Showing posts with label Geotechnical engineering. Show all posts
Showing posts with label Geotechnical engineering. Show all posts

Wednesday, 15 June 2016

Types of Stairs


The stairs may be built with wood, concrete masonry or with cast iron. Wooden stairs are not safe, because of the danger of fire. However they are used in buildings to access to small areas in the upper floors.

 Cast iron or steel stairs in the spiral forms were used commonly to reduce stair case area. In many residential buildings masonry stairs are also used. Reinforced concrete stairs are very commonly used in all types of buildings.

PLASTERING


Applying mortar on the surfaces of walls, columns, ceiling etc. to get smooth finish is termed as plastering.

Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar.

Lime mortar used lime to sand ratio of 1 : 3 or 1 : 4.

Cement mortar of 1 : 4 or 1 : 6 mix is very commonly used for plastering, richer mix being used for outer walls.

CELLULAR CONCRETE


It is a light weight concrete produced by introducing large voids in the concrete or mortar. Its density varies from 3 kN/m3 to 8 kN/m3. It is also known as aerated, foamed concrete.

Properties of cellular concrete
1. It has low weight.

2. It has good fire resistance.

Compaction factor test.


This is another test to  identify the workability of concrete. This test is conducted in the laboratory.

The test equipment consists of two hoppers and a cylinder fixed to a stand, the dimensions and the distances between the three vessels being standardized. Vessel A and B are having hinged bottoms whereas cylinder C is having fixed bottom.

What is uses of timber?


1. For heavy construction works like columns, trusses, piles.

2. For light construction works like doors, windows, flooring and roofing.

3. For other permanent works like for railway sleepers, fencing poles, electric poles and gates.

4. For temporary works in construction like scaffolding, centering, shoring and strutting, packing of materials.

Uses of Stones.


(i) Stone masonry is used for the construction of foundations, walls, columns and arches.

(ii) Stones are used for flooring.

(iii) Stone slabs are used as damp proof courses, lintels and even as roofing materials.

What are the Properties of Stones?


(i) Structure

(ii) Texture

(iii) Density

(iv) Appearance

Tuesday, 7 June 2016

DETERMINATION OF MOISTURE CONTENT


OBJECTIVE 

Determine the natural content of the given soil sample.  

NEED AND SCOPE OF THE EXPERIMENT

In almost all soil tests natural moisture content of the soil is to be determined. The knowledge of the natural moisture content is essential in all studies of soil mechanics. To sight a few, natural moisture content is used in determining the bearing capacity and settlement. The natural moisture content will give an idea of the state of soil in the field. 

DEFINITION

The natural water content also called the natural moisture content is the ratio of the weight of water to the weight of the solids in a given mass of soil. This ratio is usually expressed as percentage.

APPARATUS REQUIRED

    1. Non-corrodible air-tight container.       2. Electric oven, maintain the temperature between 1050 C to 1100 C.      3. Desiccator.      4. Balance of sufficient sensitivity. 

PROCEDURE

1. Clean the container with lid dry it and weigh it (W1).  2. Take a specimen of the sample in the container and weigh with lid (W2).  3. Keep the container in the oven with lid removed. Dry the specimen to constant weight maintaining the temperature between 1050 C to 1100 C for a period varying with the type of soil but usually 16 to 24 hours.  4. Record the final constant weight (W3) of the container with dried soil sample. Peat and other organic soils are to be dried at lower temperature (say 600 ) possibly for a longer period. 

  Certain soils contain gypsum which on heating loses its water if crystallization. If itb is suspected that gypsum is present in the soil sample used for moisture content determination it shall be dried at not more than 800 C and possibly for a longer time.

OBSERVATIONS AND RECORDING

Data and observation sheet for water content determination  

S.No. Sample No. 1 2 3

1 Weight of container with lid W1 gm

     

2 Weight of container with lid +wet soil W2 gm

     

3 Weight of container with lid +dry soil W3 gm

     

4 Water/Moisture content

W = [(W2−W3)/(W3−W1)]100

     

   

  RESULT

The natural moisture content of the soil sample is ________ 

 REMARKS

1. A container with out lid can be used, when moist sample is weighed immediately after placing the container and oven dried sample is weighed immediately after cooling in desiccator.

2. As dry soil absorbs moisture from wet soil, dried samples should be removed before placing wet samples in the oven.

Monday, 6 June 2016

Self compacting concrete.


