Construction of Bored Cast in Situ Piles Using Rotary Drilling Procedure a Case Study

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Construction of Bored Cast in Situ Piles Using Rotary Drilling Procedure a Case Study

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Construction of Bored Cast-in-Situ Piles Using Rotary Drilling Procedure—A Case Study INDIA IGC 2009, Guntur,

CONSTRUCTION OF BORED CAST-IN-SITU PILES USING ROTARY DRILLING PROCEDURE—A CASE STUDY

I.V. Anirudhan Geotechnical Solutions, Chennai, India. E-mail: [email protected] ABSTRACT: This

paper describes the defects reported for some bored cast-in-situ piles constructed using hyd operated rotary drilling rig (auger type) in a site at South Chennai, India. The analysis of the defects leads to the c that these defects were mainly due to improper construction practice and also due to improper identification of th stratum. There may be genuine difficulties in identifying the bearing stratum when rotary drilling procedure is adop construction. The construction drawings do not always specify the bearing conditions and the methods for ensu bearing condition. The conclusions are applicable to several such cases recently experienced in Chennai, even thoug not reported and analysed with an open mind. The paper calls for immediate corrective measures to overcome such the forthcoming projects in Chennai. 1. INTRODUCTION

The history of piling dates back to 200–300 BC when the timber driven piles were extensively used for supporting mostly th river side establishments and until late 19 century timber driven piles ruled the roost. The theories developed, types of piles increa increased sed,, depths, depths, capacities capacities,, etc. increased increased over over the years. years. Then came experiments with concrete piles and steel piles driven to relatively larger depths and for relatively large carrying loads. Notwithstanding the quality and integrity of driven concrete and steel piles, the capability of heavy machinery to drill holes into the ground of almost any soil formation gave rise to the development of different sizes of bored cast-in-situ piles. the time, t ime, quite different from f rom the theories of spread Special offer forBy students: Only $4.99/month. footings, empirical and semi-empirical formulae were developed

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relatively hard rock but passing through roughly soft rock with SPT N >100. These and similar stu shown that an economical pile design can be mad socketing the piles into hard rock wherever a suffici very dense residual soil and soft weathered rock are av

The excavation for placing the spread footing does ‘disturb’ the soil that is involved in realising carrying capacity. On the other hand the pile f construction produces significant changes in the so influenc the pile and at the base of30the pile. The Read Free Foron Days Sign up to vote this title  the l changes and the subsequent changes during Not in useful isUseful  foundation very significant the case of pile fou Cancel anytime. One of the difficulties in directly using the theori mechanics in the estimation of the load carrying c

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Construction of Bored Cast-in-Situ Piles Using Rotary Drilling Procedure—A C

component, the overall factor of safety for the total resistance and a ‘limiting settlement’ stipulation decide the safe capacity of a pile. However, application of partial factor of safety for friction component had to be done carefully after ensuring that the such friction component is not severely altered by the construction procedure itself. This shall also stipulates a very healthy pile tip bearing so that the total settlements are within the limits.

followed by very dense residual sand layer, very w significantly weathered granitic gneiss (Charnoc then by weathered jointed rock of same type. The th weak marine deposit varied from couple of meter than ten metres and comprises soft marine clay an very loose clayey sand. The thickness of very dens sand varies between less than a metre to five met the significantly weathered weak rock is for one three metres. Significantly jointed and partly weath follows. Hard rock with good RQD is available on significant thickness of jointed rock. The ground w is always at very shallow depth and the natural grou are such that the sites are waterlogged during Considerable thickness of filling to reach the finish level and very shallow ground water table call for deep foundation for supporting multi-storeyed con even if the highly weathered rock with very go strength is available at reasonably shallow depth.

Bored cast-in-situ pile construction in India was mainly concentrated in Mumbai region and to some extent in Madras. While most of the bridge projects used bored cast-in-situ piles, driven piles were being commonly used for industrial, commercial and large residential projects. The cities have grown and the awareness and intolerance to the noise pollution increased, pushing the construction of driven piles towards more isolated project sites. The conventional bentonite bored cast-in-situ piles were accepted as the most reliable piling system for most of the projects. The expressions still being practiced in India for the estimation of safe capacities of bored cast-in-situ piles are recognising the benefits and the 3. THE SITE SPECIFIC SUB-SOIL PROFILE THE DESIGN OF BORED CAST-IN-SITU P adverse effects of bentonite mud circulation employed in the borehole construction. The procedures for maintaining the The site under review has the average sub-soil profile consistency of drilling mud for allowing effective tremmie in Table 1. The elevation of very dense residual sa concrete operations are reasonably well established. You're Reading a Preview to some extant while the variations in the elevation During the last six to eight years, the constructions increased weathered jointed rock were significant. Unlock full access with a free trial. significantly and the demand for much faster installation of bored cast-in-situ piles also increased. Several hydraulically Table 1: Subsoil Profile of the Site operated rotary drilling machines were imported for satisfying With Free Trial Download Depth range Soil description and N such demand. Chennai also witnessed a very rapid growth in  the construction of bored cast-in-situ piles using the rotary Read Free For 30 Days Sign up to vote on this title grey or greyi GL to 0.50 m Yellowish  drilling machines. The conventional DMC chisel boring that brown silty clay, dry at Not useful  Useful  was being successfully adopted in the construction of bored Cancel anytime. cast-in-situ pilesOnly became unpopular and pushed to small Special offer for students: $4.99/month. 0.50 m to 1.50 m Greyish brown soft to projects. There was also a tendency to use larger design stiff clay, N = 1 to 10


