Seismic Risk Analysis of Building Structures in Thimphu ... · Figure 1: Typical Retaining Wall...

Design and Construction of Stone Masonry Retaining Walls – A Quick Guide

i Department of Engineering Services

Foreword

The Department of Engineering Services is pleased to bring out the

“Design and Construction of Gravity Stone Masonry Retaining

Walls: A Quick Guide” for the benefit of all engineers and builders.

Though stone masonry retaining walls are simple structures and

commonly built in every nook and corner of the country for various

earth retaining purposes, yet many problems are encountered in the

field as a result of either improper design or poor quality

constructions. These problems arise mainly due to the fact that

engineers in the field are bogged down with numerous

responsibilities and often do not find time to carry out proper design.

Also during execution, negligence of some important aspects such as

drainage and use of inferior materials add to the issues.

This Quick Guide is aimed at providing guidance to designers so that

site specific design calculations can be done. Further standard

designs are also provided which can be followed for normal soil

conditions. For all important projects, site specific designs should be

carried out and the standard designs shall only be used as a

reference.

I hope this guideline would be of some benefit to all our engineers.

Tenzin

Director


ii Department of Engineering Services

Table of Contents

1. Introduction ....................................................................................... 1

2. Scope ................................................................................................... 2

3. Typical Failure Modes Of Retaining Walls .................................... 2

4. Bearing Capacity Of Soil .................................................................. 3

5. Design Criteria ................................................................................... 5

6. Preliminary Design Of Cement Masonry Walls ............................. 6

7. Standard Designs ............................................................................... 8

8. Other Considerations ........................................................................ 9

8.1 DEPTH OF FOUNDATION ................................................................. 9

8.2 DRAINAGE ....................................................................................... 9

8.3 DIP OF FOUNDATION BASE ........................................................... 11

8.4 LENGTH OF WALL......................................................................... 11

8.5 STEPPING OF WALL BASE ............................................................. 11

8.6 THROUGH-STONES / BONDING ELEMENT ..................................... 12

8.7 CEMENT MORTAR ......................................................................... 13

8.8 CURING ......................................................................................... 13

8.9 AESTHETICS .................................................................................. 14

9. Sample Calculations ........................................................................ 16

10. References ........................................................................................ 22

11. Technical Committee Composition ................................................ 23


1 Department of Engineering Services

1. Introduction

Stone masonry retaining walls are gravity type retaining walls

commonly used to retain earth or fill materials and basically retains

earth pressures by virtue of its self-weight.

Gravity type stone masonry retaining walls are basic and simple

structures which are commonly built with locally available stones. It

can be built to varying shapes and sizes, provided proper design

procedure is followed. However, beyond 6m height, stone masonry

walls becomes un-economical as it requires large base widths.

Most often retaining walls are designed for static loads and not for

seismic forces as it is easier to repair failed walls. In this guideline,

standard designs have been provided for static loads only. It is

expected to provide quick guidance to engineers in the field both in

terms of design and construction. However, it should be noted that

engineers should verify design based on actual site conditions and

the standard designs provided in this guide should be used only as a

reference. Further, other considerations such as drainage, foundation

requirements, through stones etc. improves the functioning of the

wall and appropriate provisions shall be specified in the design.



2. Scope

This guideline provides the fundamentals of design of stone masonry

retaining walls. The main purpose is to provide standard designs of

stone masonry retaining wall that can be easily followed by

engineers in the field. It should be noted that the standard designs

are provided only for heights up to 6m as it becomes uneconomical

beyond this height.

However, these standard designs should be adopted with caution

depending on actual site conditions. For all important projects,

engineers are required to carry out site specific design calculations.

3. Typical Failure Modes of Retaining Walls

Retaining walls normally fail due to any of the following failure

modes.

i. Overturning – when the overturning moments exceed the

resisting moment offered by the components of the wall, it

fails due to overturning.

ii. Sliding – when the frictional resistance offered by the wall

components is less than the lateral earth pressure, wall will be

moved from its original position and it fails due to sliding.

iii. Bearing pressure- when the base pressure exerted, exceeds

the safe bearing capacity of the soil, the wall fails due to

bearing pressure.



.

