Chapter 5

Evaluation & Discussion

In this chapter, the data presented in Chapter 4 are summarized and the significances

are studied and highlighted. The test data from both slopes presented in Chapter 4 are

grouped and the graphs are superimposed for comparison in order to evaluate the similarities

and differences, a discussion of these combined data are made by cross-referencing with the

literature review in Chapter 2 to gain understanding of the reason for the slope failure, finally

a conclusion is derived which achieves the study objectives as stated in the Chapter 1.

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Project Summary

The purpose of the thesis is to examine the failed slope at KM 13.2 LATAR

Expressway which was caused by excessive water penetration from continuous heavy rain as

reported by the site engineer. The study involves site visiting for samples collection,

performing laboratory tests on the soil samples and analysing the results by means of

calculations and graph charts.

In order to examine the slope failure, soil samples were collected and the required

laboratory tests were performed so that a set of data containing the soil moisture properties of

the failed slope may be obtained. To aid the evaluation process, the same laboratory tests

were performed on soil samples collected from the neighbouring slope which was subjected

to the same weather condition and amount of rainfall but remains stable and did not fail. The

moisture data obtained on the stable slope enables us to determine the reasons for the failure

by means of comparison with failed slope data and cross-referencing with the established

slope design guidelines.

The first laboratory test carried out was the moisture content determination test, this

was done to obtain the natural water content of the soil at site. The sieve analysis was

performed in order to describe the grain size distribution of the soil. The Atterberg Limits test

was carried out to determine the liquid and plastic limit of the soil, these properties enables us

to figure the susceptibility of the soil towards water volume in terms of slope stability. The

compaction test was carried out to determine the optimum water content for the soil which

gives the maximum dry density, this data enables us to figure the strength of the soil and it’s

suitability for slope design. The direct shear box test was carried out to determine the shear

strength properties of the soil in terms of its cohesion and angle of friction, the data enables

us to compare the strength between soil at failed and stable slope, theoretically, the strength

of soil at failure site should be lower and the test is performed to prove this hypothesis. All

the data obtained during the laboratory experiments help to give a technical reason for the

slope failure and satisfy the study objectives stated earlier in the report.

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Summary of Laboratory Tests Results

List of Laboratory Tests Performed

i) ASTM D 2216 – Moisture Content Determination Test

ii) ASTM D 422 – Particle Size Analysis

iii) ASTM D 4318 – Atterberg Limits Test

iv) ASTM D 698 – Moisture – Density Relation (Compaction) Test

v) ASTM D 3080 – Direct Shear Test

Sample Source Failed Slope Stable Slope

Natural Water Content, w% 18.6 27.6

Liquid Limit (LL) 41 53

Plastic Limit (PL) 28 34

Plasticity Index (PI) 13 19

Optimum Water Content, w% 13

Maximum Dry Density, g/cm3 1.6

Cohesion

Friction Angle

Shear Strength

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Evaluation & Discussion

Natural Water Content

The natural water content for soil at failed slope shows lower value than the one at

stable slope i.e. failed slope at 18.6 % and stable slope at 27.6 %. However, the differences

do not give much indication to the reason for the failure, the values simply indicate the water

content of soil at its natural state when it was collected at site. The samples taken were

disturbed and were collected near the surface thus the natural water contents for both soils are

subjected to a certain degree of inaccuracy.

Liquid Limit (PL)

The liquid limit for soil at failed slope shows lower value than the one at failed slope

i.e. LL for failed and stable slope are 41 and 53 respectively. We recall the definition for

liquid limit in Chapter 2 in which McCarthy (1998, p.105) defines the liquid limit as “the

water content at which the soil ‘flows’” and Handy and Spangler (2007, p.246) stated that

“liquid limit is the moisture content at which a soil becomes liquid”.

Therefore, by comparing the liquid limit values of both samples and cross-referencing

them with the definitions given, it can be concluded that the soil at stable slope requires

higher water content before it behaves like a liquid and flows while the soil at failed slope

will possess liquid behaviour at much lower water content. Given that both slopes are

subjected to the same amount of rain, there is probability that rain volume was enough to

increase the water content of failed slope to reach or surpass its liquid limit causing it to

become liquid-like, eventually the slope became destabilized and collapsed due to its

decreased strength and heavier self-weight. In the other hand, the same rain volume was not

enough to increase the water content of the stable slope to reach its liquid limit, the water

penetration may have reduced its strength but not to the extent of flowing, thus it remains

stable.

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There is a direct link between soil liquid limit and slope collapsibility as reviewed in

Chapter 2 in which Handy and Spangler (2007, p.251) mentioned when water content of soil

exceeds its liquid limit, it can be harmful and devastating especially on slopes where the

slope may appear to be stable but can suddenly turned into flowing mudslide when disturbed.

It is also advocated by Denisov in 1953 where he claimed that if the moisture content upon

saturation exceeds the liquid limit, the soil is susceptible to collapse (Handy & Spangler,

2007, p.254).

During site investigation, it was observed that the slope failure plane lies in the slope

head and below the drain as highlighted in the photo below, judging by poor maintenance of

the drain and prolonged rainfall, it can be deduced that excess rain has overflowed and

penetrated the slope head, saturation continued until the water content reached the liquid limit

and the slope behaved like fluid, dramatically reduced its shear strength which directly

reduced its resisting force, coupled with increment in self-weight from rain penetration which

increased the driving force, undeniably all these factors combined led to the failure of the

slope.

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Figure: The Failed Slope Head & Failure Plane

Plastic Limit (PL)

The plastic limit of soil at the failed slope is 28 which is lower than the soil at stable

slope i.e. 34. The plastic limit value is

Addition to chapter 2

1. Slope Stability and Stabilization Methods By Lee W. Abramson 2002

High pi indicates:

a) higher clay particles content

b) More compressable,

c) less permeable

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2. Liquid limit and collapsibility (handy spangler, p254)Moisture content exceeds liquid limit (hand spangler, p251)

In general, higher values of PI are more indicative of poor performing soils.

0 Nonplastic 1 ‒ 5 Slightly Plastic 5 ‒ 10 Low Plasticity 10 ‒ 20 Medium Plasticity 20 ‒ 40 High Plasticity > 40 Very High Plasticity

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