Supervision of Piling Works
Assistant Director (Planning & Development)
Supervision is utmost important in piling work. The durability of the entire building depends on the amount of quality work done in the foundation with adequate precision. The confirmation of the degree of safety and level of strength of the super-structure are also performed through field tests during the piling work. This paper presents the details of a pile foundation construction in Bangladesh and the quality control methods employed.
Keywords: piling, in situ, field observations
Supervision is utmost important in piling works. The durability of the entire building depends on the amount of quality works done in the foundation with adequate precision. The confirmation of the degree of safety and level of strength of the super-structure are also performed through field tests during the piling works.
One of the leading universities of Bangladesh owns a piece of land occupying an area of 115,000 sq.ft. at Bashundhara near the Diplomatic Zone at Dhaka. From the subsoil investigation report, it is understood that the land to be developed is a filling area for about a depth of 25 to 30 feet. The original soil at a considerable depth is categorized as very stiff silty clay and a sand layer is slightly visible below 80 feet from EGL.
The university authority through its dept. of Planning & Development engaged a consortium of consultant to design, the state of the art university campus. The soil test result made the consultant more cautious in designing the foundation system. The initial design referred to 798 cast-in-situ piles with 30 m depth of boring. Considering 2-storied underground car parking, the cut-off level was fixed at 5.5m below the existing ground level. A contractor experienced in similar nature of works was also engaged to do the piling works of the proposed university campus within the stipulated completion time of 180 days.
Figure 1. Piling works of cast-in-situ bored piles
LAYOUT OF PILES
The total 8 bigha (in local term) sites was divided by X-Y grid lines where N-S grid lines named alphabetically (in lowercase letters) and E-W gridlines named numerically. The three different diameter of piles namely 750 mm, 600 mm & 500 mm were categorized as Type A, B & C. Thus the naming & positioning of each pile cap was done i.e. PC-3A at GL2-A means pile cap consists of 3 nos. of type A pile located at the intersection of grid lines 2 & A. The position of each pile was marked carefully by mass concrete base 1’-0” dia. with a 3’-0” long rebar pegged in the middle. The top surface was painted red, blue & green to differentiate pile type. Each grid line was high lighted with nameplates along the perimeter fencing. A few temporary benchmarks was also set up at possible safe places ranging datum from the finished bituminous top surface of 60’ wide road along the north side of the site.
Load Test on Test Piles
Based on the sub-soil investigation report & pile layout plan, the consultant decided to conduct five load tests on test piles at different selected places on site. The load applied after 28 days of casting on top of test pile was 2 times the design load in increment approximately 25% of allowable design capacity of the piles tested. No additional increment of load was applied not earlier than one hour after all measurable settlement due to previous reading was ceased. The maximum amount of load was kept not less than 12 hours. The settlement readings against increment of load with respect to time were recorded carefully.
The method of application of load on test pile was carried out by jacking against loaded platforms. The load transferred from the platform was measured directly on the pressure gauge of hydraulic jack. Settlements of the pile was recorded by using gauges resting on an arm clamp to the pile head, the gauges being carried by reference beams supported well clear of the pile & platform support.
The hydraulic jack set up with pressure gauge & pump was calibrated from BUET. Thus the initial jack loss was determined and the unit conversion equation is obtained from the plot.
The load test report included load test log in tabular form with observations of loads, settlements and time. The test result showing Time/Load Diag., Time/Settlement Diag. and Load/Settlement diagram confirmed that the allowable design capacity of the pile was considerably safe.
Based on the test results obtained from the load test, the boring depth and casting length of piles were reduced from 30 m to 26 m and 24.5 m to 20.5 m respectively.
Figure 2. Load Test using sand bag & steel girder
In order to complete the casting of 798 piles within the schedule time, it is required to complete 10/12 piles per day and hence the work should be carried out 24 hours a day. The space available has a limitation on the number of rigs mobilized on site. In addition, the contractor was suggested to start with boring of pile at a particular time usually after midnight, so that the washing of bore holes & casting of piles were done during daylight.
PILE NUMBERING & ZONING OF SITE
In addition to colouring & cementing of pile position in order to differentiate among different categories of piles, 352 Type-A piles were numbered first from 1 to 352, 224 Type-B piles from 400 to 624 and 224 Type-C piles from 700 to 924. The total site was divided into 12 zones and 10 rigs were mobilized in these zones. Movement of rigs were controlled by the numbering of piles and zoning of site.
BORING & WASHING
The boring of a pile starts with the event of centering of piling rigs. This includes coinciding chisel tip with the center point of cemented pile position. With the progress of boring works, 6 m casing was first placed in position. The borehole should be filled with slurry prepared from Bentonite powder to protect the hole from side caving. The density of solution should be about 1.12. Once the borehole had been drilled down to the final depth, fresh bentonite solution/slurry pumped through the borehole for more than half an hour so that it is thoroughly cleaned. The time for washing is more than usual because the slurry was mainly dominated by clay instead of sand.