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Construction of Bored Cast-in-Situ Piles Using Rotary Drilling Procedure—A Case Study

Based on the subsoil profile, bored cast-in-situ piles resting weathered jointed rock was assumed as 1000 kP in highly weathered and jointed rock using DMC chisel bearing estimation and 1200 kPa for skin friction boring method was recommended. The piles were to be from the same layer. penetrated just 0.50m to 0.60 m into the highly weathered However, when the installation of piles were jointed rock realising significant skin friction from dense to hydraulically operated piling rigs were available e very dense residual sand above apart from the end bearing the designer decided to adopt larger safe capa from highly weathered jointed rock. Typical estimation of penetrating the highly weathered jointed rock fully safe compression capacity using the procedure suggested by terminating the pile in weathered jointed rock. The Cole & Stroud (1977) was adopted for the design of piles. capacities adopted in the design was 2850 kN for The summary of estimation is given in Table 2. The diameter piles and the design safe capacity of recommended pile termination is presented in Figure 1. diameter pile was 4350 kN. The estimated safe ca longer piles is given in Table 3. The average shear s Table 2: Safe Capacity Estimation for Piles with Limited the weathered jointed rock was assumed as 1500 kP Penetration into Highly Weathered Jointed Rock (0.50 m) bearing estimation and 1700 kPa for skin friction from the same layer. The recommended pile terminat Pile diameter, mm 800 1000 piles with higher safe capacities is shown in Figure Ultimate end bearing, kN 4520 7070 Ultimate skin friction from very dense soil, kN

690

860

from highly weathered jointed rock, kN

Table 3: Safe Capacity Estimation for Piles wit Penetration into Highly Weathered Jointed Rock ( and 0.50 m Penetration into Weathered Jointed

300

380

Pile diameter, mm

total ultimate skin friction, kN

990

Safe end bearing with FS = 3 kN

1510

Safe skin friction with FS = 1.75 kN Total safe capacity, kN

570 2080

1240

Ultimate end bearing, kN

2360

Ultimate skin friction

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from very dense soil, kN Unlock 710full access with a free trial. 3070

Download With Free Trial

Master your semester with Scribd & The New York Times Overall factor safety

2.65

Pile

0.00m

Desiccated clay layer,

SPT N = 5 Special offer for students:0.80m Only $4.99/month.

from highly weathered jointed rock, kN

2.71

from weathered jointed rock, kN total Read ultimate skin friction, kN Free For 30this Days Sign up to vote on title

Safe end FSNot = 3useful kN Usefulwith bearing Cancel anytime.

Safe skin friction with FS =1.75 kN Total safe capacity, kN

800 6790 690 720 320

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1730 

2260 990 3250

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The design capacities are marginally less than the above estimated capacities and this was to account for small differences in the thickness of dense sand and the highly weathered jointed rock. However, the construction drawings did not specify the revised bearing stratum and the criteria to ensure the bearing stratum.

5. INITIAL LOAD TESTS AND OTHER INVESTIGATIONS

One initial load test was conducted on 800 mm diam The load settlement curve obtained from this initia is presented in Figure 3. Load, tonnes 0

