Lesson Note On Foundation Construction

Foundation Construction

1 Site exploration and preparation

Present economics have imposed a high degree of mechanization on IS 4991 foundation construction methods.  This demands an efficient system of temporary 

roads and good site drainage to maintain a high rate of work in all weathers.  Site preparation must include tracing and clear marking of all underground services, 

power, telephones, gas, water and sewers.  Accidental cutting of electric cables and gas mains can kill people, as well as result in claims for damages if supply is

cut off from e.g. a factory.  Repairing damaged telephone trunk lines can cost thousands of rupees.  Burst water mains will stop potable water supply to the towns

/ villages and can cause flooding and collapse of incomplete excavations with disastrous results.  Collaborate with the Supply Authorities on safety measures 

whenever the site is approached closely or crossed by overhead power cables or underground services.

2 Dilapidation and condition survey of adjacent properties 

Jointly with the owners, occupiers and their professional advisers if appointed, record and agree the condition of their properties before our operations start.  This 

record will be invaluable in the event of any subsequent damage claims and can provide information to protect.  Once fixed it will enable all movements to be

monitored.  This information assists in agreeing the causes of movement.
Record in detail the decorative and structural condition. Photograph all significant points.  Fix tell-tales across cracks and arrange to both parties to record

readings at appropriate intervals during the progress of the works.

Proprietary graduated Perspex tell-tales are available. Corner, floor and displacement types are available.  They can be read to 0.25 mm by eye.  For more 

sensitive reading or less noticeable tell-tales, small discs set in each side of a crack can be monitored using vernier gauges.

Extensive and continuous monitoring can use electronic transducers, recording meters and printers providing automatic records.  This system is useful when 

regular access to locations requiring monitoring is difficult or expensive.

Where necessary establish and agree levelling points and record in a similar manner.

Surveys also include condition of roads, paving etc.  This is particularly important where deep excavation, blasting, or dynamic compaction or piling works are 

involved.

Vibration monitoring, where appropriate can also be undertaken by specialists.

3 Foundation depths 

If the foundations are designed by in-house or outside Structural Consultant, they must be insisted that the drawings shall show the Site Investigation Report

reference and state the allowable net ground bearing pressure used for the design of foundations, the depth at which foundations are to be placed and the soils

expected at this level.  Concrete mix details will take into account any aggressive chemical conditions discovered by the Site Investigation.

No variation must be made to the specified depths of foundations unless first agreed with the Client’s Engineer / Project-in-charge / Architect as appropriate.

In case of doubt, or if ground conditions differ from those expected, notify the Client’s Engineer / Project-in-charge / Architect and obtain his instructions in writing.

If there is any likelihood of adjacent foundations being at all affected, site management must obtain written instructions from the Client’s Engineer / Project-in-

charge / Architect before proceeding with the work.

4 Safety in excavations IS 3764

It is necessary to properly support the sides of all excavations to ensure stability and safety in all weather and ground water conditions.

See Section 4 (Earthworks and excavation) of this manual and the Company Safety Instructions (if available) or refer the matter to experts on Site Safety.

See also the legal requirements relating to confined spaces and toxic atmospheres:  The Construction (General Provisions) Regulations 1961.  Any underground

space is a potential death trap due to possible poisonous, asphyxiating and/or explosive atmospheres.  Before entering such spaces test the air, and assistance

and breathing apparatus must be immediately to hand.

REMEMBER: BY LAW, ALL EXCAVATIONS DEEPER THAN 1.2 m MUST HAVE  SIDE SUPPORT OR EXCAVATED TO A SAFE BATTER.

5 Excavation for foundations and preparation of formations

Any soft spots encountered at formation level are normally replaced with lean mix concrete.  See the specified requirements in the contract.

Clays are highly susceptible to softening when in contact with water.  Subject to the approvals referred to in 4.8, clay formations should be protected with blinding

concrete or with the foundation concrete as soon as possible after completion of the excavation.  Formations in granular soils will usually have been loosened by

the excavation process and should be compacted using suitable vibratory plant prior to placing blinding concrete.

Surfaces of rock are required to be sound, completely exposed, and generally normal to the direction of load and of a capacity required in the design.  Soft rock

surfaces (eg chalk) are swept clean of loose debris then blinded with 50 mm of 10 N/sq. mm concrete to prevent softening by rain.  Hard rock formations are

preferably cleaned with water or air-water jet, followed by an air jet to remove excess water.  Remove any standing pools of water.  Keying of footings into the rock

may be required.  Study the specification and drawings thoroughly.

