why two meters deep excavation is a failure for pile cap

Excavation of shallow foundation 2m
courtesy of yd-mv.com

Type of building work

This involves construction of a 4-storey flatted factory (with no basement) located between two existing single story terrace houses which were founded on small diameter bored piles.

What went wrong

Timber planks were used to support a 2m deep excavation to construct a pilecap adjacent to an existing single story house. The ground condition was poor, comprising of soft marine clay and the timber planks to support the excavation was shoddily done.

Shoddy timber planks to support

The timber planks were not effective in resisting the earth pressure and gave way, resulting in movement of the marine clay beneath the adjacent house. The ground movement cracked the small diameter bored pile supporting the adjacent house causing the entire front section of the adjacent house to collapse.

Learning points

a) Every construction project, whether big or small should be given due attention.

b) Properly designed temporary earth-retaining structures should be provided to protect the sides of the excavation, even for shallow excavation. The design should take into consideration the effects of inclement weather and presence of foundation of existing buildings.

c) Extra precautionary measures should be taken when working close to existing building.

How to install pile to foundation

The installation process and method of installations are equally important factors as of the design process of pile foundations. In this section we will discuss the two main types of pile installation methods; installation by pile hammer and boring by mechanical auger. 

In order to avoid damages to the piles, during design, installation Methods and installation equipment should be carefully selected. If installation is to be carried out using pile-hammer, then the following factors.

should be taken in to consideration: 

• the size and the weight of the pile 
• the driving resistance which has to be overcome to achieve the design penetration 
• the available space and head room on the site 
• the availability of cranes and 
• the noise restrictions which may be in force in the locality. 

Pile driving methods (displacement piles)

Methods of pile driving can be categorised as follows:

1. Dropping weight
2. Explosion
3. Vibration
4. Jacking (restricted to micro-pilling)
5. Jetting 

Drop hammers 

A hammer with approximately the weight of the pile is raised a suitable height in a guide and released to strike the pile head. This is a simple form of hammer used in conjunction with light frames and test piling, where it may be uneconomical to bring a steam boiler or compressor on to a site to drive very limited number of piles.

 

There are two main types of drop hammers:  

1. Single-acting steam or compressed-air hammers; comprise a massive weight in the form of a cylinder. Steam or compressed air admitted to the cylinder raises it up the fixed piston rod. At the top of the stroke, or at a lesser height which can be controlled by the operator, the steam is cut off and the cylinder falls freely on the pile helmet. 
2. Double-acting pile hammers; can be driven by steam or compressed air. A pilling frame is not required with this type of hammer which can be attached to the top of the pile by leg-guides, the pile being guided by a timber framework. When used with a pile frame, back guides are bolted to the hammer to engage with leaders, and only short leg-guides are used to prevent the hammer from moving relatively to the top of the pile. Double-acting hammers are used mainly for sheet pile driving. 

Diesel hammers 

Also classified as single and double-acting, in operation, the diesel hammer employs a ram which is raised by explosion at the base of a cylinder. Alternatively, in the case of double-acting diesel hammer, a vacuum is created in a separate annular chamber as the ram moves upward, and assists in the return of the ram, almost doubling the output of the hammer over the single-acting type. In favourable ground conditions, the diesel hammer provide an efficient pile driving capacity, but they are not effective for all types of ground. 

Pile driving by vibrating

Vibratory hammers are usually electrically powered or hydraulically powered and consists of contra-rotating eccentric masses within a housing attaching to the pile head. The amplitude of the vibration is sufficient to break down the skin friction on the sides of the pile. Vibratory methods are best suited to sandy or

gravelly soil.

Jetting: to aid the penetration of piles in to sand or sandy gravel, water jetting may be employed. However, the method has very limited effect in firm to stiff clays or any soil containing much coarse gravel, cobbles, or boulders.

complete vibration hammer kit, just need a heavy excavator to attach the hammer

Boring methods ( non-displacement piles) 

Continuous Flight Auger (CFA) 

An equipment comprises of a mobile base carrier fitted with a hollow-stemmed flight auger which is rotated into the ground to required depth of pilling. To form  the pile, concrete is placed through the flight auger as it is withdrawn from the ground. The auger is fitted with protective cap on the outlet at the base of the central tube and is rotated into the ground by the top mounted rotary hydraulic motor which runs on a carrier attached to the mast. On reaching the required depth, highly workable concrete is pumped through the hollow stem of the auger, and under the pressure of the concrete the protective cap is detached. While rotating the auger in the same direction as during the boring stage, the spoil is expelled vertically as the auger is withdrawn and the pile is formed by filling with concrete.In this process, it is important that rotation of the auger and flow of concrete is matched that collapse of sides of the hole above concrete on lower flight of auger is avoided. This may lead to voids in filled with soil in concrete. 

