soil nail wall design report

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1.0INTRODUCTION

In order to construct the Accident and Emegency Ward at General Hospital, Ratnapura, it is proposed that a soil nail wall be built to retain the cut-slope situated behind the complex.

The design methodology adopted in this report is the "Load and Resitance Factor Design (LFRD)" as detailed in "Manual for Design and Construction Monitoring of Soil Nail Walls" published by FWHA of U.S. Department of Transportation.

2.0SUBSURFACE CONDITION

The soil strength parameters and other data pertaining to the geotechnical properties used in the design process are obtained from the report on "Soil Investigation for Slope Protection Works for Proposed Accident and Emergency Complex at General Hosptal, Ratnapura" prepared and submitted by Messrs. ELS (Pvt.) Ltd., on 12/01/2010.

Data pertaining to 2 boreholes drilled at the site are contained in the above report, which have been used to infer the depths and composition of the soil strata present. In addition, further reference has been made to the report on "Geotechnical Investigation on Accident and Emergency Unit at General Hospital, Ratnapura" prepared by Laboratory and Site Investigation Unit, CECB on 20/08/2008, in order to infer the geometry of the top surface of the bedrock.

3 main soil layers are identified to be present. The recommended values for their soil strength parameters are as follows:

Layer 1 (Lateritic Soil)

Report Submitted by ELS Pvt. Ltd.Average cohesion=5kPa

Friction angle (varies with stress level)

Stress level (kN/m2)f' (degrees)2528.05028.07528.0

Layer 2 (Completely Weathered Rock)

Report Submitted by ELS Pvt. Ltd.Cohesion=10kN/m2

Angle of friction=32o

Layer 3 (Highly Weathered Rock)

Although the Geotechnical Investigation Reports do not suggest any values for strength parameters in this layer, judging by the Core Recovery of approx. 40%, and RQD of approx. 10%, and an SPT value in excess of 50, it may be safely assumed that the following parameters are applicable;

Cohesion =10kN/m2Angle of friction=32oProjectAccident & Emergency Ward at General Hospital, RatnapuraFeatureSoil Nail WallDDESIGN UNITDesignedAJADateREPC DIVISIONCheckedSWJDateDCENTRAL ENGINEERING CONSULTANCY BUREAU (CECB)Job CodePage1ofReferenceCalculationsOutput

3.0DESIGN METHODOLOGY

Initially, a trial geometry is assumed, in which the trial nail lengths are arrived at using empirical relationships simplified in the form of graphs.

The nail wall design is also governed by the flexural strength of the construction facing, both shotcrete as well as the cast-in-place concrete, resistance to punching shear, and nail stud tension capacity. The adequacy of the nail head under these criteria is tested in the presence of the estimated ultimate nail head load.

Thereafter, slope geometry, soil nailing details the factored soil strength parameters, and other relevant data are fed into the computer program, SLOPEW, in order to obtain the minimum factor of safety against circular slip failure of the slope. A load vs. resistance ration of unity or above is considered satisfactory.

Finally, a manual check is performed by approximating the critical circular slip surface by a bilinear wedge. The L/R ratio is derived by considering the force equilibrium in vertical and horizontal directions for both wedges.

4.0DESIGN DATA

Section 3-3 in Hospital Plan DrawingWall Height (H)=4.63m

Angle of friction of Layer 1 (f')=28oSection 2.0Cohesion of Layer 1 (c')=5kN/m2

Cut Slope (q) (obtuse angle with horiz.)=100o

Trial slope at top of wall (b)=0o

Density of Layer 1 (g)=18kN/m3

Nail Angle=15o

Shotcrete Layer thickness=100mm

Nail spacing (vertical) (sV)=2.50m

Nail spacing (horizontal) (sH)=2.5m

CIP facing thickness=150mm

Diameter of drillhole=110mm

Grade of concrete=25N/mm2

Table 3.2, 3.3 FWHA ManualUltimate Ground-Grout resistance=75kN/m2

ProjectAccident & Emergency Ward at General Hospital, RatnapuraFeatureSoil Nail WallDDESIGN UNITDesignedAJADateREPC DIVISIONCheckedSWJDateDCENTRAL ENGINEERING CONSULTANCY BUREAU (CECB)Job CodePage2ofReferenceCalculationsOutput5.0DETAILED DESIGN5.1SELECTION OF TRIAL NAIL DESIGN

Table 4.8 of FWHA ManualF.O.S. for f' for structures=0.75

F.O.S. for c' for structures=0.9

F.O.S. for Soil weight (Tw)=1.35

4.12 of FWHA ManualFactored soil friction angle (fD)=tan-1(0.75tanf')

=21.74ofD=21.74o

tan (fD)=0.399tan (fD) = 0.399

Factored dimensionless soil cohesion cD=1.0c'/(TwgH)

