performance of soil nails in dublin glacial till
TRANSCRIPT
Performance of soil nails in Dublin glacial till
Christopher O. Menkiti and Michael Long
Abstract: Soil nailing is being used in many projects in glacial tills in Ireland, particularly to provide temporary supportto steep slopes. Little design guidance is available for such materials, and it is known that the application of design proce-dures developed for other material is conservative. Detailed nail instrumentation and field monitoring has been undertakenduring large-scale soil nailing works for the Dublin Port Tunnel project. It was found that the short-term behaviour of nailswas the reverse of that assumed in current design methods. Most of the load was induced as a results of drilling and nail-ing the lift immediately below the nail being monitored rather than due to excavation-induced stress relief. The highestforces were developed in the upper nails where the largest ground movements occurred. This is the reverse of most currentdesign methods where the highest soil–nail bond is assigned to the deepest nails. It seems that the observed short-term,prefailure behaviour of nailed slopes is governed more by the deformation pattern of the slope than by large-scale develop-ment of failed wedges. Current design procedures should be reviewed. Despite this, the trial confirmed that the currentlyused procedures are highly conservative for Dublin glacial till.
Key words: soil nailing, steep slopes, glacial till, suction, design procedures.
Resume : Le cloutage du sol est utilise dans plusieurs projets dans les tills glaciaires en Irlande, particulierement pourfournir un soutien temporaire aux pentes abruptes. Il y a peu de guides de conception disponibles pour de tels materiaux eton sait que l’application des procedures de conception developpees pour d’autres materiaux est trop securitaire pour le till.On a entrepris une instrumentation detaillee de cloutage et de mesures sur le terrain au cours des travaux de cloutage desol a grande echelle pour le projet de tunnel du Port de Dublin. On a trouve que le comportement a court terme des clousetait a l’inverse de celui assume dans les methodes de conception courantes. La plus grande partie de la charge a ete in-duite par le forage et le cloutage de la couche immediatement sous le cloutage instrumente, plutot que par le relachementde contraintes induit par l’excavation. Les forces les plus elevees ont ete developpees dans les clous les plus hauts, la oules plus importants mouvements se sont produits. Ceci est a l’inverse des methodes de conception les plus courantes ou lelien sol – clou le plus fort est attribue aux clous les plus profonds. Il semblerait que le comportement observe a courtterme de la prerupture de la pente cloutee est regi plus par le schema de deformation de la pente que par le developpementa grande echelle de coins de rupture. Les procedures de conception courantes devraient etre revisees. En depit de cela,l’essai a confirme que les procedures couramment utilisees sont hautement securitaires pour le till glaciaire de Dublin.
Mots-cles : cloutage de sol, pentes abruptes, till glaciaire, succion, procedures de conception.
[Traduit par la Redaction]
IntroductionThe Dublin Port Tunnel (DPT) provides a link from the
motorway system north of Dublin to the port area. The cen-tral section comprises twin bored tunnels driven by tunnelboring machines. The shallower sections of the tunnels at ei-ther end, where the invert level is less than about 25 m be-low ground level, were constructed using cut-and-covermethods. The sides of the cut were generally supported bypropped diaphragm walls or by bored pile walls. However,in the northern section, where the excavation was 12 m
deep or less, ground conditions were such as to suggest cutslopes supported by soil nailing.
For the soil-nailed section, there were very significantconstraints on the design. Construction was confined to anarrow corridor along the line of the existing M1 motorway.The motorway carriageways had to be reduced to one lanefor each direction and diverted to either side of the cut.Even with this configuration and using steep-sided excava-tions (up to 808), the carriageways were only 2 m from theslope crest (Fig. 1). Additionally, no disruption of the mo-torway or damage to adjacent properties could be counte-nanced, and there were tight cost and programmeconstraints. However, the temporary works design life wasrelatively short at about 6 months.
At the time of design (2000–2002), soil nailing was stillin its infancy. In particular, little experience existed on thedesign of soil nails in Dublin glacial till (known locally asDublin boulder clay, DBC). A case history had been re-ported by Pedley (2000), but there was no specific designmanual or advice note for Irish conditions or indeed for gla-cial tills. Johnson et al. (2002) have reported on the use ofsoil nails in glacial till for a modest 2 m high slope neededfor a highway widening project in the UK. A similar solu-
Received 6 July 2006. Accepted 28 July 2008. Published on theNRC Research Press Web site at cgj.nrc.ca on 9 December2008.