The method is especially effective on soft ground and enables to install a variety of bored piles of various diameters that are able to penetrate a multitude of soil conditions. Still, for successful operation of rotary auger the soil must be reasonably free of tree roots, cobbles, and boulders, and it must be self-supporting.

During operation little soil is brought upwards by the auger that lateral stresses is maintained in the soil and voiding or excessive loosening of the soil minimise. However, if the rotation of the auger and the advance of the auger is not matched, resulting in removal of soil during drilling-possibly leading to collapse of the side of the hole. 

Underreaming 

A special feature of auger bored piles which is sometimes used to enable to exploit the bearing capacity of suitable strata by providing an enlarged base. The soil has to be capable of standing open unsupported to employ this technique. Stiff and to hard clays, such as the London clay, are ideal. In its closed position, the underreaming tool is fitted inside the straight section of a pile shaft, and then expanded at the bottom of the pile to produce the underream Normally, after installation and before concrete is casted, a man carrying cage is lowered and the shaft and the underream of the pile is inspected. 

C.H.D.P 

Continuous helical displacement piles: a short, hollow tapered steel former complete with a larger diameter helical flange, the bullet head is fixed to a hallow drill pipe which is connected to a high torque rotary head running up and down the mast of a special rig. A hollow cylindrical steel shaft sealed at the 

lower end by a one-way valve and fitted with triangular steel fins is pressed into the ground by a hydraulic ram. There are no vibrations. Displaced soil is compacted in front and around the shaft. Once it reaches the a suitably resistant stratum the shaft is rotated. The triangular fins either side of its leading edge carve out a conical base cavity. At the same time concrete is pumped down the centre of the shat and through the one-way valve. Rotation of the fins is calculated so that as soil is pushed away from the pile base it is simultaneously replaced by in-flowing concrete. Rates of push, rotation and concrete injection are all controlled by an onboard computer. Torque on the shaft is also measured by the computer. When torque levels reach a constant low value the base in formed. The inventors claim that the system can install typical pile in 12 minute. A typical 6m long pile with an 800mm diameter base and 350mm shaft founded on moderately dense gravel beneath soft overlaying soils can achieve an ultimate capacity of over 200t. The pile is suitable for embankments, hard standing supports and floor slabs, where you have a soft silty layer over a gravel strata.

you may also like to check out;

>Bitumen coating on pile to educe skin friction

>Monster Foundation

>Classification of piles

Monster Foundation

The 800-meter (1,827 feet) Burj Dubai will need the mother of all foundations to support a super-structure that is expected to weigh 500,000 tons. The tower will rest on a 3.7m-thick triangular frame foundation supported by 192 rounded steel piles or support cylinders measuring 1.5m in diameter and extending 50m (164 ft.) below the ground.

How many days is it for curing?

The recommended minimum curing time is 28 days, but most contractors typically wait only 7 to 14 days just to save time and money, but depending on the size of the concrete its very important to cure for 28days cos it will gain full or almost full strength.

pouring water (curing) of concrete after it been poured

What is the name given for the procedure of pouring water on concrete?
its called curing, a processed where concrete is left moist due to the exothermic reaction that takes place which may give cracks on the concrete.

Why Cure?
The major objective of concrete curing applications is to prevent the rapid loss of water from the concrete. As concrete loses water due to evaporation from the top surface, differential drying shrinkage can occur. This is a major contributor to shrinkage cracking. The application of curing methods reduces the loss of water from the surface of the concrete. It also permits more complete hydration of cement in the concrete itself. Minimizing evaporation also helps control the temperature of the concrete during its early-age stage.

Curing operations should begin after the water sheen disappears from the surface, and after any texturing operations have been completed. In the case of a curing compound, the membrane formed by the compound should not be disturbed after it is placed.

Rapid Drying Conditions
In rapid drying conditions, a light water fog may be necessary to maintain moist surface conditions prior to the application of curing methods. Light water fogging can be accomplished during a short period of time when the concrete surface begins to dry but before the curing operations can begin, such as prior to texturing operations have been completed.

