.36 and $2.04{\mathrm{m}}^3$, which coincides reasonably well with the calculated volume loss, $\mathrm{\Delta }{V}_{1+2}=1.55{\mathrm{m}}^3$. The estimates for Drilling Methods 1 and 5 also show rather good agreement, but the results are only used as an indication of the potential volume loss due to reconsolidation. The measurements in Test Areas A, C, and E showed no clear settlement trends from reconsolidation.

This paper describes and present results from a full-scale field test program with drilling for tieback anchors through soft clay and into bedrock. Monitoring data and observations during the tests yielded new valuable information about the main installation effects due to overburden drilling, with a focus on the effect on pore water pressures and ground settlements.

Drilling through soft clay is often performed at a high penetration rate, which causes significant soil displacement and excess pore pressure. Results from Test Areas B and E shows that the excess pore pressure in the surrounding clay can grow to become much greater than expected for driven, closed-ended piles with the same diameter. However, most of the excess pore pressure seems to dissipate during the first days after drilling, with the remaining excess pressure dissipating in connection with the reconsolidation of remolded clay, which may take several months. The results from Area C (Method 3) gives reason to assume that a drilling penetration rate of approximately $1\mathrm{m}/\min$ reduces unwanted soil displacements and excess pore pressures in the surrounding clay. This is assumed to be valid for flushing with water and may be different if air flushing is used in soft soils. The soil displacements caused by high penetration could, to some extent, compensate for other effects that may cause ground settlements, i.e., overcoring and loss of soil volume.

Drilling with air flushing may cause uncontrolled outbursts of compressed air along the casing and into the surrounding ground, as observed in Area B with the DTH air hammer. Such outbursts can lead to overcoring and cavities around the casings. Air flushing can also cause temporary reductions in pore pressure due to the air-lift pump effect. Results from piezometers installed down to the moraine layer in Area E (PZ E10 and PZ E11) and just above it in Area D (PZ D10) clearly indicated that air flushing with Method 4 (Area D) caused a temporary drop in pore pressure within a thin permeable layer (sand/moraine) above the bedrock. The same effect likely occurred in Area B, but was not detected by the piezometers since they were installed approximately 5 m above the moraine layer. Reduced pore pressures are most likely the main reason for the immediate settlements that were measured in the entire test field area. Air flushing may also cause erosion and cavities around the casing due to the air-lift pump effect. Drilling with only water flushing will not cause such large so-called pumping effects, and so this method of drilling will reduce the risk of large ground settlements.

The relatively small ground settlements generated in the field tests stand in strong contrast to the large settlements (up to 40 cm) reported by Langford et al. (2015) around excavations in soft clays supported by tieback anchored sheet pile walls. This difference can likely be explained by (1) the relatively few anchors/casings with small diameter installed at the test field, meaning a limited affected soil volume; (2) that drilling at the test field was carried out from the ground level, which implies a reduced unbalanced earth pressure at the top of the casing during drilling compared to drilling from a lower level within the excavation; and, more importantly, (3) the fact that drilling from the ground level excludes effects of drainage up along the casing, which is commonly observed when drilling from below the water table within an excavation. Drilling from below the water table commonly reduces the pore pressure to the level of the top of the casing and starts a consolidation process from the bedrock and up through the clay deposit, which can cause large settlements (e.g., Langford et al. 2015; Langford and Baardvik 2016).

Overburden drilling of casings involves a combination of rotation, penetration, and flushing with air or water, thus making it a very complex process. The natural variations in ground conditions, quality of workmanship, drilling systems, and procedures used make it difficult to foresee the effects of installation on the surrounding ground. Given the limited research on this topic, the authors recommend that more field data be gathered and analyzed in connection with drilling in different ground conditions. It is particularly important to investigate further the effects of drilling with air versus water flushing and to test existing and new drilling methods from the bottom of excavations in the type of controlled manner that was applied in this study from the ground surface. In time, this may lead to the development of methods that limit the undesirably large ground movements observed as a consequence of drilling for tie-back anchors or piles into bedrock from within deep excavations involving soft clays.

Some or all data, models, or code generated or used during this study are available from the corresponding author by request.

The authors would like to thank the Norwegian Research Council and all partners in the R&D project BegrensSkade (Limiting Damage) who provided funding for this study. Special thanks go to the five drilling contractors that made the full-scale field test possible: Fundamentering AS, Brødrene Myhre AS, Nordisk Fundamentering AS, Entreprenørservice AS, and Hallingdal Bergboring AS.