C.O. Menkiti. Geotechnical Consulting Group, 1A QueensberryPlace, London SW7 2DL, UK.M. Long.1,2 School of Architecture, Landscape and CivilEngineering, University College Dublin, Newstead Building,Belfield, Dublin 4, Ireland.
1Corresponding author (e-mail: [email protected]).2Present address: Department of Civil Engineering, UniversityCollege Dublin, Earlsfort Terrace, Dublin 2, Ireland.
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tion was detailed by Nicholson and Tse (1999) for a cutslope in glacial till at Tinny Bank near Moffatt on the M6motorway between Glasgow and Carlisle. Unwin (2001)also reported on some experience with soil nailing in glacialtill in northeast England. More recently, however, guidanceon UK practice was published by Phear et al. (2004).
At the time of the DPT design, local experience in Ireland(e.g., Pedley 2000) suggested that conservatism existedwhen foreign codes were adopted. As a large soil-nailingproject in DBC was ongoing, an opportunity existed for aresearch project involving instrumentation of a number ofthe nails. Detailed records of construction, geology, instru-mentation, and design performance were already being keptby the design team for design verification purposes, whichcould be drawn upon for the research work.
The research had the following purposes:
� To instrument some of the soil nails to evaluate their per-formance as structural units.
� Using this and other complementary project data to studythe performance of the whole slope.
� As there was an increased likelihood of future use of soilnails in DBC, given their flexibility and cost savings overretaining walls, the research results could allow designimprovements to provide commercial advantage in futureworks involving DBC.
Ground conditionsDetailed descriptions of the ground conditions and the
properties of DBC are beyond the scope of this paper butare available in other publications. The reader is referred toSkipper et al. (2005) described the background geology,Long and Menkiti (2007) discussed the engineering proper-ties of the soil, and Menkiti et al. (2004) described a full-scale instrumented trial excavation at the site.
The ground conditions encountered are summarized inTable 1 and in Fig. 2. Conditions are relatively uniform andare characterized by low moisture content, high density, lowplasticity, low permeability, and very high stiffness andstrength. Ground-water pressure is hydrostatic beneath awatertable at about 2 m depth. Average values of other per-tinent soil properties are summarized in Table 2. It isthought that the coefficient of earth pressure at rest (K0) isof the order of 1.0. It seems likely that the material exhibitsa curved failure surface with high values of effective frictionangle (f0) at low effective stress.
Design philosophyTemporary support for the cut slopes was only required
for a period of 3–6 months, during which the reinforcedconcrete tunnel was built and the excavation was backfilled.Excavations would largely be in boulder clays, which wereknown locally to stand at steep slope angles in natural andman-made slopes for considerable periods without support(Long et al. 2003). Advantage could be taken of this short-term stability to provide a temporary works design solution.
However, investigations of the boulder clays had indi-cated that, while they have relatively high f0 for fine-grained materials, there is negligible c’ that could be reliedon in design. A slope 12 m high inclined at 708–808, with
Fig. 1. Typical cross section.
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equilibrium pore pressures consistent with a groundwater ta-ble close to the ground surface, would require a substantialinstallation of soil nails to maintain stability under long-term conditions. Clearly, the observed stability of steepslopes could only be explained by the presence of significantsuctions created in the slopes during excavation and main-
tained for long periods by the very low permeability of theboulder clay (Long et al. 2004).
An ‘‘observational approach’’ design solution evolved.The default design was applied for lengths of the cut thatwould be required by the construction programme to remainopen for periods in excess of 26 weeks. The default design
Table 1. Summary of ground conditions.
