cyclic load testing of pre-stressed rock anchors for slope...
TRANSCRIPT
J. Mt. Sci. (2016) 13(1): 126-136 e-mail: [email protected] http://jms.imde.ac.cn DOI: 10.1007/s11629-015-3605-8
126
Abstract: The objective of this research was to assess the characteristics of seismic induced damage and the deformation patterns of pre-stressed cement-grouted cables that are used for rock slope stabilization projects subjected to quasi-static cyclic loading. The experimental configuration includes the installation of 15 pre-stressed cables in a slope model made of concrete blocks (theoretically rigid rock mass) on top of a pre-existing sliding surface. The study showed that: (i) The pre-stressed cables exhibited great seismic performance. Rapid displacement of the model blocks was observed after the complete loss of the initial pre-stress load under continued applied cyclic loads and exceedance of the state of equilibrium, which implies the higher the initial pre-stress load, the better the seismic performance of the rock anchor; (ii) The failure of the pre-stressed cables was due to fracture at the connection of the tendons and cable heads under cyclic loading. The sequence of failure had a distinct pattern. Failure was first observed at the upper row of cables, which experienced the most severe damage, including the ejection of cable heads. No evidence of de-bonding was observed during the cyclic loading; (iii) The stress distribution of the bond length for pre-stressed cables was highly non-uniform.
High stress concentrations were observed at both the fixed end and the free end of the bond length both before and immediately after the state of equilibrium is exceeded. The results obtained can be used to evaluate the overall performance of pre-stressed rock anchors subject to seismic loading and their potential as rockfall prevention and stabilization measures. Keywords: Rock anchor; Prestressed cables; Cyclic loading; Slope stabilization; Seismic analysis
Introduction
Seismic activity has caused significant damage to existing geological disaster prevention and remedial measures, which in turn has compromised the stability of slopes and the reliability of hazard prevention structures worldwide. As the most widely used rock slope reinforcement measure, rock anchors have been used for decades in slope stabilization programs as temporary or permanent structural member to minimize the potential relaxation and loosening of the rock mass (Hoek and Bray 1981). In practice,
Cyclic load testing of pre-stressed rock anchors for slope
stabilization
ZHENG Da1 http://orcid.org/0000-0003-1640-7190; e-mail: [email protected]
LIU Fang-zhou2 http://orcid.org/0000-0001-5600-4017; e-mail: [email protected]
JU Neng-pan1 http://orcid.org/0000-0002-3159-1689; e-mail: [email protected]
FROST J. David 2 http://orcid.org/0000-0001-9625-1258; e-mail: [email protected]
HUANG Run-qiu1 http://orcid.org/0000-0003-2560-4962; e-mail: [email protected]
1 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China
2 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta 30318, GA, U.S.A.
Citation: Zheng D, Liu FZ, Ju NP, et al. (2016) Cyclic load testing of pre-stressed rock anchors for slope stabilization, Journal of Mountain Science 13. DOI: 10.1007/s11629-015-3605-8
© Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag Berlin Heidelberg 2016
Received: 25 June 2015 Accepted: 29 October 2015
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rock anchors can be either tensioned or untensioned. An untensioned rock anchor is generally grouted over its full length and installed prior to excavation, and thus its capacity is mobilized upon the displacement of the reinforced rock mass. A tensioned rock anchor is installed across the potential sliding surface with the fixed end grouted and located in the intact rock mass behind the sliding surface, and thus stabilizes the rock mass by altering the normal and shear stress along the discontinuity (Wyllie and Norrish 1996). The tensioned rock anchor consists of a bonded length and an unbonded length of an anchor tendon and can be made of different materials. A tensioned rock anchor with strand tendons is referred to as the pre-stressed cable, the engineering application of which dates back to the early work for the improvement of the seismic stability of concrete dams.
