grouting pressure overburden pressure

8/8/2019 Grouting Pressure Overburden Pressure

1/6

STUDY ON THE EFFECTS OF GROUTINGPRESSURE AND OVERBURDEN PRESSURE ONTHE PULLOUT RESISTANCE OF SOIL NAILS

Wan-Huan ZHOU, Jian-Hua YIN, Hong-Hu ZHU, Cheng-Yu HONG

Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China

ABSTRACTA series laboratory soil-nail pullout tests on complete decomposed granite at the saturated condition under differengrouting pressures and overburden pressures were carried out to study the effect of grouting pressure and overburdenpressure on the pullout resistance of soil nails. The pullout tests were fully instrumented and monitored. The bare FibBragg Grading point sensors were installed together with the strain gauges to measure the axial strain changes duringtesting. Some latest test results are presented and discussed in this paper.

RSUMDes sries dessais darrachement sol-ongle sur du granit compltement dcompos l'tat satur, sous diffrentespressions de jointoiement et de surcharge, ont t effectus en laboratoire pour tudier leurs effets sur la resistancedarrachement des sols-ongles. Les essais darrachement ont t entirement quips et surveills. Des sondesdvaluation de point de Bragg de fibre nue ont t installes ainsi que des jauges de contrainte pour mesurer leschangements axiaux de contrainte pendant l'essai. Les derniers rsultats d'essai sont prsents et discuts en cetarticle.

1 INTRODUCTION

Soil nailing has been widely used worldwide as aneconomic, effective, and simple method for stabilizationof new cut slopes and retaining structures. Since themid 1980s, the technique of soil nailing has beenapplied for improving the stability of marginally stableslopes and retaining walls in Hong Kong (GEO 2005).In Hong Kong, most slopes are composed of completelydecomposed granite (CDG) soils and soil nails areinstalled by the drill and grout method.

In the design of a soil nail system, the interface shearresistance between a soil nail and the surrounding soilis a key parameter for design and safety assessment ofthe soil nail stabilized slope (Powell and Watkins 1990).Many factors have influences on the pullout resistanceof the soil nails, such as the overburden pressure, thesoil dilation, the grouting pressure, the shear strength ofthe soil, the roughness of the nail surface, the degree ofsaturation of the soil, and etc. It has been studied andfound by previous researchers that the pull-outresistance of grouted soil nails is largely contributed bythe effect of soil dilation (Schlosser 1982).

Extensive field pull-out tests (Schlosser and Guilloux1981, Berglund and Oden 1996, and Franzn 1998)have been carried out on different types of driven nailsand grouted nails. Due to the uncertainties of the fieldconditions, the reported test results are commonlyscattered. These uncertainties include the nonuniform

properties of the in-situ soils, the cement grout integrityof nails, and some uncontrolled test parameters (e.g.vertical stresses, grouting pressure, the roughness ofthe drilled hole, etc). In addition, the field pullout testsare normally carried out in a no-rain weather conditionand in unsaturated soils. Such testing condition is notthe worst case and the measured pullout resistance is,therefore, on the unsafe side. On the other hand,laboratory soil nail pullout tests can be conducted undercontrolled conditions, so that some key influencingfactors on the fundamental interaction mechanism andshear resistance between a soil nail and the soils canbe studied. Chang and Milligan (1996) conductedlaboratory pullout tests of steel bars and rubber tubes inyellow Leighton Buzzard Sand and Baskarp Sand. InHong Kong, Leeet al. (2001) and Junaideen et al. (2004) conducted laboratory pullout tests on soil nails ina loose Completely Decomposed Granite (CDG) fill. Yinand his co-workers (Yin and Su 2006, Su 2006, Yin etal. 2006) developed a new soil nail pull-out box andcarried out studies on the interface shear resistancebetween a grouted soil nail and CDG soils. A few keyinfluencing factors, including the overburden pressure,degree of saturation of the soil were investigated.

