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Rock Weathering – Tropical climate

LO2- Characterization and classification by weathering grade

Determine the quality of rock mass

Clause 44,pp 112,1981Clause 44 – pp 132, 1999

BS 5930

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DEVELOPMENTOF ROCK MASS CLASSIFICATION BY WEATHERING GRADE FOR ENGINEERING WORK.(Zainab,2004)

soil mechanics

rock mechanics

Weathered rock

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Weathering profile of Granite rock massPuchong Quarry

Weathering profile of Granite rock massPuchong Quarry

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Weathering profile of Granite rock massLembah Klang

Weathering profile of Granite rock massIpoh

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Weathering profile of Granite rock massHulu Langat - Semenyih

Weathering profile of Granite rock massHulu Langat - Semenyih

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Weathering profile of Meta Sedimentary rock massCyberjaya

Excavation of Hard material at Puncak Alam, Selangor

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Failure of Sedimentary cut slope at Lebuhraya Pantai Timur

Schematic diagram of weathering profiles (Fookes,97)

Granite Metamorphic Carbonate

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* Rock mass deterioration with time

* Lost of strength

* Weakening of discontinuities with time

* Engineering mechanics

No MechanicsSoil Mechanics Rock Mechanics

Physical weathering

Chemical weathering

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Figure 2: Rock slope failure at Bukit Lanjan, from left; a year after construction, during the collapse and after rehabilitation work.

A year after construction During the collapse After rehabilitation work

Construction of Pergau Dam

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Pinnacle

Steep cliff

Floater

Cavity

ALLUVIUM

LIMESTONE

KARSTIC PROFILE

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PROBLEM/RISKS OF BORED PILES SOCKETED IN LIMESTONE BEDROCK

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(Mohamed, 2010)

GENETIC INFLUENCE

GeologyClimate

LandformHydrology

TimeVegetation

TROPICAL WEATHERING

IN SITU CHARACTER e.g.

MineralogyFabricStructureStrengthMoisture condition

PROJECT INFLUENCEChanges in:StressHydrologyFabric

PROJECT GEOTECHNICAL PERFORMANCEe.g.:Actual swellActual settlementWorking strengthWorking strain

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Site investigation is a process of site exploration consisting of boring, sampling and testing so as to obtain geotechnical information for a safe, practical and economical geotechnical evaluation and design.

Generally it is an exploration or discovery of the ground conditions especially on untouched site.

To study the general suitability of the site for an engineering project.

To enable a safe, practical and economic design to be prepared.

To determine the possible difficulties that may be encountered by a specific construction method for any particular civil project.

To study the suitability of construction material (soil or rock).

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Methods for study of rock masses :

1. Desk study of existing data and photo-interpretation2. Surface investigation, geological and engineering geological

mapping and sampling of outcrops3. Subsurface investigation, using geophysical and mechanical

techniques as well as methods for visual inspection and logging zones of the rock masses at depth

4. Laboratory test for the characterization of rocks and discontinuities

5. In-situ tests for the assessment of rock mass properties6. Monitoring the behaviour of rock masses

SI of rock - si of small rock samples in the lab provides only approx info. Small specimens represent stronger rock material than the rock mass in situ

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- Joints in a rock mass reduce its effective shear strength at least in a direction parallel with the discontinuities

- Strength of jointed rocks is highly anisotropics

- Joints offer no resistance to tension and high resistance to compression

- Nevertheless, they deform under compression if there are compressible filling or aperture along the joint or if the wall of the rock is weathered

- Where discontinuities dip into a rock face, they only impose a direct mechanical instability on the face when they are of the same scale

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- They allow the ingress of water into the rock mass and as a result facilitate an increase in pore water pressure, hence reduces the effective strength of rock mass

- When discontinuities daylight into a rock face they adversely affect stability. In this case the slope of the face is to a greater or lesser extent controlled by the discontinuities

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Failure modes in rock masses- depend on

Type and structure of the rock

Seepage water conditions and other

characteristics

Geometric conditions of the slope

Possible external loads (including seismic forces)

Heterogeneity and anisotropic

Structural features joints, bedding planescleavageshear zones

Voids rock porescavitiesfissurescrackspartially or fully filled with air, gas/water

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Effect of water pressure

Increasing water content effects on stability, namely

seepage pressure

lateral water pressure in tension crack

shear strength deterioration of gouge

• How do you decide on the allowable risk to rock slope failure? Degree of calculated risk is influenced by the extent to which rock investigations are take , most rock slope-slope failures occur along discontinuities or zone of weakness

• Amount of deformation that can be tolerated in the foundation of a dam

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• Allowable loads in the reinforcing cables in an underground cavern or tunnel

• Rock excavation techniques?– - drill and blast method?– - tunnel boring machine?– - especially….cost implication.

• Boulders within a decomposed rock mass lead to a stress concentration on the retaining wall

• The pressure of water in cracks, pores, joints or bedding planes causes mechanical disintegration, splitting of rock and a local overstressing of retaining structures