Figure 1: Typical Retaining Wall Section

4. Bearing Capacity of Soil

The type of soil and its bearing capacity plays a vital role in the

stability of the retaining walls. It is recommended that proper soil

tests be carried out before designing of any retaining wall. However

in the absence of soil data for design, the safe bearing capacity may

be assumed as per table 1 of IS 14458 (part 2): 1997 which is given

below:



Table 1: Safe Bearing Capacity of Different Types of Soil

Type of Bearing Material

Symbol Consistency of

place

Recommended

value of SBC

(ton/m2)

Well graded mixture of fine

and coarse grained soil,

glacial till, hard pan,

boulder clay

GW-GC,

GC, SC

Very Compact 100

Gravel, gravel-sand

mixtures, boulder gravel

mixtures

GW, GP

SW, SP

Very compact

Medium to compact

Loose

80

60

40

Coarse to medium sand,

sand with little gravel

SW, SP Very compact

Medium to compact

Loose

40

30

30

Fine to medium sand, silty

or clayey medium to coarse

sand

SW, SM,

SC

Very compact

Medium to compact

Loose

30

25

15

Homogeneous inorganic

clay, sandy or silty clay

CL, CH Very stiff to hard

Medium to stiff

Soft

40

20

5

Inorganic silt, sandy or

clayey silt, varied silt-clay-

fine sand

ML, MH Very stiff to hard

Medium to stiff

Soft

30

15

5



5. Design Criteria

In design of stone masonry retaining walls, first the anticipated

loads on the structure should be considered and then evaluate

the safety of the structure based on standard criteria.

The loads to be considered in the design are:

Self-weight of the structure;

Imposed dead loads and live loads if any;

Earth pressures

Seismic loads if it is to be designed for earthquakes forces

Pore-water pressure

Surcharge loads if any

Special loads if any

The structure then should be evaluated for safety using following

criteria:

a) Factor of Safety against overturning > 2.0 for static loads

>1.5 with seismic forces

b) Factor of safety against sliding >1.5 for static loads

>1.0 with seismic forces

c) Maximum base pressure ≤ qa(allowable bearing capacity)

≤ 1.33 qa(during earthquakes)

d) Minimum base pressure > 0 (zero)

e) Factory of safety against floatation > 1.25

f) for steep hills, factor of safety for slip surface > 1.5 static

> 1.0 seismic



Notes:

1. Retaining walls are generally designed for static loads without

considering seismic loads. The reason being, the failed retaining

walls are easy to repair than going for heavy and costly walls

considering seismic forces.

2. For checking overturning and sliding, live and imposed loads

adding to the stability of the structure shall not be considered.

6. Preliminary Design of Cement Masonry Walls

As per table 1 of IS 14458 (Part 1):1998, preliminary dimensions

shall be calculated as follows:

Table 2: Preliminary Design of Wall

Top Width (Tw) 0.5m to 1.0m

Base width (Bw) 0.6H to 0.65H

Front Batter 10:1

Inward dip of foundation Horizontal or 1:6

Foundation depth below drain 0.5m to 1.0m

Range of Height 1m to 10m

Hill slope angle 350 to 600

Toe protection in case of soft soil Boulder pitching



Figure 2: Pressure Distribution at the base



7. Standard Designs

Table 3: Soil Properties considered/assumed for standard

designs

Table 4: Standard Design without Surcharge

Note:

i) SBC= Safe Bearing Capacity of soil

ii) All dimensions are in meters

γm 17 KN/m3

γs 24 KN/m3

φ 30 °

φi 30 °

μ 0.6

Unit Weight of Soil

Unit Weight of Masonry

Angle of Repose

Soil Properties considered/Assumed

Angle of internal friction

Coefficient of friction

Base Width (m) Top Width (m) Base Width (m) Top Width (m) Base Width (m) Top Width (m)

1 1 0.5 0.45 0.5 0.45 0.5 0.45

2 2 0.9 0.5 0.9 0.5 0.9 0.5

3 3 1.5 0.5 1.4 0.6 1.4 0.6

4 4 2.3 0.6 2.1 0.6 2 0.6

5 5 3.2 0.75 2.9 0.75 2.5 0.75

6 6 4.3 1 3.8 1 3.1 1

Standard Designs

SBC 100 KN/m2 SBC 150 KN/m2

sl no Height (m)

SBC 80 KN/m2



Table 5: Standard Design with soil surcharge load

Note:

i) surcharge load implies load from the soil surcharge which

is assumed as a sloping angle as shown in figure 1.

ii) All dimensions are in meters

8. Other Considerations

8.1 Depth of Foundation

Depth of foundation shall be at least 500mm below the firm ground

level. For unusual soil conditions, foundation shall be designed as

per IS 1080 and IS 1904.