A concrete which is capable to compact itself by its own self-weight under gravity without any external efforts like vibration is called as self compacting concrete. the mix is required to have ability of passing, filling and being stable.

Following ingredient are used to prepare self compacting concrete.

1. cement :- OPC 43 or 53 grade.

2. Aggregates:- well grade rounded or cubical aggregate of size 10 to 20mm uniformly graded fine aggregates.

3. Good quality of mixing water.

4. Super plasticizer like poly-carboxylated ether to improve the workability and viscosity modifying agent.

5. Mineral admixture like fly ash, ground granulated blast furnace slag, silica flume, fiber, finely crushed lime stone, dolomite or granite

6. Under self weight , scc should level and deform itself without any compaction and external vibration. There should be not be any entrapped air in concrete.

7. SCC should be fully flowable but without segregation and bleeding. This is achieved by keeping higher viscosity of cement and mortar to ensure flowability while maintaning no sedimentation of bigger aggregates.

Slump Test


Slump test is the most commonly used method of measuring workability of concrete. the apparatus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as follows:-

Bottom diameter   20cms

Top diameter          10cms

Height                      30cms

The mould is the place on a smooth, horizontal, rigid and non absorbent surface. the mould is then filled in four layer each approximately 1\4 of the mould. each layer is tamped 25 times by the tamping rod taking care to distribute the stroke evenly over the cross sections. After the top layer has been rodded, top is struck off level with a trowel and tamping rod. the mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction. this allows the concrete to subsidence. this subsidence is referred as slump of concrete. the difference in level between the height of the mould and that of the height point of the subsided concrete is measured. this difference in height in mm is taken as slump of concrete.

slump requirement for beams and slabs 50mm to 100mm

for walls and columns 75mm to 100mm

Vibrated concrete 15mm to 25mm  

Tuesday, 31 May 2016

Standard Penetration Test (SPT)


One of the most common in-situ tests is the standard penetration test or SPT. This test which was originally developed in the late 1920s.

SPT is most commonly used in situ test, especially for cohesionless soils which cannot be easily samples. the test is extremely useful for determining the relative density and the angle of shearing resistance of  for cohesionless soils.

it can also used to determine the unconfined compressive strength of cohesive soils.

The standard penetration test is conducted in a borehole using a standard split-spoon sampler. 

When the borehole (55 to 150 mm in dia) has been drilled to the desired depth, the drilling tools are removed and the split-spoon sampler, attached to standard drill rods of required length is lowered to the bottom of the borehole and rested at the bottom.

The split-spoon sampler is then driven into the soil for a distance of 450 mm in three stages of 150 mm each by blows of a drop hammer of 63.5 kg mass falling vertically and freely through a height of 750 mm at the rate of 30 blows per minute (IS 2131 – 1981). The number of blows required to penetrate every 150-mm is recorded while driving the sampler. If full penetration is obtained, the blows for the first 150 mm is retained for reference purposes, but not used to compute the SPT value because the bottom of the boring is likely to be disturbed by the drilling process and may be covered with loose soil that may fall from the sides of the boring. The number of blows required for the next 300 mm of penetration is recorded as the SPT value. The number of blows is designated as the “Standard Penetration Value” or “Number” N.

The slit-spoon sampler is then withdrawn and is detached from the drill rods. The split barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. 

Precautions

The drill rods should be of standard specification and should not be in bent condition

The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear

 The drop hammer must be of right weight and the fall should be free, frictionless and vertical.

 The height of fall must be exactly 750 mm. Any change in this will seriously affect the N value.

 

The standard penetration number is corrected for dilatancy correction and overburdon correction.

Undisturbed soil samples


Undisturbed soil samples are those in which the in-situ soil structure and moisture content are preserved.

• They are representative and also intact

• These are used for consolidation, permeability or shear strengths test (Engineering properties)

• More complex jobs or where clay exist

• In sand is very difficult to obtain undisturbed sample

• Obtained by using Shelby tube (thin wall), piston sampler, etc., 

Disturbed soil samples


Disturbed soil samples are those in which the in-situ soil structure and moisture content are lost, but the soil particles are intact.

• They are representative 

• They can be used for grain size analysis, liquid and plastic limit, specific gravity, compaction tests, moisture content, organic content determination and soil classification test performed in the lab

 • e.g., obtained through cuttings while auguring, etc.

