GEOTECHNICAL ENGINEERING

ECG 503

LECTURE NOTE 07

TOPIC : 2.0 SLOPE RISK

ENGINEERING

AUGUST 2008

TOPIC TO BE COVERED

Slope rehabilitation works

Post mortem analysis,

rehabilitation technique such as

soil nailing, grouting,

engineered slope stability

structures

FORENSIC WORKS

• SITE INVESTIGATION (S.I.)

• SLOPE FAILURE TYPES

• SLOPE FAILURE CAUSES

• SLOPE REHABILITATION WORKS

SITE INVESTIGATION

• Slope Mapping

• Site survey by visual aspect

• Ground water observation

• Indication of geological materials

• Drainage system observation

• Water ponding

• Site Testing

SITE INVESTIGATION cont’

• Laboratory Testing

• Preparation of S.I. Report

SLOPE FAILURE TYPES

• Sliding

• Fall

• Erosion

• Toppling

• Planar

• Wedge

SLOPE TREATMENT

• Short-Term

a. Tarpaulin

• Long-Term

a. Turfing f. Masonary Wall

b. Guniting g. Sheet Pile

c. Gabions h. Rock Bolts

d. Cribwall i. Netting

e. Concrete Wall j Soil Nails

Soil Nailing In Slopes Stabilization

• Proven cost, used to cut slopes or to support deep

excavation by reinforcing the insitu ground. (Drilling,

inserting bar, grouting and nail head construction/

facing/ guniting.)

• Pull-out resistant of the reinforcing element.

– Practice : estimated based on soil data with reference

to empirical observations.

– On site : pull-out test result derived from full scale

verification/ sacrificial tests.

Soil Nailing In Slopes Stabilization

• Some slope movement is required to mobilize load

tension in the soil nail (up to 30mm).

• Lateral displacement due to stress-relief of excavated

steep soil nailed slope/ wall is about 0.1-0.3%H.

• When lateral deformation exceed 0.5%H, excessive

bending and shear in soil nails may happen, resulting in

excessive creep and tension cracks in upslope or

eventual slope failure, if left unattended.

Soil Nailing In Slopes Stabilization

• Soil nails need to extend to sufficient length beyond

the active zone or any plane of weakness to overcome

external stability including overturning, sliding,

bearing and overall slope instability, modes of failure.

• Three internal failure modes must be checked to

ensure an adequate factor of safety, i.e. nail pull-out

resistance, nail material tensile capacity and nail head/

facing capacity.

Drilling Method

• Characteristics:

– Constant straight diameter

– Stable drill hole

– Drilling debris wholly

– Cleanly removed are achievable.

• The rotary percussion method

– Using top hammer or down-the-hole hammer

– Capable of completing the drill hole within a short time (<1hour) should normally used.

Drilling Method

• Observation of the drill log

– Location

– Time/ duration

– Soil type/ strata

– Changes in penetration rate/ sound

– Flushing characteristics (wetness, color, nature and

sizes of cutting…)

Installation Of Reinforcement Bar

• For permanent works, rebars should be protected against corrosion by hot-dip galvanizing (BS729) with a minimum coat thickness of 85microns or 610gm/m2.

• Rebars only need to be protected in corrugated HDPE stealth for cases of proven aggressive soil (pH value <4.5; or sulphate >200ppm; or chlorite content >100ppm.)

Installation Of Reinforcement Bar

• Centralizers should be made from galvanized steel

or other suitably high quality material

dimensioned to fit the rebar at the centre of the

hole whilst not obstructing passage of cement

grout pipe.

Nail Head

• Guniting facing

• HDPE Geocell slope

protection system

• Grid beam system

For green vegetative

slope landscape

CASE STUDY

SLOPE FAILURE AT

KOMPLEKS IMIGRESEN,

PENGKALAN HULU,

PERAK DARUL RIDZUAN

WHAT WOULD YOU DO?

?

SITE INVESTIGATION

INTRODUCTION

The proposed site of construction for Kompleks Imigresen Pengkalan Hulu is located at the boundary of Thailand and Malaysia, about 60 km north of Grik town in the upper Perak region, through access road to Betong, Thailand.

The proposed site area is experienced slope failure and high water table problems. The landslide had occurred at the slope, close to proposed construction of Kompleks Imigresen Pengkalan Hulu

OBJECTIVE

The objectives of works to be carried out are :

– To understand subsurface condition and

geology at study area

– To understand type of slope failure.

– To determine probable causes of failure.

– To propose mitigation measures.

– To obtain soil type, soil strength

parameter for slope stability analysis.

