METHOD OF STATEMENT FOR BORED PILING INSTALLATION BY ROTARY DRILLING MACHINE

Main elements of a bored piling installation process

• Preliminary survey & setting out of pile point using coordinates (using Total Station)
• Driving temporary steel casing
• Pile boring (soil/ rock boring)
• Preparation of the reinforcement cage
• Cleaning or flushing pile toe
• Concreting Bored Pile
• Temporary casing extraction 
• Pile Hacking
• Testing of Pile

Ground Preparation and Setting Out

The pile locations shall be set out in accordance with the bored pile foundation layout plan. All setting out reference points and related contract drawings shall also be referred. Setting out of pile location, ground level and ancillary control points shall be checked and confirmed with the Engineer prior to commencement of piling works. Once the first section of pile casing is erected, the setting out with all ancillary control points and the verticality of the pile will be re-checked with the Engineer before piling works proceeds further. 

Pre-Drilling and Driving Temporary Casing

A temporary steel casing will be driven by casing driver attached to the boring rig or vibro-hammer if required. Before casing driving, the alignment and verticality of the casing shall be checked to ensure that there is no deviation from the original point.

In cases where the maintenance for stability of borehole is difficult by drilling fluid (SPT N<10) such as peat layer (weak layer) or in the places where there is aggressive action of ground water and constant change of external ground water level; permanent casing will be required to maintain the integrity of pile shaft. In such situation case by case, situation will be informed to engineer for further instruction.

Maximum Permitted Deviation

The maximum permitted deviation of the pile center from the center point shown on the setting out drawing shall be 75mm in any direction. 

Verticality

The maximum permitted deviation of the finished pile from the vertical is 1 in 100.

Pile Boring with Bentonite Slurry (Wet Boring)

Boring of the piles shall be carried out by a rotary boring rig, using special types of boring tools depending on the soil conditions encountered. The position and inclination of the pile have to be checked and documented. In case tolerances are exceeded, the design engineer has to be consulted before constructing the next neighboring pile.

The excavated material in the bore hole is taken out by boring bucket, cleaning bucket and the drilling fluid. The excavated material has to be documented by a drawn borehole profile with classification of the excavated material according to the exploration profiles. (Soil samples and Rock particles from each bore hole shall be taken and packed in polythene bags. Bags shall be labeled with pile reference number, depth, date of sampling etc). Especially the depth of the surface of un-weathered rock (fresh rock) and the rock socketing shall be documented.

Drilling Fluid

Bentonite slurry shall be prepared at site using a mixture and stored in silos to maintain an uninterrupted supply. The quality of the bentonite shall be checked regularly and the results shall be recorded.

Boring Near Recently Cast Pile

Piles shall not be bored so close to other piles which have recently been cast and which contain workable or unset concrete that a flow of concrete could be induced from or damage caused to any of the piles. Minimum distance between two adjacent piles which are recently casted should be 4d. According to that, piling sequence shall be decided. A minimum period of 72 hours shall be provided before boring near recently cast piles, unless otherwise agreed with the Engineer.

Properties of Bentonite Slurry

Drilling fluid shall be checked for following properties, during the soil boring stage and the flushing stage of a pile.

Bentonite Slurry Properties

Property to be Measured	Test Method	Compliance values measured at 20 0C
		As supplied to pile	Samples from pile prior to placing concrete (Bottom)
Relative Density	Mud Density Balance	1.10 ~ 1.15 g/ml	Less than 1.25 g/ml
Viscosity	Marsh Cone Method	30 to 90s	≤ 28s
Sand Percentage		<6%	≤8%
PH value	PH paper	7~9	7~9

In order to check the quality of the slurry the following equipment/ tests shall be used:

• Marsh funnel
• Mud balance
• PH testing paper

Prior to concreting of a pile, bentonite samples will be taken from the bottom of the pile bore to test the actual properties of the bentonite slurry. A bentonite sampler which allows collecting slurry at any depth will be lowered to pile bore in order to collect bentonite samples. It consists of a tube and two balls on top and bottom of the tube.

Bentonite Slurry Sampler and Working Principle

Identification of Sound Rock Level

With reference to the borehole logs and visual inspection of the samples obtained from the borehole at site, sound/fresh rock identified.  Inspection of samples and clarifying fresh rock starting level shall be jointly inspected with consultant’s staff to confirm the rock head level.

• Visual inspection of rock samples
• Penetration rate of coring rock
• Borehole data
• Boring details of adjacent piles

Termination of Drilling

When the boring rig reaches the specified depth of socketing, the Engineer/Engineer’s representative shall check the soil investigation information regarding the material expected to be encountered at the approximate level for formation of sockets and the tentative founding level of the pile base. Boring rig shows the pile toe level by its indicators, a further check shall be carried out at the site by dipping a tape with a plum bob attached to the end of the tape. Pile penetration rate shall also be considered as a factor for termination.

An engineer/engineer representative shall inspect the borehole profiles & the rock samples obtained from the borehole to confirm that the required depth of the pile has been reached and boring can be terminated upon reaching rock strata. The engineer/engineer’s representative shall be consulted to obtain permission to terminate the drilling operations. Inspection of samples and dipping of actual founding level shall be jointly inspected with the engineer/engineer’s representative to confirm the condition & level of pile base. Information for inspection as usual will be done through RIF supplemented with direct communications with counterpart Engineer’s representative.

Preparation of the Reinforcement Cage

The reinforcing cages will be prepared on site during the boring operation. The reinforcing cages shall be checked to ensure that they are in accordance with the approved construction drawings. Following parameters shall be complied with the approved construction drawings. 