Figure 3. Boring of cast-in-situ bored piles
LIMITATION OF BORING SEQUENCES
No boring operations should take place nearer than 10 ft. from any pile for which concreting operations were in progress or from any newly completed pile until at least two days had elapsed from the last concrete operation on that pile.
The out of position tolerance should not exceed 75 mm in any direction.
Placing of Reinforcement
The reinforcement of the pile was securely binded by welding at contact points of alternate tie so as to form a rigid case. The remaining contact points were laid with 20 gauges G.I. wire. Since the pile cut-off level was 5.5m below the EGL, the reinforcement casing was lowered with 2 nos. of Y16 bars welded and binded on both sides of it at a length not less than 5.5m. The 20.5m length of reinforcement casing should split in to two parts/sections and the over-lapping of main reinforcement was not less than 40 times the diameter of the main bar. The circular roller which is made of cement concrete at three different side of the casing was used through out the length to ensure clear cover of 75 mm. Concrete spacer circular blocks would also help the reinforcement-casing roll in side the borehole.
The consultant carried out inspection at the time of washing of borehole and placing of reinforcement casing. During the inspection, it was confirmed that the borehole was cleaned properly to the required depth and the reinforcement member, spacing, welding & binding were as per drawing and specification. Upon confirmation through inspection, the contractor was permitted to proceed with the casting/concreting works of piles.
Casting of Cast-in-Situ Pile
The casting of cast-in-situ pile was done by placing concrete of standard mix of 1:1.5:3 and the slump within the limits between 4 & 5 inches. The concrete should give a compressive strength of 24 MPa (3500 psi) at 28 days measured on cylinder test. The trial mix design of concrete was also done at the beginning of the works to confirm w/c ratio and required slump. The concrete was placed by using tremie pipes of not less than 150 mm diameter at about a feet above the base of the hole. The concrete was poured into the tremie pipes though funnel fix on top of it. It was ensured at all times that the bottom of 150 mm diameter tremie pipes were always at least 300 mm within the concrete, so that the fresh concrete was always added in the concrete.
Boring Cum Washing House
It was a two chambered house size: 14’ x 20’ x 7’ (approx.) inter-connected at a given draft equipped with submersible pump of 2 hp capacity. Using one of those chambers continued boring and sedimentation was allowed to over flow the soluble slurry in to next chamber where fresh bentolite solution and water were added to prepare fresh slurry to wash the borehole. The pump was used to carry the residual slurry from the site and dump into a safe place.
The sludge that came out during boring & washing was mainly dominated by clay. The sedimentation that were expected to happen in the boring chamber, did not take place satisfactorily. This resulted great difficulties in the removal of sludge from both the chamber. The huge amount of sludge was then taken into a tank by using pump and carried away by engaging truck. At the peak time of piling works when 10-12 piles were casting within a day, the boring & washing house were separated and worked independently.
Portland Cement should confirm to the specification for Portland Cement (ASTM C-150 or equivalent Type-1). Cement was stored temporarily in a store accommodating 3000 bags constructed along the perimeters of site. The floor of the store was raised by 12 inches from the ground to prevent possible dampness. The manufacturer’s test certificates would in general be accepted as proof of the quality of the cement.
Concrete Aggregate should confirm to ‘Specifications for Concrete Aggregate’ ASTM C-33. The fineness modulus of fine aggregate should be not less than 2.5 not more than 3.1. The coarse aggregate should be quality shingles collected from natural resources and size of aggregate should be 20 mm down grade. Screening & washing were directed to make the aggregates well graded and free from deleterious substances.
The reinforcement used in concrete should be rolled mild steel bars complying with the requirements of ASTM-A160, 40 grades. The tensile strength of reinforcement of different diameter was tested from the Bureau of Research Testing & Consultation (BRTC), BUET. All reinforcement bars should be stored on the site on timber/concrete supports/bamboo suitably spread and of sufficient height to keep the steel clear of the grounds. The steel surface was kept clean, and free from loose rust, oil, grease, earth or other harmful materials at the time the concrete was placed.
Two deep tube wells were installed to meet the requirement of water at a selected safe place on site. The layer was available at a depth of 266’ from EGL.
This test is carried out on concrete to find out compressive strength of 24 MPa at 28 days measured on cylinder. Initially 3 nos. of samples were taken for each pile to make an avg. of the compressive strength of concrete for that batch. However, the no. of sample taken for each pile were reduced to one with the satisfactory progress of work. In this regard, the test standard followed was ASTM C-39.
Structural Integrity Test
The integrity test would be performed on each pile as per standard practice. The test report & charts should be submitted to the consultant within 24 hours of the test. The cost incurred for the test, was included in the rate of casting.
Load Test on Service Pile
The change of boring depth from 30.0 m to 26.0 m based on the initial load test on test piles was confirmed by this load test on service pile as per the similar procedure ASTM D-1194 performed earlier.
It is a constant struggle by an engineer to obtain the maximum amount of durability and strength with a minimum amount of expenditure. The experience that I gathered during the supervision of a piling works has been illustrated above. Considering, this might be helpful to engineers in their day-to-day construction works to fulfill the commitment of their profession with more efficiency.
|© 2003 ejge|