Nowadays, in common practice of the soil nailconstruction in Hong Kong, gravity or low pressuregrouting is normally adopted. The effect of groutingpressure is seldom taken into account in the soil nailingdesign and studies on the contribution of the groutingpressure to the soil nail pullout resistance are limited in

OttawaGeo2007/OttawaGo2007

985


2/6

references. Yeung et al. (2005) carried out field pullouttests on Glass Fiber Reinforced Polymer (GFRP) pipesin a CDG soil slope in Hong Kong and observed asignificant increase of the pullout resistance due to thepressure grouting. Yin et al. (2006) reported somelaboratory pullout tests and discussed the influence ofcement pressure grouting on the soil nail pullout

resistances. Totally four tests with grouting pressures of80kPa and 200kPa and overburden pressures of 80kPaand 130kPa were carried out on the CDG soil with 50%degree of saturation.

In the pullout testing, the axial strains of the nail can bemeasured in order to calculate the shear resistance atthe interface between cement grout and thesurrounding soils. Normally electrical resistance andvibrating wire type strain gauges are used to measurethe strain data. The Fibre Bragg Grading (FBG) sensoris an innovative technique for measuring the strains.The concept is the wavelength changes of the FBGsensor are proportional to the strain changes at thesensor point. Comparing to traditional strain gauge, this

optical fibre sensing technology has apparentadvantages. It is immune to electromagneticinterference and highly resistant to corrosion. Thecompact size makes it easy to install without affectingthe structural integrity. This technique has been widelyapplied in structure health monitoring (SHM). Chan etal. (2006) has applied the FBG sensors in themonitoring of Tsing Ma bridge in Hong Kong. However,its application in geotechnical field is still limited. Yin etal. (2007) firstly applied this technique in field soil nailmonitoring and found that FBG sensors are morereliable than electrical strain gauges for the strainmonitoring.

Recently, further studies on the influence of groutingpressure on the shear resistance of soil-grouted nailinterface are in progress based on the previous studies(Yin and Su 2006, Su 2006, Yin et al 2006). A series oflaboratory soil nail pullout tests with different groutingpressures and overburden pressures are proposed. Allthe tests are performed on CDG soil under nearlysaturated condition, that is, the critical condition. Bothstrain gauges and FBG sensors are used to measurethe strain changes during testing. In the paper, someup to date test results are presented and discussed.

2 SOIL NAIL PULLOUT BOX WITHINSTRUMENTION AND TEST PROCEDURES

The pullout box designed by Yin and Su (2006) wasused in the present study. The internal dimensions ofthe box are 1000 mm long, 600 mm wide and 830 mmhigh. The pullout box is fully instrumented with six earthpressure cells, four pore water pressures cells, anoverburden pressure application system, an additionalchamber at the box back covering the end of the soilnail, a back pressure saturation system, a specialtriaxial cell as a waterproof front cover, a pressuregrouting device, and a pullout device. More details on

the box design and setup can be found in Yin and Su(2006).

Test procedures include box preparation, samplepreparation, placement of transducers, hole drilling, soilnail installation, pressure grouting, saturation with backpressure, pulling out of the soil nail. More descriptions

on the test procedures can be found in Su (2006) andYinet al. (2006).

3 BASIC PROPERTIES OF SOIL AND CEMENTGROUT AND RESPONSES OF SOIL AND NAILDURING PULLOUT TESTING

3.1 Basic properties of the soil and the cement grout

The Completely Decomposed Granite (CDG) used inthis study was taken from a highway construction site atTai Wai, Hong Kong and was a typical in-situ soilcommonly found in Hong Kong. The composition of theCDG soil obtained for the present study was 9.3%gravel, 62.5% sand, 25.0% silt and 3.2% clay, classified

as yellowish brown, very silty sand. The plastic andliquid limits of the soil were 27.3% and 35.5%respectively. The maximum dry density was 1.802Mg/m3. The soil compacted in the box had the initialdegree of saturation of around 75%. The density ofcement grout was 1.89 Mg/m3. The average uniaxialcompressive strength of the cement grout on the 5thday was 32.1 MPa. The secant Youngs modulus(defined as the slope from the origin to the point of 50%of the maximum axial stress in the axial stress-axialstrain curve) and the corresponding Poissons ratiowere 12.6 GPa and 0.21 respectively. Other basicparameters and the shear strength parameters of thesoil and cement grout were summarized in Table 1 andTable 2 after Yin et al. (2006).

3.2 Typical experimental results during application ofoverburden pressure, pressure grouting, and thepull-out of the soil nail

Due to page limit, only typical results of the test nailsubjected to an overburden pressure of 120 kPa and agrouting pressure of 200 kPa are presented anddiscussed in this section.