Stratum Thickness (m) NatureTopsoil/loess/made ground Typically 1.0, maximum 3.0 Variable, predominantly highly permeable coarse gravel with other
materialsUpper brown boulder clay (UBrBC) Typically 2.0 Firm to stiff friable slightly sandy clay with occasional cobbles
and fine to coarse gravel; rare thin lenses of grey sandy silt withsome medium gravel zones
Upper black boulder clay (UBkBC) 4–10 Very stiff to hard dark-grey slightly sandy clay with some graveland cobbles; rare fissures, subvertical, rough and tightly closed,spacing 0.5–0.75 m; occasional small gravel lenses, mostlyforming hydraulically isolated pockets
Lower brown boulder Clay (LBrBC) 5.0–9.0 Hard brown silty clay with gravel, cobbles, and boulders; morefrequent, larger and more complex silt–gravel lenses thanUBkBC; continuous 2 m thick layer of silty sand – fine gravel at10–16 m depth over a 350 m long stretch of the works
Lower black boulder clay (LBkBC) Patchy, up to 4.0 m thick Hard gravelly silty clay with an abundance of bouldersLimestone bedrock Encountered at 16 m depth
or belowDark-grey fine-grained limestone interbedded with black shale;
upper few metres often highly weathered
Fig. 2. Summary of ground conditions. wp, plastic limit; wL, liquid limit; nmc, natural moisture content.
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(see the following section), was also applied where the geol-ogy was problematic, usually owing to permeable layers inthe lower part of the slope (which would allow suctions todissipate quickly and thus cause instability). Elsewhere, thelower rows of nails were omitted based on criteria estab-lished from results of the trial excavation and finite elementanalyses. In all cases, detailed geological logging of excava-tion faces was undertaken and the performance of the slopewas monitored so that remedial action could be taken from apredetermined suite of measures if necessary. More detailsare given by Long et al. (2003).
Mostly for safety reasons, it was decided that the top halfof the slope would always be nailed (i.e., the top five rowsof nails would always be installed).
Soil nail designAs a fall-back position for when circumstances required,
default designs were developed based on long-term soilstrengths. The basic design was for a 12 m high slope at808 (for a theoretical slope at 758 with a construction toler-ance of 58, see Fig. 3). Soil parameters used in the designare summarized in Table 3. Partial factors of 1.1 were ap-plied to give design values; the unit weight was only re-duced when calculating vertical stresses for the assessmentof nail pull-out resistance. A surcharge of 22.5 kPa was in-cluded in the design as required by the client’s specification.
Nail design was, in general, to Advice Note HA68/94(UK Department of Transport 1994 and discussed in detailby Love and Milligan 1995) with reference to Clouterre(1991) and FHWA (1998). Nails were designed to have aninclination of 108 and to be installed in 100 mm diameterholes. Pull-out resistance (bond) was calculated from theoverburden pressure, using the method given in HA68/94but with a minimum bond stress of 50 kPa in the upper partsof the slope where the calculated overburden pressure is
Table 2. Summary of pertinent soil parameters (modified and extended from Long and Menkiti 2007).
Upper brown boulderclay (UBrBC)
Upper black boulderclay (UBkBC)
Lower brown boulderClay (LBrBC)
Lower black boulderclay (LBkBC)
Clay content (%) 12 15 18 18Undrained strength from unconsolidated
undrained triaxial tests, su(UU) (kPa)80 220 350 >250
Coefficient of permeability, k (m/s) *10–8 *10–10 10–8–10–10 *10–10
Small-strain shear modulus, Gmax (MPa) 250–1000 1000–1200 1000–1200 1000–1500Effective friction angle, (8) 36 38 38 38Effective cohesion, c’ (kPa) 0 0 0 0
Fig. 3. Default design for soil-nailed slope (nails in rows 6–10 or 8–10 omitted where allowed by observational approach).
Table 3. Summary of soil nail design parameters.
StratumBulk unit weight,g (kN/m3)
Effective frictionangle, (8)
Effective cohesion,c’ (kN/m2)
Pore pressureratio, ru
Upper brown boulder clay (UBrBC) 20 31.5 0 £0.4Upper black boulder clay (UBkBC) 20.5 32.5 0 £0.2
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low. Following extensive testing of nails during construc-tion, the minimum bond strength was raised for the laterstages of design, first to 85 kPa, then to 90 kPa for the top
two rows of nails, and to 110 kPa for the remainder. Figure 4shows this design profile together with measured bondstrength from pull-out tests on sacrificial test nails. The test
Fig. 5. Typical output from slope stability analyses.
Fig. 4. Measured ultimate skin friction versus depth for Dublin boulder clay (DBC).