=0.0400cD=0.0400

Fig. 4.27 and 4.29 of FWHA ManualTD - from Chart A for backslope 5o=0.37TD=0.37

Required nominal nail head strength (TNN)=TDTwgHsVsH/FNTable 4.8 of FWHA Manual(for FN = 1.0)=260kNTNN=260kN

Area of Bar Required=TNN/fy

=566mm2

However, for trial Nail layout, select 25mm dia. Bars

Area of 25mm dia. Bar=491mm2

Table 3.3 of FWHA ManualQD = Dimensionless nail pullout resistance=FQQU/(TwgsVsH)with FQ = 0.7Unit Ult. Bond Stresswhere QU = Ultimate Pullout resistance=Unit Ult. Bond Stress x pD75

=25.9kN/mQU=25.9kN/m

QD=0.119QD=0.119

TD/QD=3.097TD/QD=3.097L/HFig. 4.28 and 4.30 of FWHA ManualFrom chart 1C and 2C, for CD = 0.0218, the following graph may be drawn;tanfD1C2CAve.0.41.341.361.350.51.341.321.330.71.281.271.28

from the above chart, for tanfD of 0.399, L/H=1.40L/H=1.40

ProjectAccident & Emergency Ward at General Hospital, RatnapuraFeatureSoil Nail WallDDESIGN UNITDesignedAJADateREPC DIVISIONCheckedSWJDateDCENTRAL ENGINEERING CONSULTANCY BUREAU (CECB)Job CodePage3ofReferenceCalculationsOutput

Therefore trial Nail Length=L/H*H

=6.5mL=6.5

5.2COMPUTATION OF ALLOWABLE NAIL HEAD LOAD

Assume 225mm square, 25mm thk, connection plate and facing r/f of 3mm mild steel welded bars at 50mm spacing (mesh).

2 Nos. 12mm dia. Vertical bearing bars to be provided behind each bearing plate . R/f is to be nominally located at the centre of the shotcrete/concrete section. Grade 25 concrete to be used.

5.2.1TEMPORARY SHOTCRETE FACING

5.2.1.1CRITERION 1: FACING FLEXURE

Nominal Unit moment of flexure from shotcrete,

m=(Asfy/b)[d-Asfy/(1.7f'cb)]

Area of vertical steel over supports (Nail heads)

=[p(3)2/4x(103/50)](250/460)(2)+2p(12)2/4

=550mm2As neg=550

Area of vertical steel over midspan=[p(3)2/4*(103/50)](250/460)(2)

=393mm2As pos=393

mv neg=19.991kNm/mmv neg=19.991kNm/m

mv pos=12.522kNm/mmv pos=12.522kNm/m

Table 4.2 of FWHA ManualFacing pressure factor CF for 100mm thk. Temporary facing,

CF=1.0

Nominal Nail Head Strength for resisting flexure

TFN=CF(mv pos+mv neg)(8)sH/sV

=260kNTFN=260kN


5.2.1.2CRITERION 2: FACING PUNCHING SHEAR

Section 4.5.3 Eq. 4.2.4.3 FWHA ManualDepth of shotcrete,hc=100mm

D'c=bPL+hc(Where bPL = Width of bearing plate)=225

D'c=325mmD'c=325mm

Nominal internal punching shear strength,VN=0.33[sqrt(f'c(Mpa))]pD'chc

=168kNVN=168kN

Table 4.2 FWHA Manualfor 100mm thick temporary facing,Cs=1.0

Effective diameter of punching shear cone at back of facing,

Dc=hc+D'c

=425mmDc=425mm

Diameter of grout column,DGC=110mm

Area of punching shear cone base at back of facing,

Ac=0.25pDc2

=141863mm2Ac=141863mm2

Area of grout column,AGC=0.25pDGC2

=9503mm2AGC=9503mm2

Eq. 4.3 FWHA ManualNail Head strength for resisting punching shear,

TFN=VN/[1-Cs(AC-AGC)/(sVsH-AGC)]

=172kNTFN=172kN

5.2.2PERMANENT CAST IN PLACE (CIP) CONCRETE FACING

5.2.2.1CRITERION 1: FACING FLEXURE

Thickness of permanent facing=250mm

Select facing reinforcement of T10@200 in both vertical and horizontal directions. Considering both shotcrete and CIP as one composite facing,

Section 5.2.1.1As neg=550mm2As neg=550mm2

As pos=p(10)2/4/(200/1000)(2)=393mm2As pos=393mm2


Effective depth of negative reinforcement,=150 +100 - 100/2

=200mm

Effective depth of positive reinforcement,=150 + 100 - 150/2

=175mm

m=(Asfy/b)[d-Asfy/(1.7f'cb)]

Accordingly,

mv neg=20.0kNm/mmv neg=20.0kNm/m

mv pos=15.7kNm/mmv pos=15.7kNm/m

TFN for resisting flexure=CF(mv pos+mv neg)(8)sH/sV

=285kNTFN=285kN

5.2.2.2CRITERION 2: FACING PUNCHING SHEAR

Section 4.5.3 FWHA ManualSelect Nail stud arrangement with following dimensions;

body diameterdHS=20mm

head diameterdH=32mm

head thicknesstH=10mm

overall lengthL=85mm

stud spacings=125mm grade of stud steelFU=250N/mm2 Provision 1dH>sqrt(2.5) dHS

32>31.6O.K.

