Bentonite slurry is one of the most common excavation fluid used in constructing diaphragm wall. Bentonite clay (in powder form) and water are combined in a colloidal mixer and clay particles bond to each other and set to form a gel when left to stand for a period of time. When the bentonite is set in motion, it reverts back to the fluid state rapidly.
What is the difference between pad foundation, strip foundation and raft foundation?
Shallow foundation is commonly accepted as foundation with founding level less than 3m from ground surface. In case surface loads or surface conditions could still affect the bearing capacity, the foundation which sits on it is called shallow foundation.
Pad foundation refers to the foundation which is intended for sustaining concentrated loads from a single point load such as structural columns.
What are the components in contributing the bearing capacity of shallow foundation?
Based on Terzaghi’s bearing capacity equation, there are three components in contribution to the bearing capacity:
(i) Surcharge pressure
Foundations are normally not placed directly on the ground level. Instead, they are installed at a depth below the existing ground level. The soil pressure arising from the depth of soils serves as a surcharge imposing a uniform pressure at foundation level.
In driven piles, the allowable set is limited between 25mm and 50mm per 10 blows. Why?
In final set table it is commonly to limit pile set between 25mm/10 blows and 50mm/10 blows. The reason of the provision for lower limit of set value of 25mm/10 blows is to avoid possible damage at pile toe owing to compressive stress reflected at the bottom of pile.
Some piling contractors incline to use drop hammer instead of hydraulic hammers in setting of piles. Why?
The use of high grade and heavy steel pipes may not warrant the use of drop hammer. For pile driving of Grade 55C steel section, it requires heavier drop hammer with increased drop height so as to comply with penetration resistance at final set. As such, it is possible that such heavy hammer may damage the pile and endangers the worker who takes the final set record.
There is a general trend that toe defects in bored pile construction have become more serious. Why?
Direct coring method is commonly employed to check the quality and workmanship of bored piles. It is further divided into two main types, namely interface coring (pile/rock interface) and full coring (entire concrete pile length). In the past, the implementation of full-depth coring usually takes place near the pile centre to prevent physical conflicts with reinforcement cage. When placing concrete by tremie method, concrete pouring by tremie pipe is carried out near pile centre and soft and weak materials are usually displaced to the sides of bored piles.
What is the widespread usage of 45o load spread rule?
Under the 45o load spread rule, when the horizontal distance between the toe level of adjacent piles is smaller than the vertical difference between the piles, additional load is deemed to be added by the pile at lower founding level. Hence, when the pile at lower founding level is within the zone of 45o spreading from the pile at higher elevation, further load checking is required for the lower pile.
What is the purpose of maintaining excess water head during excavation for bored piles?
Excavation of bored piles is usually implemented by a hammer grab. The steel casing will be extended by welding or bolting on additional casing and is installed by hydraulic oscillator.
Why are bored piles usually cast higher than the required final level?
It appears to be a common construction practice that bored piles are normally cast up to a piling platform much higher than the formation level of pile caps. Then, the excessive pile length has to be cropped and removed to the correct level for the subsequent construction of pile cap. The reasons of such practice are as follows:
Piles Used To Retain Soil
The following picture shows how Piles are used to retain soil. Piles are very useful way to prevent the sliding soil. More over as we go deep and deep, excavation becomes a prime source of concern for site engineers as soil keeps on flowing. More over in some cases it can become a source of accident. So piles are a great relief to site engineers.