Overburden pressure was applied to the soil beforedrilling a horizontal hole in the soil for the nailinstallation. Figure 1 shows the vertical pressuresmeasured by the six pressure cells (P-Cells 1 to 6)versus time, after an overburden pressure of 120 kPahad been applied on the top soil surface in the pulloutbox. The overburden pressure was applied graduallyand then maintained at 120 kPa. As shown in Figure 1,the vertical pressures first increased to about 120 kPaand maintained constant afterwards, which indicates thesoil stresses in the pullout box were adequatelyestablished in simulation of the overburden pressures ina slope.


986


3/6

During the hole-drilling, the stresses in the surroundingsoils around the hole will release. it was observed thatthe earth pressures measured by P-Cell1 to P-Cell4dropped to lower values after the hole-drilling. The earthpressures measured by P-Cells 1,2,3 and 4 were notreduced to zero because these cells were still in somedistance (about 40mm) from the hole. The 4 cells had

to be placed not too close to the perimeter of thedrillhole in order to avoid damage during the drilling.

0

30

60

90

120

150

0 200 400 600 800 1000 1200Time (min)

P r e s s u r e

( k P a

)

P-Cell 1 P-Cell 2P-Cell 3 P-Cell 4P-Cell 5 P-Cell 6

1 2

43

5 6Overburden pressure kept constant

Overburden pressure increasing

Figure 1 Measured earth pressures versus time afterhaving applied an overburden pressure of 120 kPa onthe top soil surface in the pullout box

After the hole was drilled, a high yield steel bar of40mm in diameter was inserted in the centre of the holeand the cement slurry was grouted under pressure of200kPa. Figure 2 shows rapid increases of the earthpressures at the six pressure cells due to the groutingpressure. The pressure increases at P-Cells 1,2,3, and4 were more than those at P-Cells 5 and 6. Thepressures decrease gradually during the curing of thecement grout, which are consistent with the

observations of the tests done by Yin et al. (2006),where one can find more discussions on theobservations. Yin et al. (2006) also reported that, for thepressure grouting tests on the soil with 50% saturationdegree, the soil nail pullout resistance had significantlybeen increased even though the earth pressures onlyincreased for a short duration.

About two days after the pressure grouting, a backpressure of 30 kPa was applied from six holes at thesides, the waterproof front cap and back extensionchamber of the pullout box. After five curing days whenthe developed strength of the cement grout was about32MPa, the nail was pulled out using a hydraulic jackagainst a steel reaction frame. The load was appliedstep by step with 5 kN increment and held for about onehour for each loading step. After the peak pull-outresistance was achieved, the nail was continuouslypulled out by displacement control using displacementrates of 1mm/min for tests. The displacement at the endof the test was 100mm. During the pullout of the soilnail, the variations of pullout force, pulloutdisplacement, vertical earth pressures, pore waterpressures at four locations around the soil nail and theaxial strains along the soil nail were monitored.

-50

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250Time (min)

P r e s s u r e

( k P a

)

P-Cell 1 P-Cell 2 P-Cell 3

P-Cell 4 P-Cell 5 P-Cell 6

1 2

435 6

Figure 2 Measured earth pressures versus time duringgrouting under the pressure of 250 kPa

For the axial strain monitoring, totally four electricalresistance-type strain gauges are adhered along thesteel rebar to measure the strain distribution along thesoil nail. In addition, three FBG sensors are applied tocompare and verify the results from the strain gaugesduring testing. The FBG strain sensors weretemperature compensated by a reference sensor andmultiplexed in serials to form a quasi-distributedsensing array. The locations of the strain gauges andFBG sensors are shown in Figure 3. Using themeasured strain data, the frictional force between thenail surface and the surrounding soils during the pullouttest can be determined.

