GEOTECHNICAL ENGINEERING

ECG 503

LECTURE NOTE 9

3.0 ANALYSIS AND DESIGN OF

RETAINING STRUCTURES

LEARNING OUTCOMES

Learning outcomes:

At the end of this lecture/week the students would

be able to:

Able to design Reinforced Earth Structures

(Mechanical Stabilized Earth Walls, MSE)

FOS Against Overturning, Sliding and Bearing

Capacity Failure

TOPIC TO BE COVERED

Reinforced Earth Structures

(Mechanical Stabilized Earth

Walls, MSE)

Analysis and Design

FOS Against Overturning,

Sliding and Bearing Capacity

Failure

Types of retaining wall (after J.E. Bowles – 1977)

Gravity walls

No tensile, not economical for high > 1.5m

Cantilever wall

r.c. structure, economical for moderate hight (up to 7.5m)

Semi-gravity wall

Counterfort wall

Suitable for high wall (5 – 10m)

Bridge abutment wall

Crib wall

Precast conc. member,

granular fill, like gravity

wall

BACKFILLED WALL

i. Wall Geometry

Terminology

Geometry of historically successful wall(after

J.E. Bowles – 1977)

ii. Selection & Types

of Retaining wall

Classical Semi-gravity

Retaining Wall (JKR-1979)

Pre-cast Retaining Wall

iii. Selection of soil parameters

• Usually involves effective stress analysis (c’ & Ø’)

• Unit weight of soil 16 to 22 (granular) & 15 to 21 (stiff clay) kN/m2

• In the absence of reliable laboratory test data for clay

plasticity index Ø’ (critical)

15 30

30 25

50 20

80 15

• For sand Ø’= 30 + (A+B) where A= 0 to 4 (rounded to angular)

B = 0 to 4 (uniform to well graded)

• For rock Ø’ = 28 (weak mudstone)

42 (weak sandstone)

Earth retaining structure should not fail. The design must satisfy limit state

philosophy i.e.

i. ultimate limit states (total collapse – external & internal stability)

ii. serviceability limit states (deformations)

External stability deals with adequacy against: i. Overturning

ii. Sliding

iii. Bearing capacity

iv. Global stability

Internal stability deals with adequacy against: i. Bending moment

ii. Shear

iii. Tension

Serviceability limit states involve limiting deformations to tolerable value

It is normal to limit deformation by adopting high FOS against u.l. states

Overturning & Sliding

Bearing capacity failure

Global stability

Case histories on

design,

construction &

failures of earth

retaining

structures:

• Masonry wall

• Gabion wall

• R.C. Wall

• L-shape

• Cribwall

A lesson from an

engineering

failure is better

than 1000 theories

Construction of cantilever wall

Failure of masonry wall in Johor

i. By rupture

ii. By sliding/tilting

4.5m

0.3m

1.5m

R

S

Ov

Collapse of wall

Source: Maphilindo’s record photograph

Plastic sheet

Disintegrated rubble

Intact piece of rubble

Computation Results

Retaining Wall Height

4.5m

Type Unreinforcedmasonry

Geometry: B/H ratio

0.33 < 0.7

Bearing pressure -ve: R out of middle 3rd

FOS against Sliding S

1.0 <1.5

FOS Over turning Ov

1.1<2.0

Overall stability FOS

< 1.3

By rupture

3m

0.225m

1.22m

S

R

Ov

Gap opening at top of wall due to lateral movement(Looking from the opposite direction)

Gap opening at top of wall due to lateral movement

Source: Contractor’s record photographSource: Contractor’s record photograph

Type Unreinforcedmasonry

Geometry: B/H ratio

0.4 < 0.7

Bearing pressure -ve: R out of middle 3rd

FOS against Sliding S

1.0 < 1.5

FOS Over turning Ov

1.1 < 2.0

Overall stability FOS

< 1.3

Sliding/Tilting

Excessive movement – P.DExcessive movement/sliding – Meru

Gabion as bank erosion protection- Sik

FOS = 1.48

Gagion as slope protection –

Air Kroh

Before After

Gabion as Buffer

zone/platform

Terengganu

Buffer zone

Erection of gabion/ terramesh

Gabion as isolation/buffer zone -

Kedah

Gabion/Terramesh wall –

Cemeron Heighlands

R.c. Retaining wall - Perlis

Serviceability limit states – O.K.

Rubble wall as slope toe protection-KL

Failure of r.c. wall at bridge abutment/ bank protection – Meru

FOS < 1.0

(Failed)

Slope instability leading to wall

failure – Slim River

Pre-cast r.c. wall - Seremban

L-drain failure - Meru

L-shape drain failure - Terengganu

Inappropriate L-shape usage - Selayang

Failure of retaining structure

• Painful & expensive

• Open opportunity to advance the technology,

precision, prevent recurrence

• It is a full-scale prototype test

• It confirms that fundamental theory is true

• You pay for heavy price but must at least get

something useful

Construction &

completion of L-shape

bank protection - Sik

Construction/installation

problem of L-shape in soft

ground – Shah Alam & Meru

Cribwall construction- Selangor

In Kluang

R.E. wall

construction &

design

Causes of r.s. failure

• Lack of technology

• Inadequate S.I.

• Design inadequacy understanding real problem is more important

than accurate calculations

• Construction inadequacy

• Lack of design input during constructiondesign is incomplete until construction is; design/verify as you go; avoid blanket design;

total solution; good liaison

• Undesirable trends & practices

• Practicing beyond one’s competency

• Pressure of time & budget

REMARKS

• Many failures in general are repeat of the

past failures, but with new players

• Challenges now is to reduce recurrences

• Two main criteria: stability &

deformation

• Grd is complex natural material with wide

range of eng. properties may very 10 to 100X but FOS= 2 to 4

Mechanically Stabilized Retaining

Walls

• General Consideration :

Must satisfied the two condition of :

a. Internal stability requirements

b. Checking the external stability of

the wall

• Internal Stability

Determination of tension and pull

out resistance

• External Stability

Determination of overturning,

sliding and bearing capacity

SH = Horizontal spacing c/c

SV = Vertical spacing c/c

Procedure For Design Metallic Strip

Reinforcement

• General :

1. Determine the height of wall, H and the

properties of granular backfill materials

• Internal Stability

2. Assume values for horizontal, SH and

vertical, SV tie spacing. Also assume the

width of reinforcing strip, w to be used.