Provision 2tH0.5(dH-dHS)

106.0O.K.

Effective height of punching shear cone, hc = plate thickness + L

=210mm

Effective diameter of punching shear cone, D'C =hC + diameter of grout column

D'C=320


VN = 0.33[sqrt(f'c)]pD'ChC=348kNVN = 348kN

where VN is the nominal internal punching shear strength.

AGC = 0.25p(diameter of grout column)2=9503mm2AGC =9503mm2

AC = 0.25p(D'C + hC)2=220618mm2AC =220618mm2

Section 4.3 FWHA ManualTFN = VN[1/(1-CS(AC - AGC)/(sVsH - AGC))]=361kNTFN =361kN

5.2.2.3CRITERION 3: NAIL HEAD STUD TENSION

TFN=4AHSFU

where AHS is the cross sectional area of the stud shaft, and FU is the characteristic strength of the stud material.

TFN=314kNTFN =314kN

5.3DETERMINATION OF MINIMUM DESIGN NAIL HEAD STRENGTH

Section 2.4.5 FWHA ManualNail head service load factor=0.5

KA= Coulomb active pressure coefficient

=(sin(q+f))2sin(q-d)(sin(q))21+sin(f+d)sin(f-b)2sin(q-d)sin(q+b)

KA=0.298q100b0where, q=wall slope (measured anticlockwise from horizontald0f28b=Back slope

d=Wall friction angle

The angle of friction at the wall soil interface, and backslope angle are assumed as 18 degrees and 5 degrees respectively, which is reasonable considering the backslope geometry indicated in the drawings.

Section 4.7.2 Step 3 FWHA ManualNail head service load, tF=FFKAgHsHsv(neglecting cohesion)

deducting cohesive component, tF=FF[KAgH - 2c(KA)]sHsv

=92kNtF=92kN

where FF is the nail head service load factor, assumed as 0.5 as recommended in the absence of site specific data from walls constructed in similar soils.


Table 4.6 FWHA ManualFactored Nail Head Load, TEH(tF)=1.5tF

=137kN

Sections 5.2.1.1, 5.2.1.2, 5.2.2.1, 5.2.2.2, 5.2.2.3 Table 4.7 FWHA ManualFactored nail head strengthTF=min(260,172,285,361,314)x0.9

TF=155kN

TF>TEH(tF)O.K.

5.4DEFINING OF NAIL HEAD STRENGTH DIAGRAMS

The design nail head strength diagram for each nail is developed by determining the design pullout resistance, the design nail head strength and the design nail tendon tensile strength.

Design nail head strength, TF=155kNTF=155kN

Table 4.8 FWHA ManualDesign pullout resistance, Q=FQQU

where FQ is the ground-grout pullout resistance factor, and QU is the ultimate ground-grout pullout resistance per metre length.

Q=(0.7)(75)(p)(110)/103

=18.1kN/mQ=18.1kN/m

Table 4.8 FWHA ManualDesign nail tendon tensile strengthTN=FN TNN

=(0.9)(491)(0.460)

=203kNTN=203kN

Q=18.1TN=203Q=18.1

TF=155

5.5SELECTION OF TRIAL NAIL SPACINGS AND LENGTHS

Section 5.4Dimensionless nail pullout resistance, QD = FQQU/(TWgSVSH)

=0.119

Section 1Dimensionless nail length, L/H=1.40

QD/(L/H)=0.085

Fig. 4.11 FWHA Manual"R" value (from chart)=0.46R=0.46


Nail No.Trial LengthR(x)Trial Nail length distribution

16.516.5

26.516.5

36.516.5

46.51.15978401737.5

56.50.80989200865.2

where R(x) = [x/(H/2)](1-R)+R

5.6DEFINING OF DESIGN SOIL STRENGTHS

Section 4.0Ultimate friction angle, fU=28.0o

Section 4.0Ultimate cohesion, cU=5.0kN/m2

Table 4.8 FWHA ManualFf=0.75

Fc=0.9

Therefore design friction angle, f=tan-1[Fftan(fu)]

=21.74of=21.74o

Similarly, design cohesion, c=FCCU

=4.5kN/m2c=4.5kN/m2

5.7CALCULATION OF RESISTANCE/LOAD RATIO

A limiting equilibrium analysis is performed using computer software SLOPEW, to determine the actual nail lengths which are required to achieve a minimum resistance/load ratio of 1.00.

