Setting out and Excavation

 SETTING OUT

PROPOSING A HOUSE FOR A LAND

v  Dimensions of the land were taken by using given Land separation drawings.

v  By using Wooden pegs the corners of the lands were marked.

v  Then by using leveling machine reduce levels at 3m grids were taken.

v  Longitudinal section of the land were drawn by using reduce levels of the land.

v  There were 5 types of houses and according to the drops most suitable house type is selected for the land.

 SETTING OUT OF A PROPOSED HOUSE AT GIVEN LAND

To construct a House or any other structure, first setting out process should be carried out. The necessity of the setting out is to identify the locations of structural parts of the buildings. Mainly setting out work must be carried out with higher accuracy and the instruments should be free from defects (errors). Theodolite is the main instrument used in most places where the construction works proceed. At “Digana housing project” site theodolite was main instrument which used to setting out the boundaries of houses. By using theodolite 90⁰ angles marked on the land by using wooden pegs and also when do this task following things considered.

v  Front face of the house

v  Dimensions of the land

v  Dimensions of the house type “ width and length”

v  Costumer requirements

Then by using marked reference line (90⁰ angle pegs) boundary of the houses were marked and this case steel tape and theodolite was used. Steel tape was used to measure the horizontal distance between each pegs. Then by using string and pegs all the parts of the foundation were marked (Pythagoras theorem (3:4:5) used) and then excavation was done.   

EXCAVATION AND FOUNDATION WORK

 EXCAVATION

Excavation is an important process, in a building construction to have certain depth on earth for a foundation of column and wall. The depth of the excavation depends on some factors.

They are;

➢     The bearing strength of soil

➢     The size of building to be built over

➢     the uniform ness of soil

 EXCAVATION FOR FOUNDATION

The selection of the type of excavation depends on various factors like,

v  Nature of the Project

v  Type of Sub Soil

v  Depth of Excavation

v  Types of Available Machinery

After selecting a type of excavation, excavation was done according to the drawings.

Figure3.1: Excavated Trench for Foundation

SOIL TEST FOR EXCAVATED FOUNDATIONS

Before start any construction activity, it is essential to investigate about the existing soil, because the entire load from the structure is transferred to the soil beneath the foundation. From the result of soil test we can decide whether the soil is suitable for the construction or any improvement required for the existing soil.

The soil which we find at the site is a sandy soil and the ground water table is slightly higher than the normal situation. All the houses and building of the project are two storey or single storey. Regarding above factors, Mackintosh Test have been recommended as the soil test (to find allowable baring capacity of soil) of this site and it is preceded by quality unit of the site. The Mackintosh equipment consists of a rod and a hammer. One, two and three feet distances have been marked in the rod to determine penetration of the rod into the ground when proceed the test. The minimum depth which we should penetrate the rod into the ground is depend on the zone of influence.

Figure3.2: Zone of Influence

According to above diagram it is obvious that zone of influence depends on dimensions of the foundation. The following table shows influence zones for different type of foundations of the houses in the project.

Table3.1: Influence zones for different type of foundations

Type of Foundation	Width, B (mm)	Influence Zone 1.5B (mm)	Influence Zone 1.5B (ft)
Single Storey	350	525	1’ 7”
Two Storey	500	750	2’ 5”
Column Footing of Two Storey	750	1125	3’ 8”

According to the table, for a foundation of two storey house which has B value of 500 mm the test should be proceed up to 3 feet depth but for a foundation of single storey house which has B value of 350 mm up to 2 feet of penetration is sufficient.

The test was done on the excavated foundation trench before placing the screed layer. In this test, the bearing capacity of a particular soil was determined and then the results will be compared with the allowable one.

First of all the points which the soil test to be done was decided so that the points where the load bearing walls, columns exist according to the plan. Then blows were applied on the soil by using a steel rod and no of blows were counted to penetrate the rod into the ground up to 1^st feet. Readings were recorded. Like that the no of blows required for the 2^nd feet was counted & recorded. (For single storey house it is sufficient to precede the test up to 2 feet) Then required minimum number of blows was determined from the chart of allowable bearing capacity vs. Mackintosh value (blows/feet).

v  Single storey house     : 75 kN/m²

v  Two storey house        : 200 kN/m²

Finally the value of blows per feet from the chart was compared with the recorded value and decided whether soil is capable to bear the structure. If the test is not satisfied, soil can be improved by following ways and tested again.

v  Compact the ground

v  Remove the existing soil layer and fill with ABC (Aggregate Base Coarse) and Compact.

v  Increasing the width of the foundation

FOUNDATIONS

Foundation is the lowest part of the structure which provides a base for the supper structure properly. This term includes the portion of the structure bellow the ground level as well as the artificial arrangement of concrete blocks, piles and etc. provided to transmit the load of the structure including the dead weight of the structure itself to the soil bellow. In fact, it is a device to transmit the load of the structure to the soil bellow. Foundation is provided for the following four main purposes.

