Pile Foundations!!

Pile Foundations

 

What is Pile?

Piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. Another reason for choosing a pile foundation is the condition and quality of soil layers. Piles are structural members made of timber, steel, or concrete. Based on how they transfer the load into the subsoil, piles can be categorized as friction piles, and end-bearing piles. In friction pile, the load transfer is done through shear stress generated along the interface of pile and soil. In end-bearing pile, the load is transferred through the tip of the pile to a firm stratum. Drilled shaft, as its name implies, are drilled into the subsoil, and then filled with concrete. Generally, drilled shafts have larger cross sectional area (Barja M. Das, 2008)

Using Piles ? Why and When ?

1. Poor quality of upper soil layers
2. When we have expansive soil in construction site
3. To resist uplift forces
4. To resist lateral loads (horizontal)
5. Bridge abutment and piers

Concrete Piles

Concrete piles are of course, made of concrete! They can be either pre-cast pile, or cast in-situ. Concrete piles are generally reinforced. For pre-case piles, the reinforcement brings extra strength to resist bending moment during pile pick up, transportation, vertical loads, and bending moment as a result of lateral loads. They can be built in different sizes and shapes, as required for each specific use. Pre-cast piles can be pre-stressed as well.

Cast in-situ piles are made by drilling a hole into the soil, and then filling out with concrete.  Cast in-situ piles can be divided into two main categories: cased, or uncased. Cased concrete piles are made by driving a steel casing into the soil. In this case, the mandrel is placed inside the casing. After reaching desired depth, the mandrel is withdrawn, and the casing is filled with concrete. In the case of the uncased piles, the casing will be gradually withdrawn.

How Tall is a "Tall" Building??

