VIVEK DHAKAL

# VIVEK DHAKAL

I'm a Civil Engineer. I usually write about engineering and some topical matters.

## What are the Special Considerations for Hill Road Alignment?

According to Nepal Rural Road Standards (2055), 2nd Revision 2071, the terrain is classified as Terai & Hills based on the topography of the country. The Terai covers the plain & rolling terrain having a cross slope of 0 to 25%. Hill covers the mountainous & steep terrain having a cross slope of 25 to 60% and more. The road passing through the hilly terrain with a cross slope of 25% or more is generally termed as Hill Road. A hill road usually consists of either a river route or a ridge route.

## What are the Factors Affecting the Alignment of Hill Roads?

There are various factors affecting road alignment. Moreover, there are some special considerations to be followed while selecting a hill road alignment. The major factors to be considered while deciding the alignment of hill roads are as follows:

### 1. Geological Stability

The road alignment should pass through a stable hill slope. The area should not be prone to erosion, landslides, rockfall, etc.

### 2. Availability of Construction Materials

Availability of construction materials near the construction site will reduce the transportation cost of materials thus making the project economical.

### 3. Cross Drainage Structures

Due to numerous watercourses present in the hilly regions, there may be the necessity of intense cross drainage works. The alignment should be so selected in such a way that the number of cross-drainage structures required becomes minimum.

### 4. Geological Structures

Excessive cutting of hard rock will be expensive. So, such areas should be avoided as much as possible from the road alignment.

### 5. Geometric Design

The alignment should be chosen to minimize the ineffective rise & fall, steep gradients, number of hairpin bends, etc. Also, the geometric design parameters should comply with the design guidelines & standards for hilly regions.

### 6. Altitude of the Road

• Rainfall (or Snowfall) ∝ Altitude
• Atmospheric Pressure ∝ $\frac{1}{Altitude}$
• As the altitude decreases, the number of cross drainage works required increases.

## Basics of Tunnel Engineering | Methods of Tunneling

Tunnels are underground passages used for transportation purposes. Tunnels are the underground routes driven without disturbing the overlying soil to bypass the obstacles safely. Tunnels can be used to carry passengers & freights, water, sewers, gases, etc. Tunnels are constructed in various shapes & sizes. The shape of the tunnel cross-section is governed by the nature & type of soil to be penetrated while the size of the tunnel depends on the usage to which it is subjected. The economy of tunnel construction depends on the relative cost of open cuts vs. tunneling. The tunnel becomes more economical than an open cut beyond a certain depth.

• It reduces the route distance & travel time
• It provides easy gradients in hilly terrain
• Surace activities are not disturbed
• It remains free from the weather actions like rainfall, snow, etc
• The tunnel becomes more economical than an open cut beyond a certain depth.

• The initial cost of construction may become higher
• Construction of tunnel requires skilled manpower & sophisticated equipment
• Strick supervision is necessary during construction
• Higher safety precautions are necessary during construction
• Construction of tunnel requires more time than open cuts
• A tunnel may collapse during an earthquake

## Terminologies related to Tunnel Engineering

• Tunnel Portal: The tunnel entrance is called a tunnel portal.
• Crown: It is the topmost point of the tunnel cross-section.
• Invert: It is the lowest point of the tunnel cross-section.
• Adit: It is a horizontal or near-horizontal passage that provides access to a tunnel. It may be used for the purpose of the auxiliary entrance, ventilation, drainage, etc.
• Shaft: It is a vertical passage from the ground surface that provides access to a tunnel. It may be used to transfer the centerline from the ground surface into the tunnel.
• Tunnel Linings: These are the supports erected during & after tunnel construction to ensure a safe working environment inside the tunnels. Stone masonry, brick masonry, timber, steel, etc are used as tunnel lining materials.
• Mucking: Mucking means the removal of blasted debris from the tunnel interior to a good distance outside the tunnel entrance.
• Faces of Operation or Attack: It is the surface from which a boring operation is carried out.
• Pilot Tunnel: It is a small tunnel driven, parallel & close to the proposed main tunnel, to explore geological conditions & assist in final excavation.

