Deep excavations

Presented by :Bhargava Ram P B130899CE

Introduction

Earth retaining walls

Supporting Elements

Deep Excavation Techniques

Design steps for a Deep Excavations

TABLE OF CONTENTS

INTRODUCTION• Number of deep excavation pits

in city centers are increasing every year.

• Buildings, streets surrounding excavation locations and design of very deep basements make excavations formidable projects.

• It forms an important part in the

modern construction field

Excavation :An excavation means a man made cavity or depression in the earth’s surface formed by earth removal.

Deep Excavation :An excavation in soil or rock more than 15ft or 4.5m is called deep excavation.

Open Excavation :It means an excavation without any retaining system by providing suitable slope of excavation such as 1:2, when surrounding allows.

Earth Retaining walls :

• Retaining walls are structures designed to restrain soil to unnatural slopes.

• They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely

• The criteria for the selection of type of wall are size of excavation, ground conditions, groundwater level, vertical and horizontal displacements of adjacent ground and limitations of various structures, availability of construction, cost, speed of work and others.

Types of Earth Retained Walls :1. Braced walls, soldier pile and lagging walls 2. Sheet-piling or sheet pile walls 3. Pile walls (contiguous, secant) 4. Diaphragm walls or slurry trench walls 5. Prefabricated diaphragm walls 6. Reinforced concrete (cast-in-situ or prefabricated) retaining walls

7. Soil nail walls 8. Cofferdams 9. Caissons 10. Jet-grout and deep mixed walls 11. Top-down construction 12. Partial excavation or island method

EARTH PRESSURES ON IN-SITU RETAINING WALLS

• Earth pressures on in-situ retaining walls are rather different than those on ordinary retaining walls due to the supporting elements.

• Free displacement of walls are not allowed. • Type of support affects the distribution of earth pressure. • Earth pressure diagrams on strutted and anchored walls

are expected to be somewhat different due to stiffer support conditions in the former.

• Theoretical approaches will also be discussed.

Based on load measurements on struts, the recommended pressure distribution for cohesionless soils of uniform pressure is given by

p = 0.65 KA γt H

KA - Ative earth pressure coefficientH – Height of the wallΓt – Unit weight

Supporting Elements :Ground anchors :Ground anchor is a common type of supporting element used in the design and construction of in-situ retaining walls. It is an installation that is capable of transmitting an applied tensile load to a load bearing stratum which may be a soil or rock.

Section viewFront view

Struts :Strut is a compression member to provide temporary support to in-situ retaining walls in deep excavations. It is mostly a steel beam of various sections or a pipe. Reinforced concrete beams are seldom used. Struts usually span the width of excavation and in cases of very wide excavations vertical support is needed. It is an old support system compared to ground anchor which has been used since mid Seventies.

Some type of Structs

Retaining systems for deep excavationsFollowings are the common systems for supporting deep excavation :

CONTIGUOUS PILES :• In this technique the reinforced concrete unconnected piles are

installed in the periphery of the proposed site. • The contiguous piles wall is designed in such a way that it resists the

lateral earth pressure of retained soil. • The width of gap between piles varies between 50 and 500mm

according to ground conditions.• The dia. of these piles varies from 300 to 1200mm normally. • The soil between two piles is stabilized by shotcreting.

Contiguous bored piles supported excavation :

•Typical depth of excavation supported by this system is 5 to 10m without tie back anchors

SECANT PILES :In this technique primary piles (PCC) are installed first with secondary piles (RCC) constructed in b/w primary piles when it gained some strength as shown.

SECANT PILES :

• Pile overlap is typically in order of 3in.• The principal disadvantages of contiguous pile walls,

the gaps between piles and the resulting problems of lack of waterproofness are effectively over come by interlocking or secant piles.

Sheet PILES :Sheet pile walls are constructed by driving prefabricated sections into the ground by vibration in Sandy strata and by pressure in cohesive strata.Following are the typical steps involved :

1.Position of Sheet Pile :A guide Frame is placed on the ground to set out the position of the sheet pile wall.

Diaphram Wall : The continuous diaphragm wall (also referred to as slurry wall) is a structure cast in a slurry trench by tremied concrete. The trench is initially supported by bentonite polymer based slurries. The construction sequence is illustrated in Fig.

(A) Trenching under slurry, (B) End stop inserted (steel tube or

other), (C) Reinforcement cage lowered

into the slurry-filled trench, (D) Concreting by tremie pipes.

VERTICAL SOLDIERS AND HORIZONTAL LAGGING KING POST METHOD :

• The method consists of boring holes on the wall line, typically at 2 to 3m centers, placing vertical steel universal beam or column soldier piles within the holes and concreting the base of each joist below final formation level.

• Lean mix concrete is often used for this purpose. • In suitable soils the steel soldier pile may be driven. When the

soldier pile is to be extracted the base of each joist is supported by gravel backfilled to formation level.

VERTICAL SOLDIERS AND HORIZONTAL LAGGING KING POST METHOD :

• As Bulk excavation proceeds, horizontal lagging timbers or precast concrete units are wedged between the soldier piles.

• Steel section walings are placed to take the thrust from the soldier piles to ground anchors drilled at intervals along the length of the waling beam. Alternatively, each soldier pile is anchored and a waling is not needed

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