What is a tall building? How is a building's height measured? What do terms like "supertall" and "mixed-use" actually mean? These and other questions are answered in the CTBUH Height Criteria, the official rules whereby tall buildings are defined and heights are measured.
What is a Tall Building?
There is no absolute definition of what constitutes a “tall building.” It is a building that exhibits some element of “tallness” in one or more of the following categories:
a) Height Relative to Context:                 It is not just about height, but about the context in which it exists. Thus, whereas a 14-story building may not be considered a tall building in a high-rise city such as Chicago or Hong Kong, in a provincial European city or a suburb this may be distinctly taller than the urban norm.
b) Proportion Again, a tall building is not just about height but also about proportion. There are numerous buildings that are not particularly high, but are slender enough to give the appearance of a tall building, especially against low urban backgrounds. Conversely, there are numerous big/large footprint buildings that are quite tall but their size/floor area rules them out as being classed as a tall building.
c) Tall Building Technologies:         If a building contains technologies which may be attributed as being a product of “tall” (e.g., specific vertical transport technologies, structural wind bracing as a product of height, etc.), then this building can be classed as a tall building. Although number of floors is a poor indicator of defining a tall building due to the changing floor to floor height between differing buildings and functions (e.g., office versus residential usage), a building of perhaps 14 or more stories – or more than 50 meters (165 feet) in height – could perhaps be used as a threshold for considering it a “tall building.”
What are Supertall and Megatall Buildings? The CTBUH defines “supertall” as a building over 300 meters (984 feet) in height, and a “megatall” as a building over 600 meters (1,968 feet) in height. As of June 2015 there were 91 supertall and 2 megatall buildings fully completed and occupied globally.
How is the Height of a Tall Building Measured?
The Council on Tall Buildings and Urban Habitat (CTBUH) recognizes tall building height in three categories:
1. Height to Architectural Top: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the architectural top of the building, including spires, but not including antennae, signage, flag poles or other functional-technical equipment.5 This measurement is the most widely utilized and is employed to define the CTBUH rankings of the "World's Tallest Buildings."
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2. Highest Occupied Floor: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the finished floor level of the highest occupied6 floor within the building.
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3. Height to Tip: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the highest point of the building, irrespective of material or function of the highest element (i.e., including antennae, flagpoles, signage, and other functional-technical equipment).
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Footnotes: 1 Level:  finished floor level at threshold of the lowest entrance door. 2 Significant:  the entrance should be predominantly above existing or pre-existing grade and permit access to one or more primary uses in the building via elevators, as opposed to ground-floor retail or other uses that solely relate/connect to the immediately adjacent external environment. Thus, entrances via below-grade sunken plazas or similar are not generally recognized. Also note that access to car park and/or ancillary/support areas are not considered significant entrances. 3 Open-air: the entrance must be located directly off an external space at that level that is open to air. 4 Pedestrian: refers to common building users or occupants and is intended to exclude service, ancillary, or similar areas. 5 Functional-technical equipment: this is intended to recognize that functional-technical equipment is subject to removal/addition/change as per prevalent technologies, as is often seen in tall buildings (e.g., antennae, signage, wind turbines, etc. are periodically added, shortened, lengthened, removed and/or replaced). 6 Highest occupied floor: this is intended to recognize conditioned space which is designed to be safely and legally occupied by residents, workers or other building users on a consistent basis. It does not include service or mechanical areas which experience occasional maintenance access, etc.
Number of Floors The number of floors should include the ground-floor level and be the number of main floors above ground, including any significant mezzanine floors and major mechanical plant floors. Mechanical mezzanines should not be included if they have a significantly smaller floor area than the major floors below. Similarly, mechanical penthouses or plant rooms protruding above the general roof area should not be counted. Note: CTBUH floor counts may differ from published accounts, as it is common in some regions of the world for certain floor levels not to be included (e.g., the level 4, 14, 24, etc. in Hong Kong).
Criteria for Co-joined Buildings A building complex is considered to be a single, co-joined building (as opposed to two separate buildings connected by skybridges or other elements) when 50 percent or more of the total building height is connected. Exceptions to this 50 percent rule can be made in cases where the form of the building creates a coherent arch, creating a singular architectural expression and thus a co-joined building – to be judged by the CTBUH Height Committee.
Building Usage Building vs. Telecommunications Tower: A tall “building” can be classed as such (as opposed to a telecommunications/observation tower) and is eligible for the "Tallest” lists if at least 50 percent of its height is occupied by usable floor area.		To enlarge, click here
Single-function and mixed-use buildings
A single-function tall building is defined as one where 85 percent or more of its total floor area is dedicated to a single use. A mixed-use tall building contains two or more functions (or uses), where each of the functions occupies a significant proportion7 of the tower’s total space. Support areas, such as car parks and mechanical plant space, do not constitute mixed-use functions. Functions are denoted on CTBUH “Tallest” lists in descending order (e.g., “hotel/office” indicates hotel function above office function).		To enlarge, click here
Footnote: 7This “significant proportion” can be judged as 15 percent or greater of either: (1) the total floor area, or (2) the total building height, in terms of number of floors occupied for the function. However, care should be taken in the case of supertall towers. For example a 20-story hotel function as part of a 150-story tower does not comply with the 15 percent rule, though this would clearly constitute mixed-use.
Building Status
Complete (Completion) A building is considered to be "Complete" (and officially added to the CTBUH Tallest Buildings lists) if it fulfills all of the following three criteria: 1) Topped out structurally and architecturally8 2) Fully-clad9 3) Open for business, or at least partially occupiable  Footnotes: 8The topping out architecturally of a building implies that ALL structural and finished architectural elements are in place. 9The omission of a small number of cladding panels to allow fixing of a construction hoist while interior fit-out of some building areas is continuing does not affect the status of “fully clad.” Under Construction (Start of Construction) A building is considered to be "Under Construction" once site clearing has been completed and foundation/piling work has begun. Structurally Topped Out A building is considered to be "Structurally Topped Out" when it is under construction, and the highest primary structural element is in place. Architecturally Topped Out A building is considered to be "Architecturally Topped Out" when it is under construction, and has reached its full height both structurally and architecturally (e.g., including its spires, parapets, etc.). On Hold A building is considered to be "On Hold" when construction works had begun, but work on-site has been halted  indefinitely, however there is still an intent to complete the construction to the original design at a future date. Never Completed A building is considered to be “Never Completed” when construction works had begun, but work on-site was halted and never resumed. The site may go on to accommodate a new building, different to the original design, that may or may not retain the original construction. Proposed A building is considered to be "Proposed" (i.e., a real proposal) when it fulfills all of the following criteria: 1) Has a specific site with ownership interests within the building development team 2) Has a full professional design team progressing the design beyond the conceptual stage 3) Has obtained, or is in the process of obtaining, formal planning consent/legal permission for construction 4) Has a full intention to progress the building to construction and completion Only buildings that have been announced publicly by the client and fulfill all the above criteria are included in the CTBUH "proposed" building listings. The source of the announcement must also be credible. Due to the changing nature of early stage designs and client information restrictions, some height data for proposals may be unconfirmed. Vision A building is considered to be a "Vision" when it either: 1) Is in the early stages of inception and does not yet fulfill the criteria under the “proposal” category, or 2) Was a proposal that never advanced to the construction stages, or 3) Was a theoretical proposition Demolished A building is considered to be "Demolished" after it has been destroyed by controlled end-of-life demolition, fire, natural catastrophe, war, terrorist attack, or through other means intended or unintended.
Structural Material
A steel tall building is defined as one where the main vertical and lateral structural elements and floor systems are constructed from steel. A concrete tall building is defined as one where the main vertical and lateral structural elements and floor systems are constructed from concrete. A composite tall building utilizes a combination of both steel and concrete acting compositely in the main structural elements, thus including a steel building with a concrete core. A mixed-structure tall building is any building that utilizes distinct steel and concrete systems above or below each other. There are two main types of mixed structural systems: a steel/concrete tall building indicates a steel structural system located above a concrete structural system, with the opposite true of a concrete/steel building. Additional Notes: 1) If a tall building is of steel construction with a floor system of concrete planks on steel beams, it is considered a steel tall building. 2) If a tall building is of steel construction with a floor system of a concrete slab on steel beams, it is considered a steel tall building. 3) If a tall building has steel columns plus a floor system of concrete beams, it is considered a composite tall building. This post originally appeared on CTBUH.