If blasting is used, this must be carried out by blasting specialists who must control charges to avoid damage to other foundations or finished work.

6 Avoidance of surcharge on adjacent excavations etc.

It is essential that foundations do not surcharge existing drains, trenches, retaining walls and other excavations.  To achieve this, take undersides of new
foundations down so that a line, at 45 degrees from the nearest bottom edge of the new foundation, passes under the drainage, trenches etc.  And conversely to

prevent new excavations from undermining adjacent foundations, excavations must not be taken down below the 45 degrees line.

On similar lines, care shall be taken to avoid any surcharge between the adjoining footings within a new construction, and any such condition in the substructure

shall be discussed and a confirmation on the adequacy obtained in writing from the consultant, Architect and Engineer in charge.

 7 Records of obstructions

Ensure that all obstructions are recorded precisely.  Records must show nature, location, depth and dimensions of obstructions, and be similar to but separate

from the record of foundations.  The Engineer must be promptly told about such obstructions so that any significance these may have on design may be

investigated.

8     Bottom of excavations (formation level)


Bottoms of excavations (formation level) are to be inspected and passed by the Client’s Engineer / Project-in-charge / Architect / Statutory Authority as required by 

the Contract.  Immediately after approval all formation levels other than hard rock are to be protected normally by blinding with concrete for protection against the 

weather and to provide a firm working surface for subsequent steelfixing operations if needed.

9     Foundation in shrinkable clay

The specified formation level will normally be below the zone of cyclical shrinkage and swelling due to weather changes, and taking into account the significant 

effects of trees and hedges.

Site survey drawings normally indicate all past, (if stumps are visible) and existing trees and hedgerows.  Account of these will have been taken in the design of 

foundations.

Trees, stumps and root systems of dead trees can occur outside the site boundary, but close enough to affect new foundations. If old stumps or root systems are 

discovered or if there are any doubts about the possible effects of trees and hedgerows over the site boundary, contact the Client’s Engineer / Project-in-charge / 

Architect  before work proceeds.

For detailed information on heights and limiting distances for trees see BRE Digest No. 298 or any similar publication.

Even small ornamental and fruit trees can make clay movement problems worse when grown close to shallow strip foundations.
Protect formation surfaces in clay from the softening effects of rain and drying shrinkage until inspected, then immediately blind with 50 mm of 10N/sq mm 

concrete, or otherwise cover in accordance with the specified requirements.  Remove any soil inadvertently softened before blinding. 

DELAY IN CONCRETING A STRIP FOUNDATION MAY CAUSE PROBLEMS DUE TO SHRINKAGE OR SWELLING , PARTICULARLY IN CLAY as given below:

(a)    Sun and wind will drive the moisture out and shrink the exposed clay.  Once placed, the foundation may suffer heavy movement later when the clay takes up 

moisture and swells.

(b)   Similarly, a foundation placed on a wet, swollen clay base will settle later as the clay compresses under the wall load.

10 Concreting foundations 

Before placing concrete ensure that formation surfaces are clean and trimmed and that any side formwork is securely strutted with all gaps at joints between

shutter panels packed to prevent grout leakage.  If this occurs and the concrete contains reinforcement, make good the honeycombed concrete before backfilling


to prevent ingress of ground water to the reinforcement.

Struts bearing against the earth sides of excavations must bear on adequate spreader plates or timbers.

The materials, the mix, the water content and the methods of mixing, transporting, placing and curing the concrete are to fully conform with the specification.

Surfaces against which concrete is to be placed must be firm and free from loose material.  If concrete is to be placed upon or against, and or, is required to bound

with, old concrete surfaces, clean the surface of  the previously cast concrete of oil, grease, or other foreign matter and laitance, preferably by wet sand-blasting. 

Ultra high pressure water blasting is also an appropriate method.  Surfaces sometimes require roughening.  Roughening for its own sake is not necessary to obtain

bond if a thoroughly clean surface, comparable in cleanliness to a fresh break, is obtained.  Such a clean joint surface approaching dryness without free water is

best for bond strength.

Avoid damaging the formation if sand or water blasting methods are used.