8.2 Drainage

Drainage is an important aspect of retaining wall construction. For

proper functioning of the wall, following drainage provisions shall

be provided:

Base width Top width Base width Top width Base width Top width

1 1 0.5 0.45 0.5 0.45 0.5 0.45

2 2 0.9 0.5 0.9 0.5 0.9 0.5

3 3 1.55 0.5 1.55 0.5 1.55 0.5

4 4 2.2 0.6 2.2 0.6 2.2 0.6

5 5 3.85 0.8 3.5 0.8 2.65 0.8

6 6 4.9 1 4.7 1 4.1 1

sl no Height

SBC 80 SBC 100 SBC 150

Surcharge angle upto 150



i. weep holes of 100x100mm opening shall be provided

at 1.5m interval and should be staggered both horizontally

and vertically. Alternatively pipes of equivalent area may be

used.

Figure 3: Picture Showing Weep Holes, Separation Gap and

Bonding Element

ii. 400-500mm width of filter media composed of good quality

stones (50mm – 100mm) shall be provided. And in addition to

above filtration media, geo-textile or bamboo mats are

recommended.



8.3 Dip of foundation base

The dip of foundation base towards hill side increases

the factor of safety against sliding significantly and

therefore it is recommended to provide a dip of 6: 1

(horizontal : vertical) as shown in figure 1.

8.4 Length of Wall

A separation gap of 50-100mm shall be provided at

maximum of 10m length of wall. Also gaps shall be

provided at bends and junctions.

8.5 Stepping of wall base

The wall base shall not be stepped except when the wall

rests on rock foundation. When foundation rests on rock,

the foundation may be stepped as per the clause 6.2 of IS

14458 (part 2) as shown in figure 4:



Figure 4: Stepping of Wall base on Rocks

8.6 Through-Stones / Bonding Element

Through stones shall be provided at every 1.0m interval both

horizontally and vertically throughout the length and height

of the wall. In the absence of through stones, reinforced

concrete blocks of 100mmx100mm shall be used as shown in

figure5.The through-stones or bonding element shall be

staggered both horizontally and vertically. Figure 3 shows the

front elevation of wall with bonding element.



Figure 5: Reinforced Concrete Bonding Element

8.7 Cement Mortar

Cement mortar ratio of 1:5(cement : Sand) or richer shall be

provided.

8.8 Curing

When cementer mortar is used, the wall shall be properly

cured for a minimum of two weeks to attain good bonding

strength.



8.9 Aesthetics

The stability and safety are of primary concerns. However,

depending on site conditions and location, retaining walls

shall be aesthetically appealing as well.

Figure 6: Sample Picture of Retaining Wall with Weep holes and

separation gap



Figure 7: Sample wall with PVC pipes as weep holes

Figure 8: Sample Pictures with neat stone face



9. Sample Calculations

a) Sample calculation for 4 m high stone masonry retaining wall

without surcharge load with following data. Refer figure 9 for

pressure distribution diagram. The dimensions of walls can be

taken from preliminary design and check for standard

criterion.

Table 6: Input Data for Sample Calculation (a)

Sl No Component Symbol Value Unit

1 Height H 4 m

2 Base Width Bw or b 2.1 m

3 Top Width Tw 0.6 m

4 Unit Weight of Soil 𝛾𝑠 17 KN/m3

5 Unit Weight of Masonry 𝛾𝑚 24 KN/m3

6 Angle of Repose ∅ 30 degrees

7 Coefficient of friction 𝜇 0.6

8 Safe bearing capacity SBC 150 KN/m2



Figure 9: Distribution of Forces- Sample calculation (a)

Active earth pressure coefficient without surcharge

𝐾𝑎 =(1 − 𝑠𝑖𝑛∅)

(1 + 𝑠𝑖𝑛∅)



sl no Load Distance from Toe (m) B.M about Toe (KN-m) Remarks

1 W1 0.6*4*24 57.60 1.80 103.68

2 W2 0.5*(2.1-0.6)*4*24 72.00 1.00 72.00

∑W 129.60 175.68 ∑MR

3 Ph Ph= Ka*γ*H2/2 45.33 1.33 60.44 MO

a) Check Against Overturning:

∑MR /Mo ≥2.0 2.91 Safe

b) Check Against Sliding:

μ∑W/Ph ≥1.55 1.72 Safe

c) Check for Bearing Pressure:

x = (Resultant Moment at toe)/∑W 0.89

e = (Bw/2-x) 0.16

33.35 Safe

90.07 Safe

Horizontal Earth Pressure Ph= Ka*γ*H2/2 act at H/3 from wall base

∑W is the total vertical load

∑MR is the total restoring moment

MO is the total overturning moment

μ is the coeff. Of friction between wall and soil interface taken as 0.6

Magnitude (KN)

Design Calculation for Stone Masonry in Cement Mortar

𝑚 =

1 +

𝑚 =

1 −



b) Sample calculation for 5m high stone masonry retaining wall

with soil surcharge of 15ᶿ. Refer figure 10 for pressure

distribution diagram. The dimensions of wall can be obtained

from preliminary design and check for standard criterion.