Fig. 5.7.1 - Cut Slope Cross SectionProjectAccident & Emergency Ward at General Hospital, RatnapuraFeatureSoil Nail WallDDESIGN UNITDesignedAJADateREPC DIVISIONCheckedSWJDateDCENTRAL ENGINEERING CONSULTANCY BUREAU (CECB)Job CodePage9ofReferenceCalculationsOutput

Area of Block A7.84Area of Block B44.8

Fig. 5.7.2 - Cut Slope Force Diagram

From the above diagram, resolving forces in the horizontal and vertical directions for the two blocks gives rise to the following simplified equation, after eliminating the terms I,RA and RB:TwWA190.512TA0T0Theta0.2617993878{-TWWB-(TB+T)sin(q)+FCcUB(LB)sin(aB)/(R/L)}tan(aB- fB)+ (TB+T)cos(q) +FCcUB(LB)cos(aB)/(R/L) = {TWWA+(TA-T)sin(q)-FCcUA(LA)sin(aA)/(R/L)}tan(aA-fA)-(TA-T)cos(q)-FCcUA(LA)cos(aA)/(R/L)IzBeta0PhiC0.9CuA5CuB10where f = tan-1[Fftan(fU)/(R/L)]LA5.7940649459alphaA0.9773843811Solving the above equation using a spreadsheet,R/L0.9737281308RAxR/L=0.974PhiA0.321730162818.4337804716PhiB0.494610293228.3390823034Although this R/L value is somewhat lower than 1.136, obained from the computer software SLOPEW, considering the assumptions involved, and the fact that the R/L derived from the manual calculation is close to unity, it may be assumed that the computer generated result is accurate enough for the purpose of designing the soil nail wall.TwWB1088.64TB156.32LB8.983900053alphaB0.558505360632RBy

REDUCED EQUATION5.8EXTERNAL STABILITY CHECK OF NAILED BLOCKLHS151.9857988056An external stability check is not necessary for this wall as the subsurface soils are granular and no hydrostatic pressures are expected behind the wall. Further, it is assumed that the wall is located in an asiesmic region.RHS114.487107673

LHS-RHS37.4986911326


5.9CHECK OF THE UPPER CANTILEVER

The height of the upper cantilever above the top nail does not exceed 2.0m for both the temporary shotcrete and permanent CIP facings. Therefore the static loading is identical for both cases. Because both the facing thickness and the steel content is higher in the permanent facing, the CIP facing is less critical by inspection. Therefore, for the static loading condition, only the construction facing upper cantilever needs to be evaluated.

For the given slope geometry (assuming a backslope of 0o) and soil to wall interface friction angle of 16.67o Coulomb's earth pressure theory gives an active earth pressure coefficient KA of 0.406. The load component normal to the wall has a corresponding coefficient of (0.406)cos(16.67) = 0389.

5.9.1SHEAR CHECK

From force equilibrium, factored one way unit shear force at the upper row of nails.

Table 4.6 FHWA ManualTEHv=0.5KAgz2

=4.40kN/mTEHv=4.4015625kN/m

Nominal one way unit shear strength, VNS=0.166(f'c)(d)

=41.5kN/mVNS=41.5kN/m

Design one way unit shear strength,V=FFVNS

=37.4kN/mV=37.4kN/mTable 4.7 FHWA Manualwhere FF is the facing shear resistance factor.

TEHv0.76AG/FYO.K.

Maximum Reinforcement Ratio Requirement

Although Section 5.7.3.3 of AASHTO [29] contains a provision for maximum amount of reinforcement allowed in a flexural member, per the discussion in 5.10.1 of this report, this provision is not applicable for soill nail walls.

Check 3 - Minimum cover requirements

Section 5.12.3 AASHTO [29]Available cover to exposed surface=150mm - 25mm - 85mm

=40mm

Although this value is lesser than that recommended in AASHTO [29], 40mm is assumed to be acceptable considering the less severe conditions present in this country.O.K.

minimum cover requirement between CIP Reinforcement and shotcrete facing is 38mm.

Actual available cover=150/2 - 10mm

=65mm >38mmO.K.

ProjectAccident & Emergency Ward at General Hospital, RatnapuraFeatureSoil Nail WallDDESIGN UNITDesignedAJADateREPC DIVISIONCheckedSWJDateDCENTRAL ENGINEERING CONSULTANCY BUREAU (CECB)Job CodePage14of

Sheet2M52053179.501980561147103.75236919093924.25038866As784.9998331578fy460b1500d175f'c25k0.0453252175z165.6916464228a0.9

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soil nail wall design report

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