Piling / Piling Caps

Question 1

Definition of a pile foundation…

Answer 1

Pile foundations are a series of columns constructed or inserted into the ground to transmit the loads of a structure to a lower level of subsoil

Question 2

Piles are used due to to (4)

Answer 2

1) Natural low bearing capacity of the sub soil
2) High water table
3) Presence of layers of highly compressible sub-soils such as peat and recently placed fill materials
4) Subsoils which may be subject to moisture movement or plastic failure

Question 3

4 classifications of piles are

Answer 3

1) End bearing
2) Friction or floating
3) Replacement
4) Displacement

Question 4

End bearing pile…

Answer 4

Transmits load direct to firm strata below

Question 5

Friction or floating pile…

Answer 5

Transmit load to surrounding soil via friction between surface of pile and soil

Question 6

Replacement piles are formed by

Answer 6

Boring hole in ground with material removed from ground and resultant void filled

Question 7

Displacement piles are formed by…

Answer 7

Driving preformed piles into the ground, displacing the subsoil through which it passed

Question 8

Factors which affect the choice of piling to be used are (5)

Answer 8

1) Location
2) Nature of ground
3) Cost
4) Structural performance
5) Durability

Question 9

4 Types of piling we are focussing on are

Answer 9

1) Driven
2) CHD - Continuous Helical Displacement
3) CFA - Continuous Flight Auger
4) Rotary Bored

Question 10

Driven piles are a form of ________________ piles

Answer 10

Displacement

Question 11

Driven pile method - brief description

Answer 11

Precast segment piles are driven into the ground via a hydraulic hammer driven pile rig. A steel helmet if placed on the top of the pile section to ensure the concrete doesn’t break during the driving process. Pre-formed holes on top of sections and pin on bottom allow for sections to be joined together on site during the piling process. Can be coated to provide protection to the pile sections and increase friction properties.

Question 12

Advantages of driven piling (6)

Answer 12

1) Good choice for deep piles
2) Can be used in aggressive or contaminated soils
3) Sections pre-fabricated off site
4) Wide selection of different sizes and lengths
5) High bearing capacity due to being displacement pile
6) Minimal waste material

Question 13

Disadvantages of driven piles (5)

Answer 13

1) Cost of transport due to weights / dimensions
2) May not be suitable for ground with poor drainage
3) Not suitable for compact sites where vibrations may affect surrounding buildings
4) Noisy to install - consideration to surroundings.
5) Not always possible to determine sizes required - results in splicing or cut off techniques being used which has time / cost affect

Question 14

Item to sketch / annotate for driven piles (9)

Answer 14

1) Piling rig
2) Bracing on rig
3) Leader of rig
4) Drop hammer
5) Helmet on top of pile section
6) Pile sections
7) Joins in pile sections
8) Reasonably level ground surface
9) Precast concrete or steel show unit at bottom of piles

Question 15

CHD is also known as… (2)

Answer 15

1) Rotary Soil Displacement

Or

2) Screw Displacement

Question 16

CHD piles are a form of _____________ piles

Answer 16

Displacement

Question 17

CHD method - brief description

Answer 17

Multi flight, bullet ended shaft, driven by high torque rotary head bored in the subsoil / strata. Concrete is pumped down shaft and out of the tip of the drilling rig while the shaft is reverse rotated and withdrawn from the bore. Steel reinforcement is added once drilling rig removed and before concrete sets. Steel left protruding from pile.

Question 18

Advantages of CHD piling (4)

Answer 18

1) Minimal waste / spoil due to displacement
2) Minimal vibration and noise
3) High bearing capacity due to displacement
4) Speed of installation

Question 19

Item to sketch / annotate for CHD piles (5)

Answer 19

1) Drilling rig
2) 5 Stages - rig in position, drilling, reaching design point, removal and filing of concrete, insertion of steel reinforcement.
3) Arrows showing displacement of soil outwards to surrounding soil
4) Concrete delivery vehicle
5) 3 sections to drilling head, screw section, flat section, screw section

Question 20

Continuous Flight Auger (CFA) is a form of ___________ piling

Answer 20

Replacement

Question 21

CFA method - brief description

Answer 21

Hole is bored using CFA - hollow stemed auger. Design depth reached. Concrete pipe connected to head of auger stem before withdrawal. Concrete injected down hollow stem and out tip of auger to fill void as the stem is slowly withdrawn, bringing spoil material with it. Extraction is controlled and monitored to ensure positive pressure in the concrete being pumped, ensuring hole does not collapse. On completion of stem removal and concrete injection steel reinforcement added.