## Classification of Tunnels

### A. Based on Purpose

1. Traffic Tunnel
• Highway Tunnel
• Railway Tunnel
• Pedestrian Tunnel
2. Conveyance Tunnel
• Power Tunnel
• Water Supply Tunnel
• Sewer Tunnel

### B. Based on Shape/Cross-Section

1. Circular Tunnel
2. D Shaped Tunnel
3. Horse Shoe Tunnel
4. Square or Rectangular Tunnel
5. Elliptical Tunnel

## Methods of Tunneling

During tunnel construction, tunnels are lined with suitable materials parallelly with the boring operations. Tunnels are usually lined with timber, steel, cast iron, masonry, or concrete with suitable outlets to let out the enclosed subsoil water behind the linings. Other items of work include the provision of ventilation, drainage, lighting, etc. Tunneling may have to be done in the hard rock or soft soil based on which the method of tunneling differs. Hard rock is considered as a fully self-supporting soil that does not require much support except where a loose rock is occasionally met. On the other hand, soft soils like running grounds (eg: water-bearing sands) require instant supports all around. So, different methods of tunneling based on the nature of the soil to be penetrated are listed below:

### A. Tunneling in Soft Soils

1. Fore Poling Method
2. Needle Beam Method
3. Shield Method
4. Compressed Air Method
5. Liner Plate Method
6. Army Method
7. American Method

### B. Tunneling in Hard Rock

1. Full Face Method
4. Drift Method
5. Pilot Tunnel Method
For the detailed description of each method of tunneling listed above, the readers are kindly requested to go through ref 1.

References
1. Srinivasan, R.(1958). Harbour, dock and tunnel engineering. India: Charotall Book Stall

## What is Highway Alignment?

The process of establishing the centerline of a road is called highway alignment or Road alignment. It the direction through which the highway will pass. Highway alignment can be divided into two parts as Horizontal Alignment & Vertical Alignment. The horizontal alignment is seen in the plan of the road & it consists of the straight path, horizontal curves, etc. The vertical alignment is observed in the longitudinal profile of the road & it contains verticle curves, gradients, etc.

## What are the Requirements of Highway Alignment?

An ideal highway alignment may fulfill the following criteria:
• Short: The route between any two points should be the shortest route.
• Safety: The alignment should satisfy the safety requirements.
• Comfort: The alignment should have easy curves & gradients.
• Economy: The cost of construction should be economic.

## What are the Factors Controlling Highway Alignment?

There are various factors to be considered while selecting a road alignment. Additionally, there are some special considerations to be followed while selecting alignments in hill roads. In general, the following factors are to be considered while choosing a highway alignment.

### 1. Government Plannings

Since a road project involves heavy investments, it should comply with government requirements & planning.

### 2. Obligatory Points

Obligatory points are the governing points that control the highway alignment. These can be classified into two types viz. the points thorough which alignment should always pass (or positive obligatory points) & the points through which the alignment should never pass (or negative obligatory points). Ex: Highway alignment should always pass through the bridge site. In the case of mountains in the alignment, there may be options either to go round the hill or to construct a tunnel. Moreover, the highway alignment should never pass through the National Parks, Conservation Areas, Protected Areas, dense forest, costly agricultural lands, etc. In the case of an intermediate town, the highway alignment may get deviated slightly in order to connect the town.

### 3. Traffic Flow Pattern

The traffic flow pattern can be known from the origin & destination study (O&D Study). The lines are drawn in the data obtained from the origin & destination study & then, proper alignment is fixed.

### 4. Geometric Design

The road alignment is also affected by the geometric design. The horizontal curves, vertical curves, gradients, sight distance, etc should meet the requirements of geometric design standards.

### 5. Monotony

Due to very long straight paths in flat terrain, the driver may become monotonous & this may lead to accidents. Thus, small horizontal curves should be provided in suitable intervals to avoid monotony.

### 6. Economy

The alignment should be selected in such a way that the construction cost, maintenance cost & operation cost of the road is minimum. Excessive cuttings & fillings, the necessity of complex structures, etc should be avoided.

### 7. Railway Crossings

A highway alignment should cross the railway alignment preferably at a right angle.

## Definition of River Training

The process of controlling the flow in river & river bed configuration is called river training works. These are the structural measures adopted in rivers to avoid outflanking & shifting its thalweg due to geomorphological changes in the river. So, the river training works stabilize the river channel along a certain alignment.