Although pre-stressed cables have been used for decades to improve the seismic stability of rock slopes, the analysis and design of the cables is still largely based on the working and ultimate capacities obtained under static loading condition due to the lack of understanding on the behavior of prestressed cables under cyclic loading. Furthermore, the evaluation of the performance of pre-stressed cables after seismic events provides inadequate feedback regarding their stress-strain characteristics during the seismic loading, as the condition of the damaged cables are usually documented based on external indicators and replaced without assessment on the internal damage. Limited information on the behavior of pre-stressed cables under cyclic loading is available. Some general trends emerged from reviewing the early tests on repeated loading of plate anchors embedded in soils, and yielded the following conclusions: (i) The failure of anchors may occurred at load levels below the ultimate static pullout capacity; (ii) Repeated loading caused additional cumulative movement of the anchor; and (iii) The magnitude of the repeated loading is a governing factor in the service life of the anchor (Hanna 1982; Xanthakos 1991). Previous evaluations on the seismic damage of anchored bulkheads installed at ports indicated that the failures were primarily due to overloading of the anchors (Kitajima and Uwabe 1979). A vertical anchorage system in sand subject to cyclic loading
under tidal conditions was modelled in centrifuge test (Merrifield and Carey 1997). Field study on the repeated uplift loading of cement-grouted pre-stressed cables in rock indicated that the difference between the magnitude of the initial pre-stress load and the repeated load governed the service life of the cable (Benmokrane et al. 1995). Shake table tests have been widely used as a direct approach for assessing the seismic performance of slopes and retaining structures, but the analysis of the impact has often been concerned with the overall performance of the slope and/or the structure with limited or no specific study on the pre-stressed cables (Morin et al. 2002; Wartman et al. 2003; Ling et al. 2005; Lin and Wang 2006; Cheng 2008; Xu et al. 2010).
Pre-stressed cables are the most widely used technique for rock slope stabilization and rockfall prevention programs in China due to the rapid development of hydropower stations (Huang and Xu 2008; Huang 2009; Zheng et al. 2012). Prior to the Wenchuan Earthquake in 2008, the study primarily focused on the dynamic response of the pre-stressed cables under blasting (Lu 2000; Su and Zhang 2003; Yang et al. 2006). The severe damage caused by the widely distributed seismic-induced geohazards during the Wenchuan Earthquake emphasized the needs for exploiting the application of pre-stressed cables for seismic measures. However, the absence of large-scale laboratory experimental studies to investigate the seismic response of pre-stressed cable and their interaction with rock masses under cyclic loading resulted in their extrapolation to the seismic application often being based on numerical simulation and engineering judgment only.
Cyclic load testing has been used for decades to simulate the impact of seismic loading on structures. The tests usually employ a horizontal load with gradually increasing magnitude or displacement in successive cycles (Driver et al. 1998). The experimental study presented herein describes cyclic load testing on pre-stressed cables that were installed to reinforce a large-scale concrete block mass with in-plane discontinuities. The test offers the opportunity for detailed assessment on the stress distribution along the tendon and the interface as well as the failure process of the cable, which provides additional information to study the suitability of pre-stressed
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J. Mt. Sci. (2016) 13(1): 126-136
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model are representative of the slope at 5% scale of the prototype. The model consists of four components including a group of 15 pre-stressed anchors, and a concrete base and top separated by a thin galvanized iron sheet. As depicted in Figure 1, the concrete base is fixed within the model box, whereas the concrete top (sliding body) with a vertical slope face designed for simplicity was cast inside a wood model box (Figure 1b) and placed above the concrete base. The concrete used to simulate the rock mass was proportioned as 1 (Portland Cement): 7 (medium sand with particle size ranges between 0.35 and 0.5 mm) with the addition of 300 kg water for 1 m3 cement/sand mixture.
The heights of the front and the back of the sliding body are 2.0 m and 0.35 m, respectively. The length and width are 2.0 m. The prestressed cables have bond lengths of 500 mm and unbonded lengths ranging from 700 mm to 1100 mm. The concrete top and base were designed to simulate the theoretical condition of rigid rock mass. A geological discontinuity was designed as the potential sliding surface in order to assess the performance of the prestressed cables in a single joint rock mass. The sliding surface strikes parallel to the slope face and dips to the front at an angle of 60°, which is simulated by the interbedded iron sheet. The bearing plates were placed on the sliding body and connected to a hydraulic actuator by a set of metal rings that permits the actuator to apply both tensile and compressive forces to the model.
The principles of geometric, kinematic and dynamic similarity are critical to the design and testing of physical models. The development of the law of similitude as used in geotechnical engineering progressed as the use of shaking tables gained wide popularity globally. The similitude theory adopted in this experimental study follows the same dimensional analysis that has been applied in many other studies using scaled models, in which the ratio of the material densities between the model and the prototype are designed to be unity in one-g scale tests (Iai 1989; Gibson 1997; Meymand
1998; Lin and Wang 2006). Using dimensional analysis reduces the engineering parameters to their fundamental units and the scaling relations between the prototype and model can be expressed in the form of a geometric scaling factor λ, and the ratios of the dimensionless parameters are set to be unity to meet the similarity requirements. The scaling factor λ applied in the test is 20. The scaling relations for the primary parameters are determined and shown in Table 1, and the resulting relationship of similarity between the prototype and model is presented in Table 2.