Use of Ambient Weather Condition Information
Ambient weather conditions, such as wind speed, relative humidity, and air temperature can interact with the temperature of the concrete to cause excessive water evaporation from the concrete surface. Since different curing methods provide different levels of protection, knowing the amount of protection required is important in determining the method to use. In order to know the required level of protection, the ambient conditions and concrete temperature must be known. A portable weather station that records the ambient conditions and automatically predicts evaporative water from the concrete surface can be an invaluable tool for controlling water loss from the concrete surface. Such a tool can also warn in advance when conditions approach predefined limits of evaporation.
Curing Methods
Various concrete curing methods are available, and each provides different levels of protection. A single coat of liquid curing compound generally provides the least protection, but additional coats can improve its performance. Polyethylene sheets, cotton mats and wet burlap provide additional protection.

Liquid Curing Compound

White-pigmented, liquid membrane curing compound is used most often due to its low cost and ease of application. It does not require great amounts of labor, nor does it expensive, bulky material, such as cotton mats. It's disadvantages are that it provides the least amount of protection, and the membrane can be ruptured inadvertently.

The liquid curing compound should be white, to avoid excess heat absorption from the sun (Figure 4.1). Also, the white color enables construction workers to check more easily for coverage uniformity and gaps in the coverage. The liquid compound must be constantly agitated during application to ensure that the mixture is applied correctly. The curing compound spraying operation should be shielded from the wind throughout the process.

The compound must cover all exposed surfaces, including the sides of the pavement slab. The compound should not, however, be applied into any joints in the pavement. For ultra-thin whitetopping, curing compound should be applied at twice the normal application rate, due to its extra sensitivity to drying shrinkage.

Figure 4.1 - White-Pigmented, Liquid Membrane Curing Compound.

Plastic or Waterproof Paper

Plastic, or polyethylene, sheeting provides good protection to the concrete from water evaporation from the surface (Figure 4.2). It requires more labor than liquid curing compound, yet it is not as bulky as cotton mats or burlap. Waterproof paper may also be used in the same manner described here for plastic sheeting, but is not as common.

The plastic sheeting must not have any rips or tears through which water can escape. The sheets should overlap to provide full coverage for the concrete surface. Just as with curing compound, the sheeting should cover all exposed concrete surfaces, including the edges of the pavement slab. Active methods must be used to hold the sheeting in place. Do not assume that they will remain in place of their own accord.

Figure 4.2 - Polyethylene Sheeting Used as a Curing Method.

Cotton Mats or Burlap

Cotton mats represent a great increase in evaporation protection, both by providing additional moisture if needed, and by protecting the concrete from ambient conditions such as low humidity, high wind speeds, and high temperatures. Cotton mats and wet burlap must be kept continually moist. When the mats get dry they can become more harmful than without them due to "wicking" action which draws moisture from the concrete into the mat.

FAULTS (EARTHQUAKE) IMPACT TOWARDS BUILDING FOUNDATION

A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

TYPES OF FAULT;
1. Dip Slip Faults
The movement is up or down parallel to the dip of inclined faults surface

2. Strike slip fault

A fault where movement(or slip) is predominantly horizontal and therefore parallel to the strike of the fault
3. Oblique-Slip Fault
A fault where movement of both strike-slip and dip-slip components

IMPACTS OF FAULT

Earthquake-induced ground shaking can pose threats to people and structures (specially foundation of the building which leads to collapse the buildings) even at distant locations from the fault on which the earthquake event is occurring. Ground shaking at a particular location depends on the earthquake magnitude (e.g., a measure of total energy released by fault rupture); epicenter distance (e.g., the distance from the center of the fault rupture to the location of interest); and, subsurface conditions of the geologic and soil units at the location of interest.

EFFECTS OF EARTHQUAKE TOWARDS BUILDING FOUNDATION

Ground motion

• Is the trembling and shaking of the land
• can cause building to vibrate
• can be large enough to topple large structures such as bridges and office and apartment buildings
  

Landslide

• can be triggered by shaking of the ground
• can cause extensive damage

Liquefaction

• occurs when a water saturated soil or sediments turns from a solid to a liquid as a result of earthquake shaking
• causes building to sink and underground tanks to float as once-solid segments flow like water.
  

LOCATING AND MONITORING
Earthquakes can be monitored using the instrument that could accurately record seismic waves. The instruments measure the ground motion and can be use to find the location, depth and size of an earthquake.
- seismometer
- seismograph
- seismogram

MINIMIZE BUILDING FAILURE

• construction must meet the building codes(strictly enforced in earthquake-prone areas)
• control the location of the buildings.(building built on soft sediment are damaged more than building on hard rock)
• firmly attach house to the foundation with anchor bolts.
• Repair any deep cracks in foundations