Table 7: Input Data for sample calculation (b)

Sl No Component Symbol Value Unit

1 Height H 5 m

2 Base Width Bw or b 2.65 m

3 Top Width Tw 0.8 m

4 Unit Weight of Soil 𝛾𝑠 17 KN/m3

5 Unit Weight of Masonry 𝛾𝑚 24 KN/m3

6 Angle of Repose 𝜑 30 degrees

7 Coefficient of friction 𝜇 0.6

8 Safe bearing capacity SBC 150 KN/m2

9 angle of surcharge 𝛽 15 degrees

Active earth pressure coefficient with sloping surcharge

𝐾𝑎 = cos 𝛽𝑐𝑜𝑠𝛽 − (√𝑐𝑜𝑠2𝛽 − 𝑐𝑜𝑠2∅)

𝑐𝑜𝑠𝛽 + (√𝑐𝑜𝑠2𝛽 − 𝑐𝑜𝑠2∅)



Figure 10: Distribution of Forces - sample calculation (b)



sl no Load

Distance from

Toe (m)

B.M about

Toe (KN-m) Remarks

1 W1 0.8x5x24 96.00 2.25 216.00

2 W2 0.5x(2.65-0.8)x5x24 111.00 1.23 136.90

∑W 207.00 352.90 ∑MR

3 Ph Ph= Ka*γ*H2*Cos(β)/2 79.25 1.67 132.09 MO

a)

∑MR /Mo ≥2.0 2.67 Safe

b)

μ∑W/Ph ≥1.55 1.57 Safe

c)

x = (Resultant Moment at toe)/∑W 1.07

e = (b/2-x) 0.26

31.48 Safe

134.12 Safe

Magnitude (KN)

Design Calculation for Stone Masonry in Cement Mortar with surcharge angle of 15ᶿ

Check Against Overturning:

Check Against Sliding:

Check for Bearing Pressure:

Horizontal Earth Pressure Ph= Ka*γ*H2/2 act at

H/3 from wall base

∑W is the total vertical load

∑MR is the total restoring moment

MO is the total overturning moment

μ is the coeff. Of friction between wall and soil interface taken as 0.6

𝑚 =

1−

𝑚 =

1 +



10. References

IS 14458:Part 1: Guidelines for Retaining Wall for hilly area:

Part 1: Selection of Type of Walls


Part 2: Design of retaining/breast walls


Part 3: Construction of Dry stone walls

Guidelines on use of Standard Work Items for Common

Road Works 2010, Department of Roads, MoWHS

Basic and Applied Soil Mechanics, Gopal Ranjan& A.S.R

Rao

Design of Reinforced Concrete Structures, S. Ramamrutham



11. Technical Committee Composition

Technical Working Core Group members

1. Mr. Palden Khandu, Executive Engineer, Thimphu Thromde

2. Mr. Karchung, General Manager, NHDCL

3. Mr. Dillip Thapa, Executive Engineer, DoR, MoWHS

4. Ms. Kezang Deki, Executive Engineer, DES, MoWHS

5. Mr. Sonam Yangdhen, Dy. Executive Engineer, DES, MoWHS

6. Mr. Dechen Norbu, Dy. Executive Engineer, DES, MoWHS

7. Mr. Choki Tashi, Dy. Executive Engineer, DES, MoWHS – Team

Leader

Technical Review Group

1. Mr. Tenzin, Director, DES, MoWHS

2. Mr. Phub Rinzin, Chief Engineer, ESD, DES, MoWHS

3. Mr. Karma Namgyel, Chief Engineer, EARRD, MoWHS

4. Mr. Karma Jamtsho, District Engineer, Thimphu Dzongkhag

5. Mr. Jigme Dorji, Executive Engineer, Thimphu Thromde

6. Ms. Dago Zangmo, Executive Engineer, EARRD, DES, MoWHS

Cover page design and graphic editing by Mr. Tshering Norbu,

Architect and Mr. Tempa Gyeltshen, Architect, DES, MoWHS

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