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nails were drilled in the same manner as the majority of theworking nails, using air flush mixed with the minimumamount of foam necessary to control dust generation. Thesedesign bond values of 90–110 kPa provided a factor ofsafety of 2.5 on measured ultimate bond resistance. Thesedesign bond values can be seen to be much higher than thebond strength calculated using standard design methods in-dicating a significant level of conservatism for temporaryworks applications (Fig. 4). The high pullout capacity andfavourable monitored slope performance allowed the designnail spacing to be optimized from 1.5 to 1.7 m and up to1.9 m. However, for the 1.9 m spacing, the shotcrete facingthickness was increased from 75 to 110 mm.
Excavation was typically carried out in four stages todepths of 3, 6, 9, and 12 m (Fig. 3). Slope stability at eachexcavation stage was considered, using limit equilibriummethods, and checked for two-part wedge mechanisms usingGeosolve SLOPE program (ver. 9R) as well as for circularfailure surfaces using the SLOPE/W program (ver. 4.24). Inaddition to the partial factors on soil parameter values, apartial factor of 1.3 was applied to surcharge loads and nailpull-out resistance. With these partial factors, an overall fac-tor of safety of 1.2 was sought for circular failure surfacesand 1.0 from the biplanar wedge failure mechanism. This isbecause the two-part wedge analysis is known to be conser-vative. With this approach, both methods were generallyfound to give consistent results. Some typical analysis out-
put is given in Fig. 5 to demonstrate the failure mechanismassumed for comparison with the actual nail behaviour de-scribed later in the paper.
Layout of instrumented soil nails
Chainage 980E — good ground conditionsAt chainage (Ch) 980E, ‘‘good’’ ground conditions were
present (i.e., as defined in Table 1; UBrBC and UBkBC asexpected with no significant granular lenses) and thereforethe lower three rows of nails were omitted (Fig. 6). Instru-ments were installed on nails 2, 4, and 6. The upper nailcomprised an 8 m long, 20 mm diameter high tensile steelbar; the lower two were 10 m long, 25 mm diameter bars.Pairs of strain gauges were installed on opposite sides ofthe bar. Installation locations were at 0.3 m from the exca-vated slope face, 2 m from the excavated slope face, andthereafter at 2 m intervals along the nail length. The instru-mented nails were oriented so that the strain gauges were at12 and 6 o’clock positions to measure vertical bending. Thestrain gauges were equipped with thermistors, which al-lowed measurement of temperature at each instrument loca-tion.
Geokon VK-4100/4150 strain gauges were used with agauge length of 51 mm (2 inch), a range of 3000 micro-strain, and a sensitivity of 1 microstrain. The gauges are de-signed for use on steel structures or reinforcement bars.
Fig. 6. Layout of instrumented soil nails at Ch 980E — good ground conditions. bgl, below ground level.
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Fig. 7. Layout of instrumented soil nails at Ch 940E — poor ground conditions.
Fig. 8. Soil nail installation pictures: (a) nail with strain gauges, (b) signal pick-up device, (c) post inflatable packer, (d) face showingprotected nail head, shotcreting, previously installed nails, and connection to data logger.
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They have an accuracy of ±0.1% of full scale, a nonlinearityless than 0.5%, and are designed to operate in a temperaturerange from –20 to +80 8C.
Additional instrumentation at Ch 980E comprised piezom-eters at 4, 8, and 12 m depth as well as an inclinometer,surface-mounted prisms for detecting slope face movement,and settlement markers at the slope crest.
Chainage 940E — poor ground conditionsAt this location, as can be seen in Fig. 7, a granular layer
was proven at just below 8 m and therefore the default de-sign was applied and all nails were installed. Based on theexperience at Ch 980E, as described later in the paper, anddue to cost constraints, only nail 2 was instrumented. Theinstalled instrumentation was similar to that for Ch 980E
Fig. 9. Average tensile and bending strains in nail 980E, row 2.
Fig. 10. Average tensile loads in nail 980E, row 2.
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nail 2. The additional instrumentation at Ch 940E was alsothe same as that at Ch 980E.
Preparation and installation of instrumentednails
Nails at Ch 980EDetails of the nail instrumentation procedure are shown in
Fig. 8 and the following procedure was adopted for attach-ment of the strain gauges to the nails:
� Log face and mark out nail location.� Select nail tendon, mark out strain gauge locations, and
grind down a smooth bed at each location.� Spot weld vibrating wire gauges to top and bottom of
nail (Fig. 8a).� Waterproof gauges using Skotchkote electrical sealant.� Install pick-ups and protection (Fig. 8b).� Take zero readings of strain gauges with nail in a flat un-
stressed configuration.� Attach post-inflatable packer (Fig. 8c). (The packer was
formed from an inner tube of a motorcycle tyre, and itspurpose was (i) to protect the gauge and (ii) to allow thegrout column to be broken out post-installation so as notto influence the gauge readings.)