Earth Below the Foot : Calculation of bearing capacity of soil.

What is bearing capacity of Soil?

The bearing capacity of soil is defined as the capacity of the soil to bear the loads coming from the foundation. The pressure which the soil can easily withstand against load is called allowable bearing pressure.

Following are some types of bearing capacity of soil:

Ultimate bearing capacity of soil (q_u)

The gross pressure at the base of the foundation at which soil fails is called ultimate bearing capacity.

Net ultimate bearing capacity (q_nu)

By neglecting the overburden pressure from ultimate bearing capacity we will get net ultimate bearing capacity.

qnu = qu - γ Df

Where
qu= unit weight of soil
D_f = depth of foundation

Net safe bearing capacity of soil (q_ns)

By considering only shear failure, net ultimate bearing capacity is divided by certain factor of safety will give the net safe bearing capacity.

q_ns = q_nu/ F

Where F = factor of safety = 3 (usual value)

Gross safe bearing capacity (q_s)

When ultimate bearing capacity is divided by factor of safety it will give gross safe bearing capacity.

q_s = q_u/F

Net safe settlement pressure (q_np)

The pressure with which the soil can carry without exceeding the allowable settlement is called net safe settlement pressure.

Net allowable bearing pressure (q_na)

This is the pressure we can used for the design of foundations. This is equal to net safe bearing pressure if q_np > q_ns. In the reverse case it is equal to net safe settlement pressure.

How to Calculate Bearing Capacity of Soil?

Calculation of bearing capacity of soil:

For the calculation of bearing capacity of soil, there are so many theories. But all the theories are superseded by Terzaghi’s bearing capacity theory.

Terzaghi’s bearing capacity theory

Terzaghi’s bearing capacity theory is useful to determine the bearing capacity of soils under a strip footing. This theory is only applicable to shallow foundations. He considered some assumptions which are as follows.