SITE LAYOUT

GEOMORPHOLOGY OF

LANDSLIDE AREA

The original geomorphology of Pengkalan

Hulu and surrounding area is characterized

by mountainous terrain. Hills and valley in

this area can be clearly seen from the digital

terrain model (DTM) generated from the

topographic map of the area

GEOTECHNICAL

PARAMETERS

LOCATION OF BOREHOLES

AND PROBE MACHINTOSH

TO DETERMINE THE SOIL

PROFILE AND SOIL STRATA

DETERMINATION OF FOS

MOST PROBABLE CAUSE OF

FAILURE

• Presence of Geological Weak Material

The presence of Carbonaceous Shale probably mainly due to high organic carbon content. The Carbonaceous Shale are argillaceous deposits containing carbonaceous matter that induces a dark colour, between grey and black depending on the concentration of the carbonaceous matter. This is an extremely fine-grained rock consisting of quartz, sericite, chlorite, feldspar, clays and carbonaceous materials. This rock seems brittle, slickenside at the plain , dull luster and its recumbent fold.

• Increase in negative Pore Water Pressure

Based on the data obtained from the investigation

and analysis it was observed that the most

probable cause of failure is due to extensive

increase in negative pore water pressure in the

soil. The increase in this negative pore water

pressure will reduce the shear strength of the soil

thus weakening the integrity and stability of the

cut slope. The indication of seepage water from

the ground can be seen after the failure which

resulted in flow of water from the slope surface.

• Improper Drainage System

Improper drainage system on the cut slope area

may resulted in the penetration of runoff water

thus increasing the groundwater table in this area.

Some of the drainage system in this area seems to

be not functioning as required to channel the

runoff water to its outlet. Proper subsoil drainage

has not been installed on site to lower down the

groundwater table in this area.

• Characteristic and Weathering Process of Subsoil

The occurrence of weak zone in the subsoil

material which is highly decomposed as in contact

with water may have its contribution to the cut

slope failure. High groundwater table has

weakening the subsoil material thus creating a

weak layer to trigger the slip circle failure.

Completely Weathered (Grade V) of interbeded

shale and mudstone layer are very sensitive with

water. The occurrence of water in this layer may

weaken the characteristic strength of this material.

MECHANISM OF FAILURE

• Initially the cut slope was cut at a gradient of 1V : 1H with a 6m high slope and a 1.5m benches. Berm drain and cut off drain was constructed to channel out runoff water in this area. Hydroseeding was implemented to the slope surface however less than half of the surface areas were vegetated. This is due to the nature of the subsoil material which unable vegetation to growth on the soil.

• As the drainage system in this area are poorly constructed some of the drains has collapsed which enable runoff water to penetrate into the subsoil material.

• Circular type of slip failure occurred which then moves in retrogressively starting from the crest of the top slope downward to the toe of the slope. The increase in negative pore water pressure, which leads to weakening of the soil strength characteristic, has lead to the slip failure on the cut slope.

REMEDIAL AND MITIGATION

MEASURES

• Short Term Measures

Short term measures shall be carried out immediately to prevent slope failure area from getting worst. Tarpaulin sheet should immediately installed at all failure area to prevent further increase in negative pore water pressure to ensure the failure from propagate.

• Long Term Measures

Soil Nail, Allan Block 12 Wall and Green Terramesh

Reinforced Earth Slope

This comprises of installation of soil nail made up

of a 12m long Y25 bar inserted into a 100mm

diameter drilled hole which in turn will be grouted

to the full length. The spacing of the soil nail is

proposed to be at 1.5m c/c interval. The soil nail will

increase the existing factor of safety of the cut slope

by providing resistance to the overturning forces.

Horizontal drain of 75mm diameter perforated

UPVC pipe of 12m length shall be installed to reduce

the negative pore water pressure thus reducing the

groundwater along this area.

Green Terramesh system was to be installed on the

front face of the cut slope which has been installed by

soil nail. The installation of Green Terramesh will

increase the internal stability and provide aesthetical

features to the finishes of the cut slopes. Allan block wall

will also be installed on the front face of the cut slope at

the bottom portion as will increase the internal stability

of the cut slope. The drainage system within the area

shall be upgraded to ensure the surface erosion will not

take place after the treatments implemented. A new

drainage system will be introduced to integrate with the

existing drainage system.

ALLAN BLOCK

GREEN

TERRAMESH

UPGRADING

DRAINAGE SYSTEM

ALLAN BLOCK

GREEN

TERRAMESH

GREEN

TERRAMESH