� Check gauges at each stage.� Protect packers with geotextile (to avoid puncture during
tendon insertion).� Drill hole using rotary techniques with rock roller or
blade bits (note borehole needs no casing as soil is self-supporting).
� Grout hole.� Install nail tendon with the correct orientation and with
the strain gauges at the correct depth.� Fully inflate packer and displace fluid grout at instrumen-
ted locations.� Protect excavated face as shown in Fig. 8d with mesh
and shotcrete.� Install survey prism to nail head.� Hook up to data logger.
Initial soil nailing for the project was with a compressordelivering 12 bar (1 bar = 100 kPa) air pressure. It wasquickly observed from the detailed monitoring conductedthat the drilling process induced large lateral movements ofthe slope by a process of pneumatic fracturing. Lateralmovements of 10–15 mm per lift were observed owing tocreation of new subvertical discontinuities or opening up ofpre-existing but tightly closed fissures. This was confirmedby observed interconnectivity between nail holes with groutbeing blown out of nail holes up to 8 m apart. This drillingprocess was unsatisfactory as it damaged the soil fabric, in-creased the mass permeability of the ground, and possiblydestroyed the excavation-induced soil suctions in theground. However, drilling with air flush was preferred tomaximize skin friction. Trials were therefore carried outearly in the project, and this problem was avoided by usinga 7.5 bar compressor or a 12 bar compressor at a reduced airflow setting and routine by cleaning out the drill hole duringdrilling. (This latter point is important as it prevented block-age around the drill string; a situation that would deliver thefull air pressure to the soil mass.) This revised drilling pro-cedure was used for most of the soil nailing (including at the
instrumented nail sections). The revised drilling method re-duced drilling-induced movements to a maximum of 3–4 mm of horizontal movement for the first lift and evensmaller movements for deeper lifts.
Nail at Ch 940EAt Ch 940E, a similar procedure to that used at Ch 980E
was adopted for installing the instrumented nail except thatsome changes were made to the packer system. A disadvant-age of the post installation packers was the physical diffi-culty in dealing with and protecting the many wires andtubes that were necessary. At Ch 940E, a pre-inflated packerwas used. This was actually a child’s water-safety arm band.The packer was installed with non-return valve, inflated tojust below the borehole diameter, and subsequently pro-tected by a geotextile as before. This system proved mucheasier to install because of the reduced amount of tubing.
Results of monitoring
Chainage 980E, row 2Average tensile and bending strains induced in nail 2 at
Ch 980E are shown in Fig. 9 for a period of some 8 months.Note gauge pair A is towards the end of the nail and pair Dis towards the front (Fig. 6). It can be seen that the tensilestrains are significant but the bending strains are relativelysmall. This confirms that the nails are acting in tension asdesigned. Following the final soil excavation at this locationon 6 June 2003, both the axial strains and the bendingstrains remained relatively constant.
Average tensile loads induced in nail 2 are shown inFig. 10 with an enlarged x axis to focus on load changesduring the construction process. Immediately following nailinstallation, very small compressive loads are induced at thehead of the nail owing to the weight of the shotcrete facingthat sits on the nail. Thereafter, the effect of the next soilexcavation (lift 2) can be observed to induce relatively smalltensions. However, the effect of installing the nails in thislift on 11 April 2003 was considerable. It seems that the ef-fects of drilling, using compressed air even with the reviseddrilling method, caused ground movement and loading of al-ready installed nails in the lift above. (The situation on siteon this day is shown in Fig. 11.) In Fig. 10, it can be ob-served that lift 3 has some effects on nail 2, but lifts 4 and5 have relatively minor influence. The revised drilling pro-
Fig. 11. Situation on site on 11 April 2003 during nailing of lift 2at Ch 980E.