1. The base of the strip footing is rough.
2. The depth of footing is less than or equal to its breadth i.e., shallow footing.
3. He neglected the shear strength of soil above the base of footing and replaced it with uniform surcharge. ( D_f)
4. The load acting on the footing is uniformly distributed and is acting in vertical direction.
5. He assumed that the length of the footing is infinite.
6. He considered Mohr-coulomb equation as a governing factor for the shear strength of soil.

As shown in above figure, AB is base of the footing. He divided the shear zones into 3 categories. Zone -1 (ABC) which is under the base is acts as if it were a part of the footing itself. Zone -2 (CAF and CBD) acts as radial shear zones which is bear by the sloping edges AC and BC. Zone -3 (AFG and BDE) is named as Rankine’s passive zones which are taking surcharge (y D_f) coming from its top layer of soil.
From the equation of equilibrium,
Downward forces = upward forces

Load from footing x weight of wedge = passive pressure + cohesion x CB sinΦ

Where P_p = resultant passive pressure = (P_p)_y + (P_p)_c + (P_p)_q
(P_p)_{y is} derived by considering weight of wedge BCDE and by making cohesion and surcharge zero.
(P_p)_{c is} derived by considering cohesion and by neglecting weight and surcharge.
(P_p)_q is derived by considering surcharge and by neglecting weight and cohesion.
Therefore,

By substituting,

So, finally we get q_u = c’N_c + y D_f N_q + 0.5 y B N_y
The above equation is called as Terzaghi’s bearing capacity equation. Where q_u is the ultimate bearing capacity and N_c, N_q, N_y are the Terzaghi’s bearing capacity factors. These dimensionless factors are dependents of angle of shearing resistance ().
Equations to find the bearing capacity factors are:

Where

Kp = coefficient of passive earth pressure.
For different values of Φ , bearing capacity factors under general shear failure are arranged in the below table.

	Nc	Nq	Ny
0	5.7	1	0
5	7.3	1.6	0.5
10	9.6	2.7	1.2
15	12.9	4.4	2.5
20	17.7	7.4	5
25	25.1	12.7	9.7
30	37.2	22.5	19.7
35	57.8	41.4	42.4
40	95.7	81.3	100.4
45	172.3	173.3	297.5
50	347.5	415.1	1153.2

Finally, to determine bearing capacity under strip footing we can use

q_u = c’N_c + γD_f N_q + 0.5 γB N_y

By the modification of above equation, equations for square and circular footings are also given and they are.
For square footing

q_u = 1.2 c’N_c + γD_f N_q + 0.4 γ B N_y

For circular footing

q_u = 1.2 c’N_c +γD_f N_q + 0.3γ B N_y

Hansen’s bearing capacity theory

For cohesive soils, Values obtained by Terzaghi’s bearing capacity theory are more than the experimental values. But however it is showing same values for cohesion less soils. So Hansen modified the equation by considering shape, depth and inclination factors.
According to Hansen’s

q_u = c’N_c Sc dc ic + γD_f N_q Sq dq iq + 0.5 B γN_y Sy dy iy

Where Nc, Nq, Ny = Hansen’s bearing capacity factors
Sc, Sq, Sy = shape factors
dc, dq, dy = depth factors
ic, iq, iy = inclination factors
Bearing capacity factors are calculated by following equations.

For different values of Φ Hansen bearing capacity factors are calculated in the below table.

Φ	Nc	Nq	Ny
0	5.14	1	0
5	6.48	1.57	0.09
10	8.34	2.47	0.09
15	10.97	3.94	1.42
20	14.83	6.4	3.54
25	20.72	10.66	8.11
30	30.14	18.40	18.08
35	46.13	33.29	40.69
40	75.32	64.18	95.41
45	133.89	134.85	240.85
50	266.89	318.96	681.84

Shape factors for different shapes of footing are given in below table.

Shape of footing	Sc	Sq	Sy
Continuous	1	1	1
Rectangular	1+0.2B/L	1+0.2B/L	1-0.4B/L
Square	1.3	1.2	0.8
Circular	1.3	1.2	0.6

Depth factors are considered according to the following table.

