BY: AFIQ FARHAN ABDUL RAHIM
MOHAMAD FARUQ SYAHMI MD ARIPIN
INTRODUCTION
A check on GTM practice has been conducted under Newton-Ungku Omar Fund Initiative with gradual improved methods implemented on several GTM projects as pioneer.
RELATED DOCUMENTS
- JMG (2010) Garis Panduan Pemetaan Geologi Terain (JMG.GP.06). Jabatan Mineral dan Geosains Malaysia. Kuala Lumpur. 73p.
- GCO (1989) Geotechnical Area Studies Programme: Territory of Hong Kong. Geotechnical Control Office. Hong Kong. 346p.
TECHNICAL RECOMMENDATIONS
A proposed workflow is presented in Figure 1. The proposed workflow comprises of three main investigation components which can be conducted simultaneously, namely:
- Geological Terrain Mapping (Annex A)
- Geohazards Assessment (Annex B)
- Engineering Geology Assessment (Annex C)
which are separated into 5 stages of investigation, namely:
- Stage 1: Preliminary Study
- Stage 2: Field Data Acquisition
- Stage 3: Data Management and Processing
- Stage 4: Auditing and Verification
- Stage 5: Final Report / Product
Figure 1: Proposed workflow for GTM practice with three main investigation components: Terrain Mapping, Geohazards Assessment and Engineering Geological Assessment.
ANNEX A – GEOLOGICAL TERRAIN MAPPING
Terrain mapping aims to map the geomorphology related to Area of Interest (AOI) and its potential constraints towards planned development. The outcome of terrain mapping serves as general guidance for geotechnical works in proposed developments as in JMG.GP.06 – Table 8.
Terrain mapping utilizes polygons (in vector-type GIS data), with each polygon shall consist of terrain attributes as in JMG.GP.06 – Table 1.
The methodology in this technical guidance note serves as a supplementary document to guide geologist with adherence to procedure in JMG.GP.06 – section 3.0.
Figure 2: Proposed polygon demarcation workflow, with priority to demarcate the polygons regarding geomorphology first before other attributes. (i) delineate the catchment boundaries – ridge, crest and peak, (ii) delineating the drainage valley, (iii) refining the geomorphology based on terrain shapes, (iv) dividing polygons based on slope gradient, (v) refining polygons based on activity and erosion attributes based on satellite imagery, and (vi) draft Terrain Classification Map with attribute codes from polygon demarcations.
ANNEX B – GEOHAZARDS ASSESSMENT
Geohazards assessment has been included in JMG.GP.06 yet was briefly shown on the workflow without much elaboration.
Geohazards assessment must cover the extent of sub-catchment where AOI is located. Therefore, the study must be extended upstream or downstream from AOI, without limiting the study to AOI only.
Each geohazards (landslide, debris flow, or flood) constitutes a different approach, a competent geologist is needed for such assessment.
Interpretation of geohazards from topographic data. However, identifying geohazards feature including landslide scarp and body might be tricky especially in relic structures. The following map may prove helpful for geohazards identification:
- Hillshade Map (Figure B-1)
- Topographic Openness Map
- Slope Gradient Map
Figure 3: Hillshade map generated from topographic data, showing the relief of topography and any geohazards feature in study area. Demarcation of different components in a recent debris flow occurrence, highlighting its (i) source area, (ii) transportation area, and (iii) depositional area.
Geohazards interpretation shall be drawn on map. The followings are recommended ways of representing geohazards on map:
- Landslide: Identified landslide geohazards shall be demarcated in polygons, which each landslide is separated into two polygons – source area (scarp) and deposit area (body).
- Debris flow: Requiring a more comprehensive approach to study the catchment type, where each identified debris flow is separated into three polygons (components) – source area (landslide or other debris flow triggering source), transportation area, and deposition area.
- Rock falls: Run rock fall runout simulations or general 1H from rock slope rule as recommended by JMG. The rock fall affected area is demarcated in polygon while rock slope is shown in a polyline.
- Flood: Based on previous flood records, map the probable extent of flood in polygon.
ANNEX A – ENGINEERING GEOLOGICAL ASSESSMENT
Engineering geological assessment aims to identify engineering geology and geotechnical issues associated with geology in AOI.
Initially part of thematic map component in JMG.GP.06, the engineering geological assessment shall stand alone as a comprehensive investigation, with multiple outputs including engineering geological model and map.
Comprehensive engineering geological judgement based on preliminary study and field investigations must be done with emphasis on the following engineering geological issues:
- Engineering geological properties of soil and rock with its distribution within AOI and its implication towards any geotechnical works done on site.
- Weathering profile and depth of each layer based on field observations and borehole / geophysical data from SI and its implication towards any geotechnical works done on site.
- Structural geology (bedding / schistocity plane / fault or other discontinuities within rock or soil mass found on site) of study area and its influence towards any future geotechnical works.
- Groundwater and surface water regime on site. General groundwater table shall be established based on SI data, while surface water flow regime can be established from field investigations. The implication of groundwater and surface water regime towards future geotechnical works shall be highlighted.
- Potential instabilities on site (also addressed in geohazards assessment) and its implications toward any geotechnical works done on site.
- Other engineering geological features found on site that potentially influence (or act as constraints) towards future geotechnical works on site.
Figure 4: Engineering geological model of a study area, highlighting issues under and above surface, and relationship between engineering geological properties and human activities.