   

 

Cleaning Pile Toe

Initially, bore-hole base shall be cleaned by boring rig using cleaning bucket. If the depth and the sand content of the bore-hole attain to satisfactory level, concreting operation will be commenced. In case a satisfactory level is not attained, the sediments of the pile base will be pumped out through tremie pipes and simultaneously the pile will be filled with bentonite slurry. This cleaning operation shall be continued until the sand content of the pile attain to satisfactory level.

However, after lowering the rebar cage the bore-hole will be measured and verified again prior to concreting. If there is major soil collapse encountered, rebar cage will be lifted up and the bore-hole will be clean again. The pile base will be cleaned using fresh bentonite slurry circulating through trimie pipes (flushing stage) continuously until the sand content satisfy the requirement mentioned in table no 02.

Working sequence of pile base cleaning in flushing stage;

Step 01          Insert reinforcement cage to the pile.

Step 02         Insert tremie pipes to the bore hole with attached submersible mud pump of 4” to the bottom of                           the tremie pipe.

Step 03         Sediments of the pile base shall be pumped out by submersible pump. Meanwhile bentonite will                         be pumped to the borehole to maintain the bentonite head in required level.

Step 04          After attain the sand content of the pile to satisfactory level, Bottom pump will be removed and                           ready the pile for concreting.

De-Sander & Bentonite mixer

Bentonite Handling System

Bentonite slurry is required to stabilize the bore hole. During the drilling process fresh bentonite is pumped to the borehole to keep it full (inside the casing should be kept always at least 1.0m above ground water level) in order to guarantee its stability.  Prior to concreting the bore, bentonite that is charged with particles of soil must be cleaned or replaced. The System is made up of several components.

A leveled area of about 40 x 20 m is required. The storage volume is substantial and is made up of a series of tanks, silos or can be in the form of earth ponds. The system consists of 1No. de-sanding unit, Bentonite mixing unit, pumps and pipelines.

Bentonite handling system (Silos, tanks, mixer, desander, pumps and pipelines)

Mixing Unit

An efficient mixing unit (fully automatic or manual operation mode) mixes bentonite powder with water. The freshly mixed slurry is pumped to a holding and hydration tank where the slurry is kept in motion. Hydrated bentonite slurry is then transferred by a pump to the main reservoir / pile.

Bentonite Mixing Unit

The De-sanding Unit

The de-sanding unit separates from the slurry all particles down to a cut point d50 of 60 microns and is made up of four main items: one or more hydro cyclones, driers, distribution boxes and pumps. De-sanded slurry is pumped back to the storage reservoir for reuse. 

De-sander

The Storage Unit

The storage unit can be made up of a series of ponds excavated in the ground, steel tanks stacked parallel or on top of one other or, if space is at a premium, a series of silos. The layout can have different configurations to best suit the geometry of the site but it is important, in order to guarantee continuity in the work that the total capacity of storage is at least 1.5 x the volume of pile. In the design of storage capacity, consideration should be given to the local geology.

If there are indications of the presence of formations that could lead to a sudden loss of bentonite during the boring, that might be affected to the stability of the bore hole. The size of the storage basins should take into account a surplus supply that can be used in these emergency situations.

The Conveying Unit

The conveying unit is made up of a series of pumps, pipes, valves and controls designed to facilitate conveying bentonite to and from the bore hole. On return lines, charged slurry can have solids content higher than 8% and may need higher pumping capacity to convey it to the de-sanding unit. The diameter of all pipelines is normally 100 mm (4”) or 150 mm (6"). 

De-sanding and Bentonite testing

Prior to concreting, bentonite in the bore hole must be sampled and checked to ensure that it satisfies the minimum criteria for concreting. When this is not the case, bentonite in the bore hole is circulated through the de-sanding plant; alternatively it may be partly or completely replaced by fresh bentonite so that its characteristics satisfy the contract requirements. The decision on whether to circulate or to replace the slurry is a function of the properties of the slurry in the bore, the contract specifications that characterize this material, the availability of fresh bentonite and time.

Concreting

Concreting the fluid filled bore hole is carried out using a “Tremie pipe” that introduces fresh concrete to the bottom of the bore and allows it to rise upwards displacing the fluid in the bore hole. Tremie pipes will be installed into the pile bore up to the pile toe. The tremie pipes will be internally free of any old and hardened concrete as to allow for a smooth concreting procedure. O-rings may have to be inserted to the joints of the tremie pipes to ensure adequate water tightness.

Concreting shall only commence once a sufficient number of loaded mixer trucks are available on site ready to discharge the concrete and all other trucks are confirmed for arrival. The slump of the piling concrete shall be 180 ± 25 mm before the discharge into the pile bore.

The funnel with appropriate capacity shall then be filled with concrete up to brim level. The concrete shall be released by removing the lid fixed at the bottom of the funnel and the concrete shall be discharged without interruption in one operation unit. Then the tremie pipe is raised and the concrete is placed at the bottom of the pile if needed only. Whilst concreting, the tremie pipe shall be raised in such a manner so that the embedded tremie length in to the concrete is at least 3.0 m of fresh concrete.

The concrete may be retarded to ensure that setting does not commence within four (04) hours of arrival at site. A retardation of 4 hours is recommended to avoid technical problems during concreting.

On-site sampling and testing of concrete usually consists of taking:

•  Sets of cubes - The amount and frequency of sampling and testing 
•  Slump tests - carried out regularly on site to check consistency in the workability of the concrete

Process of concreting in brief

Concrete pouring

Concrete funnel handling using a crane