The designing for example roads was usually based on the conventional technique of balancing cut and fills with the slope gradient of 1V:1H to 1V:1.5H for the cut areas and 1:2 for the fill areas. Landslide records from years 1966 to 2003 show that 42% of landslides occurred in hilly terrain areas and more than 90% occurred in developed areas (infra/residential/commercial)-Slope design.
Most of the investigations carried out thus far revealed that causes of landslides were due to physical, geological and human elements. Based on landslide forensic statistical data for large scale failure from the year 2004 to 2007, it was found that about 57% of landslides were due to human factor, whereas 29% were attributed to physical factor and 14% caused by various geological factors. It was also discovered that most of the landslides occurred at a man-made slopes.
The designer shall search and study all reports on the feasibility study, development plans and investigations related to the works so as to enable a full understanding of factors which may affect the works. Notwithstanding requirements stipulated herein the designer should satisfy the aspects of aesthetics, functional and safety requirements, suitability and effectiveness, completed to the intent of the works.
Survey Data for Slope design
The Designer shall carry out detailed topographical, hydrologic and land surveys of the proposed site and areas that may be affected by the works, which may be necessary to supplement available survey information for the satisfactory execution of design and construction of the works. Survey plans shall be prepared in scales appropriate to their purpose and follow “Guidelines for Presentation of Engineering Drawing”. The designer shall be responsible for the accuracy of survey data that is used in design work.
The designer shall undertake his own additional soil investigation and material surveys for the purpose of preparation of the engineering design and construction of the works. The preliminary site investigation results, if provided by the client are to be used as a preliminary/general guide only. The client is not obliged to guarantee the completeness and accuracy of the preliminary soil investigation results.
Slope design data
All details of the geotechnical Slope design shall be based on the data interpreted from the preliminary SI report and any additional Soil investigation carried out by the designer. All detail design shall be accompanied by a summary of the results of field exploration and laboratory investigation.
Design Soil Parameters
Design soil parameters, shall be shown in figures/ photos together with selected values that include but not limited to the following:
1. Basic soil properties, e.g., unit weight, liquid and plastic limit, etc.
2. Chemical properties of subsoil and its effect on the foundation structures
3. Consolidation parameters, compression and recompression indices, drainage path, coefficient of consolidation (cv and ch) and permeability of subsoil, etc.
4. Shear strength parameters include effective (c’ and Ø’) and total stress strength (su).
5.Groundwater level/regime and prediction.
Engineering Geological Mapping and Investigation
Independent geological mapping of the subject area should be carried out at an appropriate scale which shows sufficient detail to adequately define the geologic conditions present such as rock type, structural geology, the nature of the rock slope and groundwater conditions. Existing geological maps should be treated as a basis for understanding the site conditions.
If available geological maps are used to portray site conditions, they must be field checked and updated to reflect geologic, topographic, and/or changes which have occurred since the map publication. It is necessary for the geologist to extend mapping into adjacent areas where mapping have not been carried out previously to adequately define geological conditions relevant to the project area.
For the rock slope and adjacent rock exposure, discontinuity data collection and analysis should be carried out to aid in identifying the possible modes of failure.
Rock outcrop mapping is the best field way to obtain discontinuity data. If little or no exposure is available on the slope, knowledge of local geology may permit extrapolation from outside the slope. Where extrapolation is necessary, the designer should determine whether the rock mass and discontinuity pattern in the area of the data collection is akin to those of the slope by considering local geological conditions.
Materials used in the construction of fill slopes and embankments shall if it is suitable, as far as possible be those excavated from adjacent cuts.
Rocks excavated from the cuts may be used as material for fills if they are crushed to acceptable grading envelopes, with a maximum size of individual pieces not larger than 100mm. Drying out of the fill material during hauling and handling from cut to position of placing shall have to be allowed for.
Settlement analysis shall be carried out for the fill slopes and embankments depending on the subsoil conditions encountered. Design of fill slopes or embankment shall be based on 90% settlement during construction.
If ground improvement methods are used, the settlement analysis will consider the type of ground improvement method used.
Slope Stability Analysis
Slope stability analysis, including establishing design criteria and performing calculations, will be required for all cut, fill and natural slopes. The data to be utilised in the slope stability analysis shall be based on detailed site plans, detailed field descriptions, on-site exploration data and laboratory test data. It is the responsibility of the geotechnical engineer to determine the weakest potential failure surface based on the above factors. In performing any analysis, the worst possible conditions must be utilised.
Slope stability analysis shall include;
-Stability analysis for the temporary stability measured during construction.
-Cut and fill slope stability analysis should include both circular and non-circular analysis and in multi-mode of failure.
-Any slope that is influenced by surcharge load shall be analysed taking into consideration of this surcharge load.
All Rock slopes shall be analysed and designed. Preliminary consideration can be used using 4V:1H for weathering grade I and 3V:1H for weathering grade II. If analysis indicates that it is unstable, it shall be designed to a better gradient and/or requiring extensive stabilisation measures. The type of stabilisation measures to be used can be one of the following:
• permanent rock anchors
• rock dowels
• rock bolting
• buttress walls
• counter forts
• relieved drains, etc.
These include cut slopes in residual soils and in completely decomposed rock.
All untreated slopes shall be designed with a minimum of 2m berm width and maximum 6m berm height with a Factor of Safety greater than 1.3. Stabilisation measures can be considered when the design is inadequate. Stabilisation measures may include the following:
• soil nailing with slope surface protection
• permanent ground anchors
• retaining walls, etc.
The minimum global Factor of Safety for treated slopes shall be 1.5.
Due to maintenance reasons and to minimise risk to the users, the maximum number of berms for cut slopes shall be restricted to 6 berms. If the design shows that more than 6 berms are required, other solutions such as tunnel, rock shade, bridges etc. shall be considered.
Fill Slopes and Embankments
All untreated fill slopes and embankments shall be designed with 2m berm width and 6m berm height with a minimum Factor of Safety of Stabilisation measures can be considered when the design is inadequate. Stabilisation measures may include the following:
• geogrid/geotextiles reinforcement
• reinforced concrete retaining structure
• reinforced fill structure
• replacing the fills with elevated structures
The minimum global Factor of Safety for treated slopes shall be 1.5.
The types of foundations for the reinforced structures shall be designed based on the subsoil profile and geotechnical properties of footing subsoil at each location.
Surface drains in slope faces shall be provided in addition to normal cut off drains (interceptor drains) at the top of the slope. Downslope surface drains shall be provided for all cut and filled slope surfaces.
The drains shall be cascade drain with handrail for ease of maintenance in the future.
Subsurface drains such as horizontal drains and drainage blankets shall be provided for cut and fill slopes and for areas where the groundwater level is found to be high.
Adapted Jabatan Kerja Raya “Guidelines for slope design” ’10.edition