4. PILE EXECUTION AND PERFORMANCE 0

100

200

300

400

More than 600 piles, both 800 mm diameter and 1000 mm diameter, were installed for supporting the proposed G+10 5 commercial building with large column loads. The piles were in groups of 4 piles to 10 piles. Hydraulically operated rotary piling procedure was adopted in the execution of these piles. m10 , t The boreholes were advanced using augers fitted to the n e m rotary stem and the soil cuts were frequently removed from e l t t 15 the borehole by withdrawing the auger and then re e S introducing it for further advancement. Bentonite mud and bentonite powder were periodically introduced for stabilising 20 the borehole sides and bottom. Temporary casing was provided to a depth of roughly 7.0 m from the working level. 25 The ground water table was at about 1.50 m below the Fig. 3: Load-Settlement Curve for Test Pil ground level during the construction of piles. The soil cuts obtained from the auger blades were collected and identified You're large Reading Preview Thea test load was planned as 638.4 tonnes (2.25 for establishing the pile termination depths. Relatively size stones (pieces of weathered rock) were collected by safe capacity 285 tonnes). However, the test was di Unlock full accessafter with a freetonnes trial. even though the pile was initial tes 427 segregating large chunk of soil retrieved from the borehole. The depth of pile thus executed was 10.50 m with respect to load settlement curve will show that after 23 mm recorded up to 342 tonnes, the pile refused to mo the typical profile shown in Figure 1. Download With Free Trial under next load increments. The settlement of p The cut off level for these piles was decided as 2.0 m to 3.0 427 tonnes load was 23.36 mm, about 0.40mm only m below present GL and the steel and concrete was provided Free For 30 Days Sign up to vote this title tonne loadRead increment. Theon reason for discontinuing  up to a depth equal to 1.0 m above the proposed cut-off level. spite of almost nil incremental settlement is not kn Useful  Not useful  The concrete as per the theoretical quantity was poured. It anytime. behaviour of the Cancel test pile was explained as soft muc Special offer for students: Only $4.99/month. was reported that most of the piles consumed more than one between the pile tip and the borehole bottom. Furthe cum concrete to make the concrete up to the stipulated level. this load test data will be given later in the paper.


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range of 100 tonnes to 120 tonnes against 285 tonnes design capacity used for 800 mm diameter piles.

Fig. 4: PIT Results for Two Defective Piles

level. Comparing the static load test result and the pil test, there is a possibility that the pile have an section at about 7.50 m below the working level, under-reaming. The soil at this depth is dense resi derived from complete weathering of granitic gne or less static dial gauge readings over a loading of m 80 tonnes should suggest failure of the measuring sy

Another practical possibility here is the absence o round the reinforcement for some length allo reinforcement to buckle outward resulting large dis while supporting a significant amount of load. How load settlement curve clearly suggests development amount of friction before transferring load to the rein bars. But the surface area to develop about 120 tonn should extent well into the very dense stratum possibility of reinforcement buckling is fairly rem particular case.

6. ANALYSIS OF INVESTIGATION DATA

The pile integrity test results further established tha Many studies have established that the mobilisation of almost were resting only in the highly weathered and fract full skin friction needs only a very small movement of the rock and not in the weathered jointed rock as stip pile, say 4 mm to 6 mm. Thus the skin friction mobilised here the revised safe capacities. This inference is made is roughly 120 tonnes. However, if the assumption of having pile lengths established by the pile integrity tests. soft muck between the pile tip and the good bearing stratum, earlier, the construction drawings did not clearly spec You're Reading a Preview the pile has to move further without developing any significant bearing stratum and the criterion for ensuring resistance. A most probable load settlement curve forfull such Unlock accessstratum. with a free trial. pile may be as illustrated in Figure 5. Load, tonnes 0

100

200

300

7. PROBABLE CAUSE OF THE DEFECTS

400

Download With Free Trial 500

Master your semester with Scribd & The New York Times 0

5

Field curve Series1 Series2 curve with Probable soft muck at tip

Special offer for students: Only $4.99/month. m m 10

As stated earlier, there was an observation of concrete down by about 2.0 in 30 tothis 40 minutes after the c Read Free For 30 Days Sign up tomvote on title  and removing the temporary casing. The conclusio Useful Not useful   concrete was flowing into the soft sandy clay Cancel anytime. clayey sand at bout 4.0 m below natural ground lev confirmed by the pile integrity tests. Assuming a

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the removal of the casing pipe can only suggest that this part of ‘excess consumption’ took place above the temporary casing tip level.

The final shape of the pile expected under the above is also presented in Figure 6.

The rectification measures included construction piles, almost 130 percent of the original number. additional piles were being installed close to th piles, the cutting tool hit the concrete outgrowth the existing piles confirming the above hypothesis.