Give all reinforcement the specified amount of  cover of concrete.  See Engineer’s drawings for details. Use adequate spacer blocks and supports.

When concreting foundations, arrange for competent and continuous supervision of this operations until completion, then check and record the finished concrete

levels.  Comply with the procedures in your Company Quality Systems and Procedures or Standing Instructions or Project Instructions.

Lesson Note On Coordinate Calculations

Coordinate Calculations

1. Co-ordinate calculations

Calculations involving rectangular co-ordinates are:

a)      Calculation of Whole Circle Bearing (WCB) and distance between two co-ordinated points.
i)                    Calculate the difference in easting( E) and difference in northings ( N) between the two points.

ii)                   Determine the Reduced Bearing from:

                                                     E
                  Reduced bearing=  tan ------
                                                      N
Iii) Convert the reduced bearing to a Whole Circle Bearing.

            1st quadrant                  same

            2nd quadrant                 180 – rb

            3rd quadrant                  180 + rb

            4th quadrant                  360 – rb

The correct quadrant can be found by inspection of the relative positions of the co-ordinated points.
iii)Calculate the horizontal distance from:

                       E                         N
Distance =   --------------  and  --------------
                    Sin WCB            Cos WCB

     Using both these formulae separately gives a check on the calculation of the WCB
      A further formulae could be used to determine distance but this does no check WCB calculation.
      Distance =   E +  N

Example calculation

Given: Stn A                 179.724 mE                 414.132 mN
            Stn B                142.171 mE                 372.916 mN
  A         179.724                                   414.132

B          142.171                                   372.916
            ------------                                   -----------
E          37.553     N                              41.216
                                             
Reduced bearing = tan –1  37.553
                                        41.216
                               = 42.2015
                        WCB= 180 + 42 20 15
                                 = 222 20 15
            Distance = 37.553            =  -55.758 m
                             Sin 222 20 15
                             =     41,216  =     -55.758 m
                              cos 222 20 15

The two distances should be the same and of course, the negative sign is ignored.

a)      Calculation of rectangular co-ordinates of a point (B) knowing the co-ordinates of another point (A), the WCB from that point to B (A to B) and the horizontal distance between them.

The procedure is as follows:
i)                    Calculate the difference in eastings and northings using the following formulae:
E = distance x sin WCB
N = distance x cos WCB
ii)                   Add  E and  N to the eastings and northings respectively of the known point.

Example- calculation
Given: Co-ords A               137.629 mE                 473.126 mN
            WCB A to B                      136 27 19
            Distance A to B                  53.249
E = 53.249 x sin 136 27 19
                      = 36.684 m
                  N = 53.249 x cos 136 27 19
                      = 38.597 m
Co-ords A                         137.629                       473.126

Difference                             36.684                    -38.597
                                         ------------                  --------------
Co-ords B                          174.313E                     434.529N
                                         --------------               --------------
2. Traverse calculation

The commonest form of traverse is the ring traverse, which starts and finishes at the same point, and is calculated as follows:

a) The following data is required to compute the traverse.

 

i)    All internal horizontal angles

ii)    All horizontal distances

iii)    The WCB of one leg.

iv)     The rectangular co-ordinates of one point.

i)and (ii) measured in the field

(iii)   Usually an assumed bearing but could be magnetic, grid or true.

(vi)      Usually an assumed value unless the traverse is to be tied to the Ordnance survey horizontal control or similar.

b) Distribute angular misclose equally to each measured angle.

c) Calculate the forward bearing of each line, working anticlockwise around the traverse, using:

Forward bearing = back bearing of previous line + included angle

 Ensure you finish back on the original bearing as a check against error.

 d) Calculate  E and    N for each leg using the forward bearing and measured distance.

e) Add up all  E’s and   N’s to check for large errors in observation or calculation.  The sums should be close to zero.

  f)Starting at the origin station, compute the co-ordinates of each station around the traverse  (working anticlockwise) finishing back at the origin.  Any variation in the co-ordinates of this station is the eastings and northings misclose.

g)Distribute the misclosures using either the Bowditch or Transit methods.

            Bowditch correction = Distance from origin  x E (or N) misclose

                                                     Traverse length

            Transit correction (for eastings) =        E from origin x easting misclose

                                                                                        E
            Note that all   E are treated as positive when calculating corrections.

h) Calculate corrected co-ordinates.