SG 1 SG 2 & FBG 2

300 30030050 50

SG 3 & FBG 3 SG 4 & FBG 4

Figure 3 Locations of strain gauges and FBG sensorsalong the soil nail

Figure 4 shows variations of (a) soil nail pullout force,(b) effective vertical stresses at six locations and (c)axial strains along the soil nail measured by straingauges and FBG sensors, versus time during thepullout for the soil nail pullout test under an overburdenpressure of 120 kPa and a grouting pressure of 200

kPa. Effective vertical stresses were calculated bysubtracting the average pore water pressure from thetotal earth pressure at the six locations. It can be seenthat the effective vertical pressures around the soil nail(measured by P-Cells 1 to 4) and axial strains in thesteel bar showed corresponding response at the sametime when the pullout force was applied step by step atthe increment of 5kN,. The increase of normalpressures around the soil nail demonstrated theconstrained dilation of the surrounding soil duringpullout of the soil nail. The pressures at P-Cells 5 and 6

Pulling out


987


4/6

changed little during pullout as they were further awayfrom the nail, suggesting that the effect of pulling of thesoil nail to the surrounding soil was localized. Someinconsistent variations were observed in the measuredearth pressures at the pressure cells above and belowthe soil nail (Figure 4b). A possible explanation is thatthe nail or the line of action of the pullout was not

perfectly horizontal due to the self weight of the steelnail bar, and the nail tilted slightly when being pulledout. The tilt of the nail induced different earth pressuresabove and below the nail.

Relationship between pull out force and time

0

5

10

15

20

25

30

0 50 100 150 200 250 300Time (min)

P u

l l

o u

t

f o r c e

( k N )

0

40

80

120

160

200

0 50 100 150 200 250 300 350Time (min)

E f f

e c

t i

v e

s

t r e s s

( k P

a )


1 2

43

5 6

Relationship between strain and time

-30

0

30

60

90

120

150

0 50 100 150 200 250 300Time (min)

M i

c r o - s

t r a

i n

SG1

FBG3 SG3 FBG2 SG2

FBG4 SG4

Figure 4 Variations of (a) soil nail pullout force, (b)effective vertical stresses at six locations and (c) axialstrains along the soil nail measured by strain gaugesand FBG sensors versus time during pulling out

Figure 5 shows variations of (a) average shear stress,(b) effective vertical stresses at six locations and (c)axial strains along the soil nail measured by straingauges and FBG sensors, versus pullout displacementduring pulling out the soil nail. The deduced averageshear stress was calculated using the measured axialforce at nail head divided by the total surface area of

the nail with measured diameter. One can see clearly astrength softening behavior of the soil nail shearresistance during pulling out. In this test, the peak shearstress occurred at a pullout displacement of 7mm.

Relationship between pullout shear resistance and displacement

0

20

40

60

80

100

0 20 40 60 80Pullout displacement (mm)

A v e r a g e p u

l l o u

t s h e a r s t r e s s

( k P a )

100

(a)

(a)

Relationship between effective stress and pullout displacement

0

50

100

150

200

250

0 20 40 60 80Pullout displacement (mm)

E f f

e c

t i

v e

s

t r e s s

( k P

a )

100


1 2

43

5 6

(b)

(b)

Relationship between strain and pullout displacement

-30

0

30

60

90

120

150

0 20 40 60 80 1Pullout displacement (mm)

M i

c r o - s

t r a

i n

00

SG1

FBG3SG3

FBG2 SG2

FBG4 SG4

(c)

(c)

Figure 5 Variations of (a) average interface shearstress, (b) effective vertical stresses at six locations and(c) axial strains along the soil nail measured by straingauges and FBG sensors, versus pullout displacementduring pulling out

The diameter of the steel rebar was 40mm. Although itis commonly used in Hong Kong soil nailing practice,the strain responses in the present soil nail pullouttesting are quite small, with the maximum strain ofaround 100 micro strains during pull-out. Two methodswere used to measure the strain changes along the soilnail, which are traditional strain gauges (SG) and FibreBragg Grading (FBG) sensors. As shown in Figure 3,the axial strains at 4 locations along the steel bar weremonitored by 4 strain gauges. To increase the reliabilityof the testing results, FBG strain sensors were


988


5/6

employed and located beside SG2, SG3, and SG4respectively. The variations of the axial strains (Figure4c and Figure 5c) along the steel bar show that, as thesoil nail was pulled out the strains at Location 1 (nearthe soil nail head) responded distinctly and reached thehighest value, the strains at Location 2 responded thesecond, while the strains at Location 4 (near the end of

the soil nail) changed little and the values were below 5micro strains. From the comparison of the measuredresults between strain gauges and FBG sensors, itshows that for the small value strain monitoring, theFBG sensors provide higher sensitivity than thetraditional strain gauges.