The assumption for the tests was that the sliding body reaches a state of equilibrium at a 0.25 , which represents the corresponding PGA for the aforementioned level of earthquake acceleration. The seismic loading is modeled as a cyclic horizontal force exerted on the sliding body based on the pseudo-static analysis defined as (Jibson 2011):
c ∙ W ∙ cosα ksinα ∙ tanϕW ∙ sinα kW ∙ cosα
(1)
where is the pseudo-static factor of safety, W is the weight of the sliding body, and α, ϕ and c are the slope angle, angle of friction, and cohesion of the failure surface, respectively. Area of the failure surface is represented by A, and k is the seismic coefficient defined as
k a / (2)
where a is the horizontal ground acceleration and is the gravitational acceleration. The uncertainty
in the magnitude of the earthquake and the associated a commonly result in the selection of the seismic coefficient that plays a decisive role in
Table 1 The law of similitude (ratio of prototype to model) in terms of the geometric scaling factor λ (after Meymand 1998). Derivations are presented in Appendix 1.
Density 1 Acceleration 1 Strain 1 Angle of friction 1 Poisson ratio 1 Length λ Mass λ3 Force λ3 Stress λ Time λ1/2 Stiffness λ2 Modulus λ Frequency λ-1/2 Cohesion λ Share wave velocity λ1/2 Table 2 The relationship of similarity between the prototype and model
Parameter Scale Prototype Model Slope Geometry L λ 52m(W) × 38m(H) 4m × 2.5m × 2m
Density ρ 1 1.80 – 2.42 g/cm3 1.978 g/cm3 Cohesion c λ 3 – 6 MPa 0.22 MPa Friction Angle ϕ 1 38° – 49° 45°
Cable Tensile Strength σ λ 7440 MPa 322.45 MPa
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the accuraccoefficient (Seed 1979California Kramer anThe value study wasaverage PGseismic resunique to The angle obe 30° usinvalues of (1).
The mgalvanized (length/widbetween thand the parametersthe iron she
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Figure 2 Tafter groutpanel of thnot connec
2016) 13(1): 12
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J. Mt. Sci. (2016) 13(1): 126-136
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200 kN. Four observational cables were monitored throughout the test to provide quantitative evidence to explore the analytical models of the structure. Load sensors were connected to the cable heads with 62 strain gauges installed on the surface of the observational cables (5 were damaged in the grouting process). The strain gauges were installed at 5 cm intervals over the bond length and 10 cm, 15 cm and 20 cm intervals over the unbonded length.
The horizontal loads applied to the model slope by the hydraulic actuator were resisted by a laboratory reaction wall. The horizontal loads were exerted on the sliding body through the bearing plates. The conventional application of quasi-static cyclic loading can be controlled by either load or displacement, of which the load-controlled cyclic loading is often used prior to the development of joints and/or cracks in the rock mass and is followed by displacement-controlled loading. Following the same line of reasoning, displacement-controlled loading was considered sufficient for this test program due to the presence of a fully developed sliding surface. An alternating load- and displacement-controlled cyclic loading was applied, as the application of load-controlled loading permitted direct assessment on the prestressed cables in light of the equivalent earthquake intensity levels. The subsequent displacement-controlled loading pull the sliding body back to its original location to ensure the recorded displacement remained positive throughout the experiment.
2 Application of Loads and Model Behavior
The load-controlled cyclic loading was designed to perform each cycle with an increment of 5 kN, which simulates a step increase of PGA starting from 0.05 in an idealized earthquake. No apparent deformation of the model and pre-stressed cables was observed as the first cycle started with the application of 5 kN horizontal load (a 0.05 ). A displacement of 0.1 mm for the sliding body was measured in the load-controlled loading excursion, and the sliding body was subsequently pulled back to the origin in the displacement-controlled loading excursion.
In cycle 5 (a 0.25 ), the equilibrium was exceeded. The displacement was measured as 1.19 mm with light cracking noise from the cable heads as the result of stress concentrations.