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cedure was observed to induce tensions in the outermost 5–6 m of the row 2 nails during drilling for lift 2 (locations D,C, and B). Induced tensions were nominally about 30 kN.The nominal tension was computed by applying the meas-ured strain to the nominal cross section of the soil nail.However, at the strain gauge locations, the bar diameter isslightly reduced as the strain gauges were bonded onto flatsurfaces on the bars to promote good bonding (see Fig. 8a).Correction for the reduced bar cross section would implythat the actual bar tension is 12.5% lower for the 20 mm di-ameter row 2 nails and 6.1% lower for the 25 mm diameterrow 4 and row 6 nails. Only nominal tensile forces arequoted in this paper.
The development of shear stress along nail 2 is shown inFig. 12. Again, it can be seen that most of the skin frictionis developed during nailing for lift 2 and the changes in skinfriction are much smaller at the other construction stages.
Most of the skin friction is developed towards the distalend of the nail where the tension in the bar is changingmost rapidly with distance along the bar. Maximum mobi-lized shear stress is of the order of 60 kPa, which is signifi-cantly lower than the minimum design bond stress of90 kPa and measured bond values of 250–350 kPa at thislevel (Fig. 4).
Load in all nailsThe average nail load for all instrumented nails at the
strain gauge pair closest to the shotcrete face is shown inFig. 13. A similar pattern to that developed for Ch 980Enail 2 can be observed, that is,
� excavation of next lift has only minor effect,� much of the load is developed at an early stage during
nailing works for the subsequent lift,
Fig. 12. Development of shear stress along nail 2 Ch 980E.
Fig. 13. Development of load — all nails.
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� developed load generally remains constant during themonitoring period, and
� the developed load generally decreases with an increasein depth from nail 2 to nail 6.The load induced in nail 2 at Ch 940E is slightly lower
than that at Ch 980E. A maximum load of 40 kN was meas-ured in 980E row 2, compared with 35 kN at 940E row 2.This may reflect variability in the ground and the construc-tion process but may also be due to more rows of nailsbeing present at Ch 940E and the subsequent greater avail-able resistance capacity.
Lateral slope displacementHorizontal slope movement at Ch 980E and Ch 940E is
shown in Figs. 14a and 14b, respectively. For Ch 980E, it canbe seen that a maximum movement of about 8 mm occurredowing to excavation-induced stress relief following excava-
tion to about 5.5 m for lifts 1 and 2 and nailing lift 1. Duringnailing of lift 2, further significant movements of about 3 mmoccurred. This is consistent with the period during which mostload was induced in the nails. Subsequently, during lifts 3, 4,and 5, some further movement up to a maximum of 3 mm oc-curred. Following the installation of nails in lift 1, the slopebehaved like a ‘‘propped cantilever’’ retaining wall with thenails restraining the movements towards the top of the slope.No movement occurred below the final excavation level.
A similar pattern is again evident for Ch 940E. Here, theslope behaviour resembles a ‘‘cantilever’’ retaining wall un-til the nails in lift 3 are installed. Maximum movement isagain about 16 mm and no movement occurred below exca-vation level.
Loads on the shotcrete facingLoading on the shotcrete facing has been inferred from
Fig. 14. Lateral slope movement from inclinometers.
Table 4. As-built nail spacing at the locations of instrumented nails.
As-built spacing at nail location
Instrumented nail Vertical spacing (m) Horizontal spacing (m) Assumed effective area (m2)980E row 2 1.25 1.5 1.2 � 1.5 = 1.875980E row 4 0.9 1.5 0.9 � 1.5 = 1.35980E row 6 1.55* (1.9{ + 1.5{)/2 1.55 � 1.7 = 2.635940E row 2 1.35 1.7 1.35 � 1.7 = 2.295
*There are no nails below nail 980E row 6 (see Fig. 6). The vertical spacing is taken to extend from thetheoretical location of row 7 to the mid-point between rows 5 and 6.
{A change in horizontal spacing occurred at nail 980E row 6. To the right of this nail, a nail spacing of 1.5 m wasused. To the left of the nail, design optimization allowed a wider spacing of 1.9 m to be used.