4 INFLUENCES OF GROUTING PRESSURESAND OVERBURDEN PRESSURES ON THEBEHAVIOUR AND RESISTANCE OF SOIL NAIL

Results from the five latest soil-nail pullout tests underdifferent grouting pressures and overburden pressuresare presented here. Su (2006) have conducted the testsunder different overburden pressures without grouting

pressure for the same soil at the saturated condition. Inthe following section, those results are interpretedtogether with the newly test results to examine theinfluences of grouting pressures and overburdenpressures on the soil nail behaviour and pulloutresistance for the CDG soil at the saturated condition.

Figure 6 summarized the average peak shear stressesof the latest five pullout tests under grouting pressure of80kPa, 130kPa and 250kPa together with the resultsfrom Su (2006) under no grouting pressure andsaturated condition. From the figure we can see that,comparing to Sus results, generally the groutingpressure has positive effect on the soil nail pulloutresistance. For the tests under VP=80kPa, the peakshear stress increased 23.5% when the groutingpressure increased to 80kPa. For the tests underVP=120kPa, the peak shear stress increased 8.7% and35.2% when the grouting pressure increased to 130kPaand 250kPa, respectively.

Figure 7 shows the average shear stress versus pulloutdisplacement for grouting pressures of 0, 130 and 250kPa under the same overburden pressure of 120kPa. Itcan be clearly seen the increase of shear stress withthe increase of grouting pressure. However, theincrease is much less than the case for the soil at Sr =50% (Yin et al, 2006). In Yin et al.s study, for the testsunder overburden pressure of 200kPa, comparing tothe results without grouting pressure, the peak shearstresses increased about 60% and 150% for thegrouting pressure of 80kPa and 130kPa, respectively.That is to say, the effect of grouting pressure on thepullout resistance for the saturated soil is much lessthan that for the unsaturated soil. On the other hand, itwas also observed that for the tests under pressuregrouting and saturated condition, the measureddiameter of the soil nail after testing is about 105 to 115mm while the diameter of the drilled hole is about100mm. This shows that the strength of the soil

decreased as the soil is saturated and the pullout failureis more likely to occur in the soil rather than at the soil-grout interface.

Figure 8 plots the increasing trends of peak shearstress, shear stress at the pullout displacement of50mm, and shear stress at the pullout displacement of

100mm in relation to the grouting pressure under thesame overburden pressure (a) for VP=80kPa and (b)for VP=120kPa.

0

20

40

60

80

100

120

0 50 100 150 200 250 300 350 400

Overburden pressure (kPa)

P e a

k

s h

e a r

s

t r e s s

( k P

a )

GP=0 kPa (after Su 2006) GP=80 kPa

GP=130 kPa GP=250 kPa

Figure 6 Relationship between average peak shearstress and overburden pressure with grouting pressures(GP) of 0kPa, 80kPa, 130kPa and 250kPa

0 20 40 60 80 1000

20

40

60

80

100GP=0 kPa; VP=120 kPa (After Su 2006)GP=130 kPa; VP=120 kPaGP=250 kPa; VP=120 kPa

A v e r a g e

i n t e r f a c e s

h e a r s

t r e s s

( k P a

)

Nail head displacement (mm)

Figure 7 Measured average interface shear stressversus pullout nail head displacement for groutingpressures (GP) of 0, 130 and 250 kPa with the sameoverburden pressure (VP) of 120 kPa

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140Grouting pressure (kPa)

A v e r a g e

i n t e r f a c e s h e a r s t r e s s

( k P a ) a Peak shear stress: VP=80kPa

Shear stress at disp=50mm: VP=80kPaShear stress at disp=100mm: VP=80kPa

(a)Peak shear stress

Shear stress at disp=50mmShear stress at disp=100mm


989


6/6

0

20

40

60

80

100

120

0 50 100 150 200 250Grouting pressure (kPa)

A v e r a g e

i n t e r f a c e s h e a r s t r e s s

( k P a ) a Peak shear stress: VP=120kPa

Shear stress at disp=50mm: VP=120kPaShear stress at disp=100mm: VP=120kPa

(b)Peak shear stress

Shear stress at disp=50mmShear stress at disp=100mm

Figure 8 Relationship of average interface shear stress(at peak, at 50mm pullout disp. and at 100mm pulloutdisp.) and different grouting pressures with the sameoverburden pressure (a) VP= 80kPa (b) VP=120kPa

5 CONCLUSIONS

The authors present some latest results oflaboratorysoil nail pullout tests under different grouting pressureand overburden pressure under the saturated condition.The following conclusions and can be drawn from the

above:The constrained dilation of the surrounding soils wasclearly observed during the pullout.