In cycle 6 (a 0.30 ), a consistent stress concentration was postulated at the cable heads, as strong cracking noises were detected from the cable heads in both the load-controlled and displacement-controlled loading process. A displacement of 2.24 mm was achieved with a rapid increase in the axial force measured in the pre-stressed cables. A number of strain gauges were damaged by the end of this cycle.
In cycle 7 (a 0.35 ), a displacement of 3.08 mm was reached. Surface peeling was first observed on the front face of the sliding body with intensive cracking noises coming from the cable heads. Significant changes in the axial forces were reported throughout this cycle. More than 20% of the strain gauges were damaged. Small partings and cracks (max. length < 10 mm) were formed during this cycle, but did not propagate during subsequent cycles.
As loading continued, the increase in the displacement of the sliding body and the axial load in the pre-stressed cables became more pronounced during each of the remaining cycles. During cycle 10 (a 0.50 ), a displacement of 16.55 mm was recorded with more than 50% of the strain gauges damaged. Failure of the pre-stressed cables was observed during the load-controlled loading excursion of cycle 13 (a 0.65 ). The measured displacement of the sliding body exceeded 143 mm. All of the failures involved damage of the cable heads, including four of the cable heads which were ejected from the model. Five of the cable heads were detached from the model and the connections of the rest of the cable heads were loosened. Small partings and cracks (max. length < 20 mm) could be seen on the surface of the sliding body, although no propagations were observed during the loading cycles, which suggest the development of cracks have a negligible effect on the behavior of the model.
3 Results and Failure Mode
The fracture at the connection between the
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cable headloading excobservationfailure was3b, followeA4, B3 anresulted intravelled a subsequentC4, but thedue to theCable head(travel dist~0.5 m). loosening which causfront face o
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2016) 13(1): 12
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3(1): 126-136
133
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per row and under cyclic g may cause pre-stressed does, and in es.
J. Mt. Sci. (2
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4 Discu
Numerconducted interactionsuch as earthquakein the loaincreasing lead to th(Driver et aadopted insimulate tinfluence desired accin the labknowledge intensity orsequence dloading hcomplexityThe loadintest in lighrate of the cwhich permdata acquiunforeseenirreversible
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2016) 13(1): 12
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e-type loadinading sequen load cycle ahe test resal. 1998). Alt
n the experimthe typical on the stru
curacy, they boratory env is requiredr the capacit
does not relyhistory, whiy for the cang excursionht of the syscyclic loadinmits sufficieisition as w
n problems e damage to tuse of a s of complex
Change of axi loading excur
26-136
static cyclic ical engineeroil and struc
anchors ung. Howevernce betweenand actual esults being though the loments in this
seismic loucture mem offer two mvironment.
d either for ty of the systy on the inpuich reduces
andidate strun can be altestem respon
ng can be defient time to well as respduring the the test specscaled mode systems un
Loa
ial load negatrsion (i.e. disp
tests have bring to studyctural membunder idealr, the differen the graduearthquake m
unconservaoading seques study may ading and
mbers with major advanta
First, no pthe earthqu
em. The loadut of earthqus the loaducture memered during
nse. Second, ined by the u ensure reliponding to
test to previmen. el permits der a contro
ading
ive number oplacement-con
been y the bers, lized ence ually may ative ence
y not the the
ages prior uake ding uake ding
mber. the the
user, able any
vent
the olled
for
simsurwasmatnatimpthedesstreper
laboandtheAn preinstcorloadcabfor rocdesreqlocathemathe
on loading axintrolled loadi
modificationThe concre
mulate intact face. Even ths not explicitterial for th
turally not portant to co prototype r
sign of slopessed cables,rformance of
The constroratory tests
d the model c displaceme exact dup
eferred for cotalled presrelations bed and the ult
bles were esta exploring thk anchors in
sign of the onquired for boation of the experimennuscript fail pre-stressed
is represents ng) for each
environmthe phencannot bthe protoThe resmodels predictionperformacables, wcalibratioanalytic study on rock anstressed cslope stacan finterpretafailure tydifferent which ma
n on the currete of the slo rock masseshough the datly consideredhe rock mas
homogenoonsider the prock mass, spe stabilizat, as it may c the pre-stresuction of the
s consumed acannot be rent occurringlication of
onducting anstressed catween the atimate staticablished. Addhe seismic ren a laboratongoing experoth the insta
strain gaugental resultsled to captud cable head
ment, especnomena beibe achieved otype (Meymsults from
provide qns of th
ance of which can bon benchm
models fr the seismic
nchors. For cables impleabilization, facilitate ation on thype and pat earthquakeay be used asrent measureope model ws separated bamage of thed in the expess in the p
ous. Therefpossibility ofsuch as cracktion project compromise ssed cables. e scaled moa great amou
e-used after ag at the slidi
the model n uplift staticables. Theramplitude oc load of the ditional workesponse of u
ory environmriments, impallation methes. As previos described
ure the displds due to the
ially when ing studied ‘at-will’ in
mand 1998). the scale
quantitative he seismic
prestressed be used as marks for rom future response of the pre-emented for the results qualitative
he potential ttern under e intensity, s a guideline e. was made to
by a sliding e rock mass eriment, the prototype is fore, it is f damage of king, in the with pre- the seismic dels for the unt of time, a test due to ing surface. would be c test on the refore, no f the cyclic prestressed k is ongoing untensioned
ment. In the rovement is
hod and the ously noted, d in this lacement of e absence of
J. Mt. Sci. (2016) 13(1): 126-136
135
strain gauges near the connections between the tendons and cable heads.