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the nail head loads, assuming that the shotcrete is permeablerelative to the clay. (This assumption is consistent with theobservations on this site and the presence of numerous con-struction joints and shotcrete permeability measurements atother sites.) The loading on the shotcrete facing is presentedas equivalent earth pressure based on the as-built nail spac-ing. The earth pressure has been calculated from an effectivearea around each nail as shown in Table 4. It is assumedthat the effective area around each nail extends from thatnail to a point midway between it and the neighbouringnails. No nails exist below nail 6 at Ch 980E (Fig. 6).Therefore, as a first approximation, the equivalent area forthis nail has been taken to extend down to the locationwhere nail 7 would have been installed. The earth pressurespresented in Fig. 15 are similar to the active pressures inDBC (for which a K0 value is thought to be about 1.0 orhigher; e.g., Long and Menkiti 2007).
It appears that slope movements of only 16 mm or 0.13%of the total slope height are sufficient to reduce the short-term pressures on the shotcrete facing towards active values.
TemperatureThe thermistors incorporated in the strain gauges showed
some changes in temperature during the monitoring period.The largest temperature changes occurred at the straingauges nearest the nail head, which recorded temperatureranging from 6 to 20 8C, primarily reflecting small dailychanges superimposed on a more gradual seasonal change.The deepest strain gauges experienced the most muted tem-perature changes, recording temperatures of 10–14 8C, whichreflected only a seasonal change (daily changes being fullymuted at this depth). Temperature changes only have asecond-order effect on the measured tendon strains becausethe gauges are temperature-compensated for steel. Closescrutiny of the daily temperature changes of the shallowestgauges indicates a temperature effect of about 3.5 micro-strains/8C, compared with a coefficient of thermal expansionof steel of 12.2 microstrains/8C.
Crest settlementCrest settlements measured by conventional geodetic sur-
veying were small and generally less than 3 mm.
Pore pressurePore pressure development in the slopes was similar to
that described by Long et al. (2004). On excavation,pore pressures reduced from the in situ hydrostatic val-ues by small amounts due to stress relief. For example,at Ch 940E, pore pressure at 4, 8, and 12 m reduced by25 kPa to 35 kPa (ru reduced by 0.3 to 0.06), that is,pore pressures remained slightly positive. The situationis similar at Ch 980E where pore pressures reduced by18 kPa to 45 kPa (i.e., ru reduced by 0.28 to 0.13).
Comparison with designA comparison between ultimate available nail-tendon ca-
pacity and maximum mobilized force is shown in Fig. 16. Itcan be seen that the mobilized capacity of the nails is fromsome 25% to 30% of the available ultimate capacity. Thissuggests that, although the overall nail–slope behaviour is
not as assumed in the calculations, the actual design is con-servative. The same conclusion is true for nail bond capacityas shown in Figs. 4 and 12. The measured short-term bondstress in DBC is very high, allowing design optimization fortemporary works problems.
Fig. 15. Earth pressures on shotcrete facing inferred from the nailhead forces.
Fig. 16. Comparison of ultimate and mobilized nail-tendon capa-city.
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Link between short-term and long-termbehaviour
Little research has been done on the link between short-term and long-term behaviour of soil-nailed slopes in clays.As reported by Phear et al. (2004), the UK Transport Re-search Laboratory attempted to confirm the relationship be-tween short-term tests and longer-term values by carryingout long-term pull-out tests to failure (Johnson et al.2002). In these tests, the load was raised incrementallyevery 3–4 weeks. The total test periods at these two siteswere 8 and 20 months, respectively. These sites were inlow-to-medium plasticity glacial till and high plasticity Ox-ford clay, respectively. The results were inconclusive be-cause 20 months may not have been long enough for thepore pressures to reach equilibrium.
Long-term behaviour in DBC is also uncertain and re-quires further research to see how much of the short-termstrength can be relied upon in the long term. Considerationwould have to be given to issues such as pore pressure dis-sipation, durability, and creep. Figure 17 illustrates the be-haviour of a cut slope with a uniform distribution of soilnails that progresses to failure. The pattern of slope defor-mation at a horizon I–I’ close to the slope crest is shownalong with the pattern of nail forces. In the prefailure phase,the maximum deformations and the maximum nail forcesoccur at the top of the slope. A similar pattern of behaviourwas observed in full-scale model tests reported in Clouterre(1991) and in this paper. As the hypothetical slope pro-gresses towards failure, deformations accumulate and local-ize and the maximum deformation migrates down to a pointat some depth. The distribution of nail forces also changesin a similar pattern. If the hypothetical slope is adequatelyreinforced, then it will not progress to failure and the patternof slope displacements and nail forces will only evolve to astate part way between the initial prefailure conditions andthe distribution at failure.