These piles were executed using rotary drilling which had no provision for continuous mud circulation. The bentonite mud was supplied intermittently as the auguring advanced. When the step by step procedure of the pile construction operation is reviewed, one would realise that the cutting tool, auger in this case, was taken out of the borehole several times for 8. CONCLUDING REMARKS removing the soil cuts. When the auger along with the soil cuts between the fins of the auger was being lifted, a suction There were several other cases of underperformanc was created below the auger bottom which produces cast-in-situ piles reported from the same region wh tremendous upward pressure within the borehole. Moreover, drilling procedure was adopted. Rotary drilling ma a significant amount of water and drilling mud poured from such cannot be blamed for this failure, since such the top were brought out of the borehole creating excess working with success in many places. The overall hydrostatic head outside the borehole. The dense to very adopted in these cases shall be defective. The maj dense residual sand present at the casing tip level being highly for the defects can be identified as improper stabil permeable, sand blow condition took place. At the same time, the borehole sides and bottom during different boring. the soil just above the temporary casing tip flowed downward creating a cavity around the temporary casing at least for some In this case, the piles were also not taken to the length. This cave like annular was filled with ground water or bearing stratum because of the difficulties in ident drilling mud which had no escape towards sides or upward. The bearing stratum during boring process. The tempora process continued creating larger cavity around the casing provided up to an intermediate depth resulted mi You're Reading a Preview pipe and loosening the dense sandy soil till the completion of around the casing tip and below the casing becau the boring operation. The loosening during the boring process is hydrostatic imbalance created within the boreho Unlock full access with a free trial. frequent withdrawal of the cutting tool as a pa described in Figure 6. construction programme. During the tremmie operation keeping the temporary casing Download With Free Trial in place, the water/drilling mud within the cave portion Very dense residual sand and the highly weath around the temporary casing pipe had no escape leaving the significantly fractured rock below can offer  tremen Read Free For 30 Days Sign up to vote on this title cavity as it is. When the casing pipe was withdrawn allowing friction for the piles resting in the jointed rock  bel being highly permeable and the ground w gradual escape for the trapped water/drilling mud along the formations  Useful  Not useful Cancel anytime. soil-concrete interface allowing the green concrete from the being very shallow, the temporary casing shall adva Special offer for students: Only $4.99/month. pile to flow into the cavity. The concrete in the borehole boring and should penetrate up to weathered where sand mining is not expected. The cutting


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Use of temporary casing to intermediate depths will lead to the hazard described above. Another method to stabilise the sides and bottom of the borehole is to provide continuous mud circulation during the drilling process avoiding frequent withdrawal of the cutting tool. Pouring of drilling mud intermittently can be effective only if the mud level in the borehole is always maintained much above the ground water table.

very light and not capable of handling the large v mud in the case of large diameter piles, the mu limited to small diameter around the DMC pipe. result suspension of large size particles at deeper l will readily settle at the borehole bottom when the c is stopped.

It is to be noted that the construction of bored c piles using rotary drilling procedure has several The system of collection of representative samples from over the conventional DMC chisel boring proce different depths along the pile bore depth need to be construction is three to four times faster and the pile modernized. The wash samples collected from the cuttings and length can be larger for realising large safe cap will not be representing the actual formation. Thick tube also has the advantage of proper quality control c sampler driven to the founding stratum can bring representative when provided with advanced gadgets like torque me samples in the case of highly weathered and fractured rock system, etc. And of course the necessity of a trained bearing stratum. There are gadgets available for measuring the cannot be underestimated. resistance to cutting that can reasonably identify the bearing stratum. REFERENCES

The indications to improper boring operations usually come Cole, K.W. and Stroud, M.A. (1977). “Rock Sock from local subsidence of the ground, extra or short consumption, at Coventry Point, Market Way, Coventry”, Pro difficulty in lowering and positioning the reinforcement cage, in Weak Rock, Institution of Civil Engineers, Lo etc. Extra consumption at any stage can result tremmie lifted 47–62. above the concrete. This can readily be observed by sudden S.R, Raju, V.S. and Rawat, P.C. (1987). “Loa rise of drilling mud/water in the tremmie. It isYou're essential to Gandhi, Reading a Preview in Piles—Analysis and Field Test on Instrumen observe the concrete level in the tremmie after every lifting Indian Conference, IGC 1987 operation. Unlock full access with a free Geotechnical trial. pp. 151–154. The non-performance of large diameter bored piles constructed Meyerhof, G.G. (1956). “Penetration Tests and using the conventional DMC chisel boring is mainly due to With Download Free Trial Capacity of Cohesionless Soils”, Journal improper use of the drilling mud and the inferior quality of Mechanics and Foundation Division,  ASCE, Vo the mud itself. This is true in the case of rotary drilling Read Free For 30 Days Sign up to vote on this title SM1, pp. 866–1–19. equipments also. Substitutes for bentonite mud are now 

Master your semester with Scribd & Theavailable NewthatYork Times can offer stability under different sub-soil conditions. When the$4.99/month. pumps used for mud circulation are Special offer for students: Only

useful  Useful  Not Tomlinson, M.J.Cancel (1981). “Pile Design and Co anytime. Practice”, Viewpoint Publication, p. 119.

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Construction of Bored Cast in Situ Piles Using Rotary Drilling Procedure a Case Study

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