An example traverse calculation is tabulated in Figure 4.10.

3 Co-ordinate setting out

Co-ordinated points can be set out from a horizontal control by either:

a)      Using bearing and distance.

b)      By intersecting theodolite rays

 The following notes detail the general approach.

a)      Bearing and distance method

i)   Two control stations are required.  Calculate the WCB and distance from each station to the point to be set out and the WCB between the two stations.

ii)   Set a theodolite over one control station and sight the other station (RO) with the computed WCB between the two stations.

iii)  Release the theodolite upper plate and rotate the instrument until the computed WCB to the point is obtained.

iv)   Mark the direction with a nail in a peg at approximately he correct distance away.

v)                    Change face and repeat to check for instrument error.

vi)                   Measure the distance to the peg at the trial position applying whatever corrections are deemed necessary (see paragraph-4).  Using a pocket tape, position a new peg at the correct position from the trial position.

vii)                 Check by using a WCB or distance from another station.

a)      Intersection by theodolite

i)                    Calculate WCB and distance as in (a) (i)

ii)                   Two theodolites are required one set up over each control station.

iii)                 Each theodolite is sighted on to the opposite station with the computed bearing on the lower plate.

iv)                 Each theodolite is then turned until the required WCB to the point is found.  Establish a peg on line and change face to check.

v)                  The required point is at the intersection of the two theoidolite lines

vi)                 The set out position must be checked by either a distance measured from a station or a WCB from a third station.

vii)               This method should only be used if the resultant triangle formed by the two control stations and the set out point is well onditioned.

(b)    Other checks

Once all the points are set out from the control, it is essential that they are checked relative to each other.  Some slight adjustment to peg position may be required. 

Lesson Note On Theodolite: Part B

Theodolite:
Axes of theodolite:
The most important relationships are as follows:
1.The axis of plate bubble should be in a plane perpendicular to the vertical axis.(main axis order).
2.The line of sight should be perpendicular to the horizontal axis.
3.The horizontal axis should be perpendicular to the vertical axis.

Instrument setup:
At each station point, before taking any observation, it is required to carryout some operations in sequence. The set of operations those are required to be done on an instrument in order to make it ready for taking observation.

1. Setting
2. Centering
3. Leveling
4. Focusing

Instrument setup:

1.Tripod height –upper about chest height to make observation

easily.Place the instrument over the point with the tripod plate as

level aspossible. Then place the theodolite on the top of tripod.

Theodolite must be hold by hand until the theodolite is attached to

tripod head.

2.Check that the station point can be seen

through the optical plummet. (Rotate the

focus reticle –pull in or out to focus

on the ground-monument)

Then push in the tripod legs firmly by pressing down on the tripod shoe spurs. If the point is now not visible in the optical plumb sight, leave one leg in the ground, lift the other two legs, and rotate the instrument, all the while looking through the optical plumb sight. When the point is sighted,carefully lower the two legs to the ground and reseat them keeping the station point view.

While looking through the optical plumb, manipulate the leveling screws until the crosshair of the optical plummet is directly on the station mark.

3.Level the theodolite circular (pond) bubble by adjusting the tripod legs up or down (approximate leveling). This is accomplished by noting which leg, when slid up or down, moves the circular bubble to ward the bull’s eye. Upon adjusting the leg, either the bubble will move into the circle, or it will slide around until it is exactly opposite another tripod leg. That leg should then be adjusted up or down until the bubble moves into the circle. If the bubble does not move into the circle, repeat the process. If this manipulation has been done correctly, the bubble will be centered after the second leg has been adjusted;

Performa check through the optical plummet to confirm that it is still close to being over the station mark/turn one or more leveling screws to been sure that circular bubble is now exactly centered (if necessary).

The instrument can be now be leveled precisely by centering the plate (tubular) bubble.

a) Set the plate bubble so that it is aligned in the same direction as two of the foots crews. Turns these two screws in opposite direction until the bubble is centered.

b)Turn the instrument 100g, at which plate bubble will be aligned with the third leveling (foot) screw. Finally turn that third screw to center bubble.

Finally,check the axis of plate bubble should be in a plane perpendicular to the vertical axis. main axis order). It is always checked by turning the instrument through 200g. If the plate bubble is centered, the instrument is leveled.