In the strain monitoring during pullout of the soil nail, theFBG sensors provide higher sensitivity than thetraditional strain gauges.

The effect of grouting pressure on the pullout resistancefor saturated soil is much less than that for theunsaturated soil because the pullout failure is morelikely to occur in the soil rather than at the soil-groutinterface at the saturated condition.

It should be noted that more tests under higher groutingpressures and overburden pressures are to be carriedout and reported in the near future.

ACKNOWLEDGEMENTS

Financial supports from The Hong Kong PolytechnicUniversity and a grant from Research GrantsCommittee (RGC: PolyU 5174/04E) of the Hong KongSpecial Administrative Region Government of China aregratefully acknowledged.

REFERENCES

Berglund, C. and Oden, K. 1996. The pullout resistanceof different types of nails. Department ofGeotechnical Engineering, Chalmers University ofTechnology, Report No. X 1995:6.

Chan, T. H. T., Yu, L., Tam, H. Y., Ni, Y. Q., Liu, S. Y.,Chung, W. H., and Cheng, L. K. 2006. Fiber Bragggrating sensors for structural health monitoring ofTsing Ma bridge: Background and experimentalobservation. Engineering Structures , 28(5), 648-659.

Chang, K.T. and Milligan, G.W.E. 1996. Effects of thetransition zone in a nailed wall model test.Proc. of Earth Reinforcement , Ochiai, Yasufuku & Omie(eds.), Balkema, 333-338.

Franzen, G. 1998. Soil nailing A laboratory and fieldstudy of pullout capacity. Doctoral thesis ,Department of Geotechnical Engineering, Chalmers

University of Technology, Sweden.Junaideen. S.M., Tham L.G., Law K.T., Lee C.F., andYue Z.Q. 2004. Laboratory study of soil-nailinteraction in loose, completely decomposedgranite. Canadian Geotechnical Journal , 41, 274-286.

Lee, C.F., Law, K.T., Tham, L.G., Yue, Z.Q. andJunaideen, S.M. 2001. Design of a large soil boxforstudying soil-nail interaction in loose fill.Soft Soil Engineering , Lee et al (eds.), 413-418

Powell, G.E. and Watkins, A.T. 1990. Improvement ofmarginally stable existing slopes by soil nailing inHong Kong.Proc. of the Int. on Reinforced Soil ,Glasgow, 241-247.

Schlosser, F. 1982. Behaviour and design of soil

nailing.Proc. on Recent Developments in Ground Improvement Techniques , Bangkok, Thailand, 399-413.

Schlosser, F. and Guilloux, A. 1981. Le frottement densles sols. Revue Francaise de Geotechnique , (16),65-77.

Su, L.J. 2006. Laboratory pull-out testing study on soilnails in compacted completely decomposed granitefill. Ph.D. Thesis , The Hong Kong PolytechnicUniversity.

Yeung, A.T., Cheng Y.M., Lau C.K., Mak L.M., YuR.S.M., Choi Y.K., and Kim J.H. 2005. Aninnovative Korean system of pressure-grouted soilnailing as a slope stabilization measure.The HKIE Geotechnical Division 25th Annual Seminar , HongKong, published by HKIE-GDC and HKGES, 43-49.

Yin, J.H., and Su, L.J. 2006 An innovative laboratory box for testing nail pull-out resistance in soil.ASTM Geotechnical Testing Journal , 29(6): 1-11.

Yin J.H., Su L.J., Cheung R.W.M., Shiu Y.K., and TangC. 2006. The Influence of Grouting Pressure on thePullout Resistance of Soil Nail in CompactedCompletely Decomposed Granite Fill. Submitted toGeotechnique .

Yin, J.H., Zhu, H.H., Jin, W, Yeung, A.T., and Mak, L.M.2007. Performance evaluation of electrical straingauges and optical fiber sensors in field soil nailpullout tests. Geotechnical Advancements in HongKong since 1970s,The HKIE Geotechnical Division 27th Annual Seminar , Hong Kong, published byHKIE-GDC and HKGES 249-254.


990

grouting pressure overburden pressure

Documents