Ø  To distribute the load of the structure over large area  so as to overloading of the             Soil beneath.

Ø  To load sub stratum evenly and thus prevent unequal settlement.

Ø  To provide a level surface for building operations

Ø  To take the structure deep in to the ground and thus increase stability preventing over tuning.

SETTING UP THE SCREED TOP LEVEL

After proceeding soil test, pegs were placed on trenches to mark screed top level. The screed top level of all houses in the project was independent each houses from the land and peg tops were adjusted to this level from TBM by using dummy level and staff. This is very important because the finished floor level of the house depends on the accuracy of screed top level.

Elevation of the TBM                      = z

Initial Reading                                 = x

Level of the Line of Collimation     = x + z

Screed Top Level                             = 99.670 m

The Final Reading                            = (x + z) – 99.670

  RANDOM RUBBLE MASONRY

Rubble is built over the concrete foundation in a specified shape that it will not create tensile force with stones from approval quarry. Rubble masonry strength depends on:

v  The stone should be laid in their well balanced position

v  Stones should be offset in specified interval vertically and horizontally to have homogeneous bond.

v  All stones should be well watered before build.

      Rubble masonry is many types such as Random Rubble Masonry, Coursed Rubble Masonry etc.

In my work site Random Rubble Masonry was done for foundations. In this type of masonry, Rubble or irregular sizes are used to build. The stones are put in irregular way and bonded with mortar 1:6 cement: sand. The foundation make up to 230mm below the DPC level. Then infill blocks were laid on foundation and it   wall thickness. These blocks were filling Grade 15 concrete.

Figure3.4: Infill Block Layer on Foundation

 SETTING OUT FOR SUPERSTRUCTURE

After finishing the foundation work, using the Theodolite two lines with perpendicular to each other (grid lines) was marked on foundation. It was done by using the pegs which were used for the foundation setting out. The setting out of the houses was carried out by using these two grid lines. Then blocks are placed on these two grid lines and after that by using them other walls are marked by putting blocks and also in this case used given drawings. After placing all the blocks, 1st block layer was checked for diagonals in the each room to make sure that corners are at 90⁰.

BOUNDARY WALL

The boundary wall was constructed for a separation of two lands. It was constructed by using random rubble masonry. Boundary walls also constructed as same as retaining wall construction.

  RETAINING WALL

Retaining walls are constructed to stop falling down of soil in embankments. The dimensions of the retaining wall are depending on the height of the embankments. Initially the soil was excavated to designed depth for retaining wall foundation. If the soil was loose, the excavating depth should be increased. Then the gage 250 polythene is laid on top of soil in order to prevent the absorption of water from the concrete to soil. Then 1:3:6(150mm) cement concrete (screed) was placed and compacted. If the soil was loose, reinforcement also provided. In After one day, water was poured on the concrete for curing and random rubblework was started. The rubble used for retaining wall was 150mm to 225mm and it was constructed with 1:6 cement sand mortar. When constructing, some 75mm diameter weep holes also constructed. Then 25mm aggregate filter media was placed in front of the weep holes as shown in figure…. The filter media is to filter the water without soil and prevent soil from entering through the weep holes. After the construction of rubblework, 20mm thick rendering was applied on the retaining wall with 1:3 cement sand mortar. The dimensions of a retaining wall were given below.

Figure3.5: Detail of a Retaining Wall

   BLOCK WORKS

Ø  1:5 cement sand mortar were used.

Ø  Before starting block workers, blocks should be wetted.

Ø  Mortar layer thickness should be 10 mm and not exceeding 15mm.

Ø  First the Connor blocks are placed and plumb the blocks and then with the help of string other blocks are laid on a line.

Ø  After lying the Blocks the vertical joints must be filling with mortar very well to keep the proper bonding between Blocks.

Ø  The number of Blocks that can be laid for day is maximally up to eight. (5”)

Ø  When continuance the Block work there are two tie beams available at the forth layer of the Block work and lintel level.

The tie beam is consisting with two 10 mm diameter for steel bars and G–25 concrete and block work was carrying out as mention above up to the required level.