Lesson Note On Theodolite Part A

Theodolite:
A theodolite is an instrument which is used primarily to measure angles, both horizontal and vertical. It is also used for many other subsidiary work during surveying such as setting up of intermediate points between intervisible points, establishment of intervisible points,prolonging a line, laying out traverse etc.
Types of theodolites:
Classification with respect to a construction:

• Open-faced,vernier-equipped engineer’s transit
• Optical theodolites with direct digital read-out sormicrometer-equipped read-outs(formoreprecisereadings)
• Electronic theodolites

Classification with respect to accuracy:

1. One-minute theodolites:the least division of the scale is 1 or 2 minutes
2. One-second theodolites: the least division of the scale is 1 or 2 seconds

Components of theodolite:

The vertical axis of these instruments goes up through the center of the spindles and is oriented over a specific point on the earth surface. The circle assembly and alidade rotate about this axis.

Horizontal axis of the telescope is perpendicular to the vertical axis, and telescope and vertical circle tiltonit.

The line of sight (line of collimation) is a line joining the intersection of the reticle crosshairs and the center of the objective lens.The line of sight is perpendicular to the horizontal axis and should be exactly horizontal when the telescope level bubble is centered and when the vertical circle is set at 100g–300g or 0g for vernier transits.

Plate bubble axis is assumed to be tangent to the plate bubble.

Axes of theodolite:

SS:Vertical(standing)axis

TT: Horizontal (Trunnion)axis

PP: Plate bubble axis

CC:Collimation axis (line of sight)

LESSON NOTE ON SETTING OUT WORKS - FOUNDATION MARKING

SETTING OUT WORKS - FOUNDATION MARKING

AIM

To set out the foundation marking for the proposed construction of building.

APPARATUS USED

1.Theodolite, 2. Chain (or) Tape 3.Ranging rods, 3. Pegs or Arrows, 4. String.

GENERAL

The operation of the marking on the site the centr e lines of the foundation of a building is called

setting out. Setting out of a foundation is the first step in the construction of any structure.

PROCEDURE

1.

A centre line sketch of the building is prepared. (The centres of cross walls are also to be indicated.)

2.

The base line is set out with reference to given reference points.

3.

The ends of the centre line of the walls, points A and B from the base line are marked.

4.

As the end marks A,B,C,etc. are disturbed during excavation, stakes are fixed at L,M,N etc., a little

away (about 2 to 3 m) for end mark and tied accur ately using a string.

5.

The centre line for all other walls AD,BC,etc  are marked by dropping perpendicular. For small

buildings the perpendiculars may be set out by using a chain or a tape by ‘3-4-5’ method. For an

important and big building when sides are long a Theodolite may be employed to accurately set out the

perpendiculars and to range the lines.

6.

For every wall, the pegs are driven a little away for marking the end and tied accurately using a string.

7.

Diagonals ar e measured and checked with their corresponding calculated lengths.

8.

Width of foundation from the centerline are marked and the corners 1,2,3,4,5 etc., are fix up. Pegs are

driven at these corners. The cord is stretched and lime is spread along the chords.

RESULT:

Thus the trench plan being marked on the ground, and excavation may be started.

What Is Visual Survey?

What Is Visual Survey?
This is also called reconnaissance survey.
⇒It is the preliminary inspection of an area to be surveyed.
⇒ It is a see-for-yourself walk-over of the ground to be used for a fish pond or a
fish farm. It is first done with a view to visualise the work to be done.
⇒ It is the venture taken to note and identify all the parameters to be measured or surveyed.
⇒ It is a rough sketch of the field or fields in which all positions and stations are made in the field book.
⇒It is preliminary work done whereby the routes of the main chain lines are noted.
What do you do during visual survey?
(i) The purposes of the survey should be noted.
This includes (a) is it for pond construction?
(b) Is it for damming? (c) Is it for irrigation purposes; (d) Is it for Hydro-electric
power (HEP)?

The purpose will determine the extent of the reconnaissance survey.
(ii) The water parameters to be measured should be noted as from the
beginning. Such parameters include : (a) Water level; (b) Geological attributes;
(c) Soil conditions (texture, structure and permeability); (d) Water pH, hardness,
alkalinity, chloride, phosphate, ammonia, sulphide, sulphite, dissolved oxygen
etc