Question 22

Advantages of CFA piling (5)

Answer 22

1) Virtually vibration free
2) One of quietest forms of piling
3) Suitable for most strata
4) Can be used in water bearing strata
5) Quick and economical install

Question 23

Item to sketch / annotate for CFA piles (5)

Answer 23

1) Drilling rig begins drilling - spiral drill section extends entire length of hole.
2) Design depth reached - pump, concrete mixer and silo connected.
3) Auger removal, hole beginning to fill with concrete, spoil at top of hole either side.
4) Complete removal of auger and hole filled with concrete
5) Steel reinforced cage added.

Question 24

Rotary Bored piles are a form of ___________ piling

Answer 24

Replacement

Question 25

Rotary Bored Pile method - brief description

Answer 25

Drilling rig with telescopic Kelly bar and auger head bores hole in ground. Steel guide casings inserted as hole formed -Tremmie pipe - sometimes full depth of hole, sometimes on first few metres. Drill down to design depth. Remove drilling auger - bringing spoil back to ground level. Concrete poured in hole. Steel guide casings removed. Steel reinforcement added if required. Optional - toe of pile can be under-reamed up to 3x shaft diameter (conical shape) to increase end bearing capacity.

Question 26

Advantages of Rotary Bored piling ()

Answer 26

1) Bigger diameter piles that CFA can be formed.
2) Virtually no vibration
3) Higher load capacity than driven piles

Question 27

Item to sketch / annotate for Rotary Bored piles (9)

Answer 27

1) Heavy duty crane / rig
2) Suspension rope
3) Kelly bar swivel connection (box at top)
4) Rotary telescopic Kelly bar
5) Kelly bar drive unit
6) Rotary auger
7) Bore hole lined with casings
8) Steel reinforcement
9) Underreamed section at toe of pile

Question 28

What are pile caps

Answer 28

Pile caps finish off a cluster of piles near formation level with a reinforced concrete pad which often incorporates the holding bolts for a steel frame or concrete kicker for a concrete frame

Question 29

There are 3 main shapes of pile caps which normally incorporate a set amount of piles. These are (3)

Answer 29

1) Square shape with 4 piles
2) Triangle shape with 3 piles
3) Rectangular shape with 2 piles

Question 30

The reinforcement bars in the piles are….

Answer 30

Linked into the reinforcement bars in the pile cap to create continuity

Question 31

Pile caps are linked together with…

Answer 31

Reinforced concrete ground beams

Question 32

The stages to forming pile caps are (8)

Answer 32

1) Excavation around the pile to create a pit for the pile cap. Soil is removed to landfill.
2) Top section of concrete removed from pile using cropper to expose steel rebar.
3) Blinding concrete is poured to the base of the pit to provide smooth platform for reinforcement cage.
4) Steel fixers prepare reinforcement cages.
5) Steel reinforcement cages placed with spacers at base separating cage from blinding concrete.
6) Permanent formwork installed along outside of pit with spacers separating from steel cage.
7) Backfill to formwork with spoil to provide support.
8) Concrete poured/placed, compacted and cured in pit. Holding down bolts / bolt boxes can be cast at this stage for continuity.

Question 33

3 types of testing piles are

Answer 33

1) Dynamic Load Test
2) Static Load Test
4) Sonic Integrity Test

Question 34

Dynamic load testing is carried out by… (6 Parts)

Answer 34

1) Two strain transducers and accelerometers are firmly attached to the head of the pile.
2) The pile is restruck using a pile hammer.
3) Force and acceleration of the pile is measured.
4) This information is relayed to the pile driving analyser.
5) Data digitally sorted.
6) Stress wave analysis conducted on data giving shaft friction distribution, bearing capacity and load settlement behaviour information.

Question 35

Advantage of dynamic load test…

Answer 35

1 pile can be done in a matter of minutes meaning a large number of piles can be tested on one visit to site.

Question 36

2 most common methods of static load testing are

Answer 36

1) Kentlidge block system

2) Anchor pile method

Question 37

Static load testing is carried out by… (4 parts)

Answer 37

1) Hydraulic jack mounted on test pile
2) Load placed on top
3) Measurements taken and recorded - data stored electronically
4) Information presented in graph detailing displacement characteristics of pile.

Question 38

Sonic integrity testing is carried out by… (5 parts)

Answer 38

1) Using small hand held hammer a series of low strain acoustic showcase waves are passed down the pile.
2) The wave rebounds where changes in impedance occur.
3) This rebound or echo is then recorded by a small accelerometer held against the pile head.
4) Responses stored digitally.
5) Information plotted in graph.

Question 39

Sonic integrity test is used to determine the (3)

Answer 39

1) reliability
2) morphology
3) quality of construction

Of a pile

Question 40

An advantage of sonic integrity testing is…

Answer 40

Test can be carried out quickly meaning a large number of piles can be tested in one visit to site.

Question 41

Disadvantage of static load testing is…

Answer 41

It is more time consuming to set up and carry out in comparison to other methods.

Question 42

As concrete is delivered to site it should be tested by doing a ______ ______ to make sure it is the correct grade

Answer 42

Slump test