## Methods of Soil Compaction | Types of Soil Compaction

Compaction of soil is necessary for various types of foundations used in civil engineering constructions. It improves the engineering properties of soil. Compaction is the process of reducing air voids in soil by means of mechanical compressions. During compaction, the air is expelled from the voids in the soil. It increases the dry density of soil, improves shear strength & hence stability and bearing capacity. The various methods of soil compaction are as follows:
• Tamper / Rammer
• Hand Operated Tamper
• Mechanical Tamper
• Roller
• Smooth Wheeled Roller
• Pneumatic Tyred Roller
• Sheep Foot Roller
• Vibrator

## Types of Foundation Used in Civil Engineering Constructions

Every civil engineering structure, whether it is a building, a bridge, or a dam, is founded on or below the surface of the earth. Foundations are required to transmit the load of the structure to the foundation soil safely & efficiently. Different types of foundations used in civil engineering constructions can be classified as follows:

## Shallow Foundation

According to Terzaghi, a shallow foundation is one whose width is greater than its depth. i.e. Df/B<1. Such a foundation transmits the load to the upper strata of the earth & is generally provided to the lightweight structures. It is preferred when foundation soil has sufficient bearing capacity at shallow depth. When the sum of areas covered by each isolated footings is more than 50% of the total area of the foundation, mat foundation is adopted.

## Deep Foundation

A deep foundation is one whose width is less than its depth i.e. Df/B>1. Such a foundation transmits the load to the strata at a considerable depth below the surface of the earth & is provided to the heavyweight structure. It is preferred when the soil at the surface of the earth does not possess a considerable bearing capacity. The most common types of deep foundations are piles, piers & caissons. Well foundations are the special case of open caissons.

## What are the Criteria for Selection of Ideal Bridge Site?

It is necessary to select an ideal bridge site at which the bridge can be made economically. As construction of a bridge requires a heavy investment, the bridge site should be selected wisely. A poor bridge site may increase the project's cost, making it susceptible to damage, in the long run, thus decreasing the life span of bridges. Thus, the following are the factors that require attention while selecting a bridge site.

## Principles & Techniques of Bioengineering for Civil Engineers

In a broader sense, Bioengineering is the use of life science & engineering to solve human life problems. Here, in this article, we are using the term bioengineering in the context of civil engineering & it basically refers to soil bioengineering. So, Bioengineering can be defined as the use of vegetative measures & small civil engineering structures in order to reduce the shallow seated instability. The living plants or non-living plant materials are used alone or in conjunction with small civil engineering structures for slope stabilization & erosion control. It utilizes locally available resources & is a cost-effective method.

## Principles of Bioengineering

Initially, stability is obtained from the small civil engineering structures. The strength of those structures decreases gradually. After the handover point, stability is derived from the vegetative measures. This can be depicted from the graph shown below:

## Functions of Bioengineering

Engineering functions performed by vegetation on a slope are as follows:
• Catch
• Armor
• Reinforce
• Anchor
• Support
• Drain

• Immediate slope stabilization & erosion control
• Utilization of locally available resources (local tools, local manpower, local materials)
• It is a cost-effective method
• No need for frequent maintenance
• It also provides an opportunity for wildlife habitat
• It also improves the aesthetic beauty of the site

## Commonly Used Techniques of Bioengineering

• Fascine: Bundle of live branches laid in shallow trenches
• Palisade: Woody cuttings planted across the slope.
• Wattling: Fence made out of vegetative materials.
• Bamboo Planting: Planting of bamboo for soil conservation
• Grass Planting: Planting of grass across the slope
• Brush Layering: Layers of woody cuttings planted in line following the contour
• RipRap: Stone pitching with vegetation interplanted between them
• Retaining Wall: Wall built to resist the pressure of earth filling or backing
• Toe Wall
• Breast Wall or Revetment Wall
• Check Dam: Dams constructed across the gullies to retard the flow
• Gabion Wall: Walls made up of gabion wire filled with stones
• Stone Masonry: Masonry construction using stones & mortar
• Jute Netting: Protecting the slope with standard jute mesh
• Rock Netting: Wire mesh of reliable material used to control the rockfall
• Rock Bolting: Reinforcement of rock slope by inserting steel bars
• French Drain: Subsurface drainage channel filled with aggregates

## Degree of Static & Kinematic Indeterminacy of Structures

A structural system that can be analyzed by using the equation of static equilibrium only is called statically determinate structure i.e. reaction components and internal stresses can be calculated using static equilibrium equations only. Eg: Simply supported beam. If it cannot be analyzed by the equation of static equilibrium alone, then it is called a statically indeterminate structure. Eg: Fixed beam. A structural system is said to be kinematically indeterminate if the displacement components of its joints cannot be determined by the compatibility equation alone. Eg: Simply supported beam. If those unknown quantities can be found by using compatibility equations alone then the structure is called kinematically determinate structure. Eg: Fixed beam. But before calculating the degree of indeterminacy of a structure, it is good to know about its stability. If a structure is unstable, then it doesn't matter whether it is statically determinate or indeterminate. In all cases, such types of structures should be avoided in practice.