One of the limitations of pseudo-static analysis, is the assumption of considering the earthquake load as a permanent unidirectional force and results in the analysis being conservative (Jibson 2011). Although the possibility of failure cannot be judged from limit-equilibrium analysis, the potential effectiveness of pre-stressed cables in slope stabilization was clearly demonstrated by the results of the experiment.
5 Summary and Conclusions
A quasi-static cyclic loading test was performed on 15 pre-stressed cables installed on a scaled rock slope model with a distinct planar sliding surface. The scaled model was constructed using the dimensional analysis to closely simulate an actual slope from an ongoing stabilization project in China. The strand system of the cables was prestressed with P 200 N. The loading excursion of the cyclic loading was controlled by both load and displacement with a step increase of 5 kN to simulate an idealized earthquake load. The state of equilibrium was set at F 25 kN based on the result of a pseudo-static analysis. Prior to the failure of the test model, high stress concentration were observed at the fixed end of the bonded section of the prestressed cables.
The pre-stressed cables in the test model exhibited realistic and very good performance. The change of P and the displacement behavior of the model indicated that rapid displacement of rock slope occurred after the complete loss of the initial pre-stress load. This phenomenon implies that the application of pre-stress loads can improve the service life of a rock anchor subject to cyclic load, even after exceeding the state of equilibrium and the amplitude of the cyclic load can be much greater than the initial pre-stress load. The higher the initial pre-stress load, the greater the seismic performance of the rock anchors. Therefore, the pre-stressed cables can serve as an effective measure for stabilizing rock slopes under seismic loading.
The failure mode of the pre-stressed cables
revealed that the connection between the tendons and the cable heads may be more vulnerable under cyclic loading. The sequence of failure occurred from the upper to the lower rows of the cable group. The damage of cables was also found to be more severe on the upper row. No debonding was generated in the experiment, but since the mechanism of bonding interaction of the pre-stressed cable – grouting material – rock mass remains unclear, the bonding effect in the bond length of the rock anchor cannot be neglected under cyclic loading.
The stress distribution along the bond length of pre-stressed cables was found to be highly non-uniform under cyclic loading. High stress concentrations were observed at both the fixed end and the free end (averaging 30% of the stress at the fixed end) of the bonded length before and immediately after exceeding the state of equilibrium. The stress distribution along the pre-stressed cable was not fully captured in the late stage of the experiment due to failure of the strain gauges. No significant stress concentrations along the unbonded length of the cable were recorded with the available strain gauges located 25 cm away from the cable head, but the failure of the cable heads implies that high stress concentrations may be occurring at the connection between the tendons and cable heads.
Acknowledgements
This study is financially supported by the National Basic Research Program of China (973 Program) (Grant No. 2013CB733202), the National Natural Science Foundation of China (Grant No.41102191), the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology) (Grant No. SKLGP2011Z019), and the National Natural Science Foundation of China (Grant No. 11670589).
Electronic Supplementary Material: Supplementary material (Appendix 1) is available in the online version of this article at http://dx.doi.org./10.1007/s11629-015-3605-8
J. Mt. Sci. (2016) 13(1): 126-136
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