Depth of exploration


The depth of the exploration required at a particular site depend on the degree of variation of the sub-surface data in the horizontal and vertical directions. It is not possible to fix the number, disposition and depth of borings without making a few preliminary borings or sounding at the site.

Generally exploration should be carried out to a depth upto which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure. such a depth known as the significant depth, depent upon the type of the structure, its weight.  It is generally safe to assume the significant depth upto a level at which the net increase in vertical pressure become less than 10% of the initial overburden pressure.

Stages of the sub-surface exploration


1. Reconnaissance

site Reconnaissance is the 1st step in the exploration. it include a visit to the site and to study the maps and other relevant records. it helps in deciding the future programme of the site investigation. types of sample adopted to be taken and the laboratory testing and in-situ testing.

2. Preliminary or general exploration

the aim of the general exploration is to get an approximate picture of the sub-soil condition at the relatively low cost. the information so obtained should suffice for the design and execution of minor and routing engineering works.

the preliminary exploration are generally in the form of few borings or test pits. test are conducted inthe form of the borings or test pits. test are conducted with cone penetrometers and sounding rods to obtain information about strength and compressibility of the soil

3. Detailed exploration

it is a supplement to general exploration when large engineering works, heavy loads and complex costly foundation are involved, such as bridge, dam, and multistory building. however for small projects especially at sites where the strata are uniform, detailed exploration may not be required.

the purpose of detailed exploration is to determine the engineering properties of the soils in different strata. it include the  extensive boring programme, sampling and testing of the samples in the a laboratory.

field test, vane shear test, plate load test and permeability test are conducted to determine the properties of the soil in natural state.

the test for the determination of the dynamic properties are also carried out.

 

what is a Scope of soil investigation ?


An investigation of the site is essential for judging its suitability for the purposed engineering works and for preparing adequate and economic design. it consists of determining the profile of natural soil deposits at the site taking the soil samples and determining the engineering properties of the soils. It also include the in-situ testing of the soil.

1. to select the type and depth of the foundation for a given structure.

2. to determine the bearing capacity of the soil.

3. to estimate the probable maximum and differential settlement.

4. to predict lateral earth pressure against retaining walls and abutments.

5. to select the construction techniques.

6. to select the safety of the existing structure and to suggest the remedial measures.

Objectives of soil exploration


soil exploration is very useful for civil engineer to execute the design of construction, planning of construction, and for following purposes.

1. To know the geological condition of rock and soil formation.

2. To establish the groundwater levels and determine the properties of water.

3. To select the type and depth of foundation for proposed structure

4. To determine the bearing capacity of the site.

5. To estimate the probable maximum and differential settlements.

6. To predict the lateral earth pressure against retaining walls and abutments.

7. To select suitable construction techniques

8. To predict and to solve potential foundation problems

9. To ascertain the suitability of the soil as a construction material.

10. To determine soil properties required for design

11. Establish procedures for soil improvement to suit design purpose

12. To investigate the safety of existing structures and to suggest the remedial measures.

13. To observe the soil the soil performance after construction.

14. To locate suitable transportation routes.

Sunday, 24 January 2016

Different types of the soil samples


The soil samples can be of two types:-

1) A disturbed sample

2) Undisturbed sample

 

1) A disturbed sample is that in which the natural structure of soil get partly or fully modified and destroyed although with suitable precautions the natural water content may be get preserved.

such a sample however be representative of the natural soil by maintaining the original proportion of various soil particle impact.

it can be used to determine the  index properties of the soil. Such as grain size, plasticity characteristics specific gravity.

however it is impossible to get truly undisturbed sample. some disturbance is inevitable during sampling even when almost care is taken. even the removal of the sample from the ground produces a change in the stresses and cause disturbance. it is used for determining the compressibility shear strength permeability shrinkage limit etc. the smaller disturbance the greater would be the reliability of the result.

 

2) An undisturbed sample is that in which the natural structure and properties remain preserved.

Negative skin friction

Negative skin friction is a downward drag acting on the pile due to downward movement of the surrounding compressible soil relative to the pile.

This happens when the surrounding, compressible soil has been recently filled of formed.

 AS the soil consolidates the earth filed moves downward developing the friction forces on the perimeter of the pile which tends to carry the pile further into the ground.

The negative skin friction may also be developed by lowering of the ground water the increase in effective stress causing consolidation of the soil, with the resulting settlement and friction force being developed on the pile.