Depth factors	Values
dc	1+0.35(D/B)
dq	1+0.35(D/B)
dy	1.0

Similarly inclination factors are considered from below table.

Inclination factors	Values
ic	1 – [H/(2 c B L)]
iq	1 – 1.5 (H/V)
iy	(iq)2

Where H = horizontal component of inclined load
B = width of footing
L = length of footing.

Quality Control Tests Of Construction Materials: Rocks

Quality Control Tests Of Construction Materials: Rocks

Quality Control: The control of material properties by compensating for over or underperformance.

Quality control of materials needs testing of materials produced or available beforehand.
A  few of the most commonly adapted tests for determining strength of rocks are stated as Follows.

Laboratory tests for determining strength of Rocks.

 

1.   Unconfined compression tests for rocks
2.   Tri-axial compression test for Rocks.
3.   Splitting tension test for Rocks
4.   Beam bending test for rocks.
5.   Ring shear test for rocks.

Unconfined Compression Test on Rocks

• It is more commonly used test for rocks to determine its strength but it should be done carefully for accurate results.
• The samples obtained during exploration are trimmed as per requirements.
• The specimen should in the cylindrical form and length to width ratio of specimen should be varying from 2 to 2.5.
• The ends of specimen should be exactly perpendicular to the axis of cylinder and ends should be flat, smooth and parallel.
• The dia of specimen should not be less than 35mm. generally 45 mm dia is preferable.
• The specimen is placed in compression testing machine and subjected to compression with a stress rate of 0.5 to 1.0MPa per second.
• Note down the peak load value (P) where it fails and find out the strength of rock using below formula.

                      Where A = initial c/s area of specimen.

                                                                            q _u = P/A

 

Tri axial Compression Test on Rocks

• This test is similar to that used for soils. Here also cylindrical specimen is required and is first subjected to lateral pressure and then deviator stress.
• As the stresses are quite large, a special type of equipment is required.
• In the usual procedure, the specimen is subjected to confining pressure (p) and then deviatoric stress is applied when P is kept constant.
• The confining pressure P increases the strength of rock. However, the increase in strength is realized only when the specimen is immersed in impervious jacket.
• Generally hydraulic oil is used as confining fluid. The jacket is made of polyurethane which is oil resistant.

 

Splitting Tension Test on Rocks

• This is also called as Brazilian test. In this test, the specimen is split along the diametric plane by applying load.
• The specimen having length to diameter ratio of unity when loaded on its side it splits along the diameter and parallel to the cylindrical axis. The horizontal stresses perpendicular to the loaded diameter are uniform and tensile. The tensile stress is given by

                                                                                

 

  where,
             d = diameter, 

             t = thickness or length of cylinder, 

             p = applied load.

 

• IS: 10082-1981 recommends that the diameter of specimen should be at least 45mm and thickness of the disc shall be approximately equal to half the diameter. The load should applied at the rate of 0.2KN/sec. and the load should be measured at least to an accuracy of 1%.

 

 

Beam Bending Test on Rocks

• In this test beam is subjected to bending till failure occurs. This is also called as flexural test.
• Generally 4 –point flexural loading system is used in this test. The bottom surface of the beam is supported at two points, one near each end.
• The top portion of the beam is loaded at the third points. This system produces pure bending in the middle third of the beam.

                   The flexural strength (modulus of rupture) is given by

 

 where,

             P = load , 

             L = length of specimen, 

             d = dia of specimen

 

Ring Shear Test on Rocks

 

• This test generally used to test Insitu rocks. It gives the shear strength of rock as a function of confining pressure.
• In this test the specimen doesn’t require perfect square or smooth ends.
• Load is applied parallel to the axis of core. As the load is applied to the plunger, 2 sets of complex fractures surfaces form along the two planes of imposed shear.

                 The shear strength is calculated by

                                                                                    Tp = P/2A 

where,

           p = load , 

           A = area of cross section of specimen.