The pattern of nail pullout capacity assumed in most de-sign methods is similar to that at failure and may be dissim-ilar to the short-term or prefailure pattern of nail forces. Theforces and deformation patterns in many engineering struc-
tures are different under working conditions than those as-sumed at failure. There is nothing wrong with this as longas the behaviour is ductile and failure does not occur as aresult of brittle behaviour, instability, or progressive failure.
The results reported here confirm that in the short term, theconstruction process dominates with large loads being devel-oped in the upper rows of nails owing to drilling–excavation.In the long term, the slope will tend towards a drained condi-tion such that the conventional design assumptions apply,with higher loads developing in the lower nails. The data pre-sented here contribute to an understanding of the short-termserviceability performance of temporary soil-nailed slopes inrelatively impermeable ground and also confirm that bothshort term and long term should be considered in design.
Conclusions
(1) The instrumented nail project worked well with all in-struments responding and with no failures. This successwas due to close attention to detail.
(2) Following the final excavation, little change occurred inthe instrument readings over the monitoring period ofabout 10 months. This is due to the low permeability ofthe DBC and the corresponding very slow equalizationof pore pressures.
(3) Nails act mostly in tension with only limited bending in-duced.
(4) The behaviour of nails is the reverse of that assumed incurrent design methods. (i) Most load was induced dueto drilling and nailing the lift immediately below thenail rather than due to excavation-induced stress relief.(ii) The highest forces were developed in the upper nails,whereas the maximum bond capacity is assumed to beavailable at depth in most design methods. (iii) Mobi-lized earth pressure on the shotcrete facing approachesactive values after slope movements of only 0.13% ofthe slope height. This is consistent with the expected be-haviour for dense soils, which typically achieve activeconditions at about 0.1% of slope height.
Fig. 17. Deformations and nail forces in a typical cut slope with a uniform distribution of nails.
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(5) Maximum shear stress is developed towards the distalend of the nail.
(6) Load development in the nails mirrors the deformationpattern of the slope.
(7) It would seem that current design procedures do not re-flect these aspects of behaviour. A more sophisticatedapproach (e.g., finite element methods) may be consid-ered.
(8) Despite this, current design methods are still very con-servative for DBC. Further design optimization is possi-ble as experience with using this method of constructionis gained.
AcknowledgementsThe authors are very grateful to the sponsors of the proj-
ect, namely Geotechnical Consulting Group, UK; UniversityCollege Dublin; Applied Ground Engineering ConsultantsLimited; ARUP Consulting Engineers; PJ Edwards & Com-pany Ltd.; John Barnett & Associates Limited (part of CSAGroup); and Byrne Looby & Partners. In addition, Mowlem–Irishenco JV (main contractor for the Dublin Port Tunnelproject) provided men and materials free of charge andITM-Soils supplied the instrumentation at discounted re-search rates. The contribution and assistance of the authors,colleagues, including Ben Follett, George Milligan, and SamPatterson are greatfully acknowledged.
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FHWA. 1998. Manual for design and construction monitoring ofsoil nail walls. US Department of Transportation, Washington,D.C. FHWA-SA-96-069R.
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List of symbols
c’ effective cohesionFmax maximum force induce in soil nailGmax small-strain shear modulus
k coefficient of permeabilityK0 coefficient of earth pressure at rest ()ru pore pressure ratio (= u /sv)su undrained shear strength
su(UU) undrained strength from unconsolidated undrainedtriaxial tests
u pore pressurewL liquid limitwp plastic limit
31, 32 strain in gauges on top and bottom of nail, respec-tively
g bulk unit weightka coefficient of active earth pressuref’ effective friction angle
s0h0 horizontal effective stress
s0v0 vertical effective stresssv total vertical stresst shear stress
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This article has been cited by:
1. C. O. Menkiti, M. Long, G. W. E. Milligan, P. Higgins. 2013. Soil Nailing in Dublin Boulder Clay. Geotechnical and GeologicalEngineering . [CrossRef]
2. Sang-Soo Jeon. 2012. Pull-out tests and slope stability analyses of nailing systems comprising single and multi rebars with groutedcement. Journal of Central South University 19:1, 262-272. [CrossRef]
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