Retaining walls

Retaining wall prevents the earth from sliding or eroding away. And to resist the material pressure of the material that it is holding back. It constructs with heavy blocks, angled stones, offset cinder blocks, wood or reinforced concrete frames backfilled or filled with the well-graded stone designed to create the geological resistance or mass required to counteract the earth pressures and surcharge loading. And also, anchor into the ground with steel I beam vertically inserted into the ground and reinforced with concrete and linked in between with planks or reinforced, Pre-fabricated concrete blocks.

Types of Retaining wall

Classification of retaining walls:

Following are the different types of retaining walls based on the shape and the mode of resisting the pressure.

1. Gravity wall-Masonry or Plain concrete
2. Cantilever retaining wall-RCC (Inverted T and L)
3. Counterfort retaining wall-RCC

1. Gravity wall – Masonry or Plain concrete

Gravity retaining Wall constructs of concrete, stone or brick masonry and are thicker in section. The geometry of these walls also helps them to maintain the stability. Mass concrete walls are suitable for retained heights of up to 3 m. The cross-section shape of the wall affects by stability, the use of space in front of the wall, the require wall appearance and the method of construction.

2. Cantilever retaining wall-RCC (Inverted T and L)

A cantilever retaining wall is one that consists of a wall which connects to the foundation. They are the most common type engages. Cantilever wall rest on a slab foundation. A cantilever wall holds back a significant amount of soil. It consists of a stem, toe slab, and heel slab.

3. Counterfort retaining wall-RCC

counterfort types adapted to larger heights. It consists of a stem, toe slab and heel slab as in case of cantilever retaining wall. But it also consists of counterforts are a regular interval which divides the stem. The stem with the combination of counterfort behaves like a tee-beam with varying width. The stem and heel slabs are effectively fixed to counterforts so that the stem bends horizontally between the counterforts due to lateral earth pressure. Thus the thickness of stem and the heel slab is considerably reduced due to the reduction of the moment due to fixity of these slabs between counterforts.

PILE TYPES BASED ON CONSTRUCTION METHOD

Three pile types of pile foundations according to their construction methods:

1. Driven piles,
2. Cast-in-situ piles, and
3. Driven and cast-in-situ piles.

1. Driven Pile Foundations:

It is of concrete, steel or timber. These piles are prefabricating before placing at the construction site. If drive piles are of concrete, they are precast. These piles are drive using a pile hammer.

When piles drive into the granular soils, they displace the equal volume of soil. This helps in compaction of soil around the sides of piles and results in the densification of soil. The piles which compact the soil adjacent to it is compaction pile. This compaction of the soil increases its bearing capacity.

Saturated silty soils and cohesive soils have poor drainage capability. So these soils are not compacts when piles are drill through it. The water is to be a drain on the soil to be compact. Stresses develop adjacent to the piles have to be borne by pore water only. This results in an increase in pore water pressure and a decrease in bearing capacity of the soil.

2. Cast-in-situ Pile Foundations:

Cast-in-situ piles are the concrete pile. These piles construct by drilling holes in the ground to the required depth. And then filling the hole with concrete. Reinforcements utilize in the concrete as per the requirements. These piles are of small diameter compare to drill piers.

 

Cast-in-situ piles are straight bore piles or with one or more bulbs at intervals are cast. The piles with one or more bulbs are under-reamed piles.

3. Driven and Cast-in-situ Piles

Driven and cast-in-situ piles have the advantages of both driven and cast-in-situ piles. The procedure of installing a driven and cast-in-situ pile is as follows:

A steel shell of the diameter of a pile driven into the ground with the aid of a mandrel inserting into the shell. After driving the shell, the mandrel removes and concrete pours into the shell.

The shell is of corrugate and reinforces thin sheet steel or pipes. The piles of this type are shell type piles.

The shell-less type formes by withdrawing the shell while the concrete places. In both the types, the bottom of the shell is close to a conical tip which separates from the shell. By driving the concrete out of the shell an enlarge bulb form in both the types of piles. Th shells left in place and the tube concretes. This type of pile is very much use in piling over water.

A pile cap is a mass concrete structure contains more piles embeds in it. A group of pill drives into clay, soft, and loose soils to provide a strong, stable and suitable foundation.

The pile designs considering the punching shea around the heads of the piles and column base. It also designs for bending moment due to the transmission of loads from columns to the individual piles.

Pile cap Necessity

Pile is necessary when the bearing capacity of soil below the structure is insufficient for a spread footing. It transfers the load to deeper, firmer strata. Piles used where the soil particularly affects by seasonal changes, to transfer the load below the level of such influence.

The load support exceeds the bearing capacity of a single pile, a group of piles uses.

The group capped by a spread footing or a cap to distribute load to all piles in the group. Where there are a large number of closely space piles, provide individual caps. It is more economical to provide just one large cap, forming a piled raft.

SHAPES

The shape and plan dimensions of the pile cap depend on two factors.

1. Number of piles in the group and
2. The spacing between each pile.

The most common shapes are

1. Number of piles in the group

A group of piles accommodates in a pile cap.

Two pile

In two pile cap, two piles are embeds in one pile cap. The picture shows a two and one pile cap PCC arrangments for pile cap.

Three pile

The picture shows a typical three pile arrangement PCC for pile cap.

Three pile cap

The image shows a three pile cap concreting.

The shapes of the pile caps minimize the plan area for symmetrical pile arrangement about the load.

It is overhand the outer piles by at least 150mm and not excessive. And not more than the diameter of the pile diameter.

There are different shapes of pile caps according to the number of piles.

DEPTH 

The overall depth provides sufficient bond length for the pile reinforcement and the column reinforcement.

The depth decides by the following criteria.

1. Punching shear.
2. Pile anchorage.
3. Shrinkage and swelling of the soil.
4. Frost attack.
5. Groundwater table.

The most important thing is a shear capacity of the pile which affects the selection of the pile depth.

SECONDARY REINFORCEMENT:

The secondary reinforcement provides to prevent the piles from splaying outwards from the pile cap.

1. This reinforcement provided at the bottom of the pile cap running around the longitudinal reinforcement. Projecting from the piles into the pile cap.
2. The direction of the secondary reinforcement is changing at the head of each pile.

The amount of secondary reinforcement changing its direction at the head of each pile. And is not less than 20% of the main tensile reinforcement and well bond.