A Record of Pile Load Test and Ground Improvement within the Coastal Plain Sands, Southeastern Nigeria

 

G. A. Akwagiobe

Civil Engineer, Cross River State Water Limited, Calabar Nigeria.

J.C. Becket

Project Civil Engineer For SGI consulting.

and

A.O. Ilori

Civil Engineer, Federal Airports of Nigeria. Civil Engineering Department,
Calabar Airport, Calabar. Nigeria
e-mail bidemiini@yahoo.com

 

ABSRACT

A maintained load test was carried out on randomly selected grouted small diameter steel end piles at a water treatment plant site in Calabar, southeastern Nigeria. At a load of 75tons, which is about one and half times the design working load of the pile, settlement value obtained was 3.75mm and 2.95mm for the filter and clarifier respectively. The maximum estimated settlement for the pile group based on Skempton’s relation is 37.8mm thus satisfying the designed permissible settlement of 60mm allowed for the structure. Settlement of the structures monitored over eight months indicates values between 6mm to 21mm.

INTRODUCTION

Every piling works should normally include a load test carried out to estimate limiting values for safe working load and settlement on piles proposed to be installed. Although

Piling works is not very common within the area being reported upon especially for building structures except for long bridges across some of the major rivers in the locality. The record of a load test programme is therefore thought to be a valuable asset for practicing engineers within the locality; serving as reference as there is not much in published works; also a comparison of similar load test within the Coastal Plain Sands from other parts of the world is welcome.

 

LOCATION AND SITE DESCRIPTION

At a water treatment plant site located in Calabar, Southeastern Nigeria,(Calabar has a geographical coordinates of 040 55’ -050 55” N and 0800 15’-0800 25’ E) ,two of the treatment units (filter and clarifiers 3 in number) have to be founded on small diameter end bearing grouted piles. The site in question is underlain with what is known as the Benin Formation, a geological Formation locally referred to as the Coastal Plain Sands. The Benin Formation, which is Tertiary, is the terminal stratigraghic unit of the three Formations that make up the Niger Delta. Its type section according to Allen (1967) is made up of fine-grained, poorly cemented sands and gravels with clay and shale intercalation. The sands are sub-angular to well-rounded; although there are local variations of the type section. Calabar generally is underlain with a water table aquifer with water table averaging 54.0m below the surface.

The filter and the clarifiers (three numbers) were to be reinforced concrete. The filter structure is 79.10m x 19.7m x 4.0m high with uniform wall thickness of 0.4m and imposes a surcharge of 60kPa. The clarifiers each with a diameter of 40m, 5.0m high with center of each located at the apex of a common triangle.. Each clarifier imposes a load of 58kPa. Due to the presence of compressible clayey sand and silty clay on the site the two treatment structures are to be founded on piles.

From few exploratory drillings carried out, the small diameter Tubix piles, grouted, were to be founded on gravel bed, which were deposited with clayey sand. This bearing stratum occurs at two significantly different depth ranges on the site. The first ranges between 6.0-13.0m from the ground surface. Within this depth range the gravel bed varies in lateral continuity and vertical thickness around the site. The second depth range is at around 23.0m. This resulted in piles of different lengths for different parts of the filter and clarifiers. Based on the above, the filter structure has six zones (Table1)

 

 

Table 1. Pile lengths for different zones
of the filter structure
Zone Size of zone
(m x m)
Pile length
(m)
A 10.58 x 19.35 9
B 5.95 x 67.50 11
C 14.0 x 67.00 13
D 2.60 x 2.40 11
E 7.05 x 19.75 11
F 0.40 x 12.60 6

 

All piles were founded in first depth range, and pile lengths ranges from 10.0-13.0m

The average cone resistance qc, for this gravel layer from cone penetration campaign around the site is 12MPa.

TUBIX MICRO-PILE DESIGN

For the pile design, the filter load was employed. The expected total load from the filter structure is 85,704.85KN. Axial pile capacity determined for both drained and undrained condition considering skin friction and end bearing resistance is estimated at 604KN.

Buckling analysis of micro pile using a design length of 23.0m, axial load of 604KN,and lateral pressure, gives a buckling load of 425KN(42.5tons).

For the pile group a pile inter-axis of 3.0m is assumed with a radial axis of 2.60m.

Grouting of the soil surrounding each pile was to be carried out; this essentially was to reduce the void ratio in the soil. Grouting pressure was determined by estimating the lateral pressure surrounding the pile and a mortar core. The grouting pressure was made equal to the lateral pressure with a maximum of 10bars for the longest pile.

With a buckling load of 425KN,and a total load of 86,000KN, a total of 202 micro-piles will be required. The spatial arrangement of micro-piles using the earlier specified radial axis leads to a total of 253 piles for the filter structure.

PILE INSTALLATION PROGRAMME

For each zone of the filter and clarifier units dry drilling using with a auger was employed. 150mm diameter holes were bored to the bearing stratum. Small diameter steel piles (micro-Tubix piles, Table 2) with two holes made on opposite sides of the piles at

every 0.5m intervals along the length of each pile. Rubber tubing was used to cover the holes.

 

Table 2. Micro-pile and grout specifications
Borehole diameter 150mm
Length of boring 6-9-11-13m
Pile- steel grade 55
Steel pile external diameter/thickness 88.9mm/8.0mm
Minimum volume of grout 60 liters
Cement type Type1 (100kg)
Plasticizer agent 1kg

 

The piles were then lowered into drilled holes. The hole drilling and installation of piles were carried out in different zones. After the piles have been lowered, bentonite slurry with a setting retarder-‘Rheobuild 501’ was poured down each hole around the piles to hold the pile in position until grouting. The retarder prevents the bentonite slurry from setting until grouting material, which displaces the slurry, is forced into place.

A packer device placed inside the piles was systematically used to break the rubber ‘seal’ enclosing the perforated holes starting from the bottom part of the pile upward. The packer device ensures that only one rubber is broken at a time, thereafter grouting of that length of the pile is carried out under pressure into the pile hole, the hole surrounding the pile, and the surrounding soil at that level.

Grouting at each of the hole was carried out at pressures between five to six Bars.

When grouting reaches ten Bars or 6oliters of grouting materials was used at a level, the grout is considered to have filled and fully penetrate the available spaces around the pile and voids in the surrounding soil at that horizon. Further grouting could lead to a blow out .The pressure of 10 Bars or 60 liters of grout volume is therefore considered as refusal.

A total numbers of 253 steel piles were install for the filter using the procedure describe above.

The pile heads were at about 400 mm above ground level. Near the top were welded brackets, which serves to carry the top reinforcement for the structures.

THE PILE LOAD TEST

A pile load test was conducted about six weeks after the commencement of pile installation. ‘A maintained’ load test as against constant rate penetration test was carried on randomly selected piles. Load on piles for testing is usually one-half to two times the expected working load. For the piles under discussion the test load was between 50 and 75 tons.The typical maintained load test is described as follows:

“A block weighing 50 tons was carefully set up on supporting blocks in a rectangular pattern around the pile to be tested. A manual hydraulic jack was set up against the 50ton concrete block with the other end on a steel plate placed on the pile to be tested. Four dial gauges placed on a relatively flat surface were set up in a rectangular fashion against the steel plate placed on the pile such that the initial readings of all the dial gauges were at zero. The hydraulic was first loaded to 15tons and subsequently in steps of ten tons up to 45tons. For each load the four dial gauges readings were noted at every five minutes, with readings averaged to give a settlement value. Each load was maintained for 20minutes giving four settlement readings at each load increment or reduction (the unloading cycle). A second cycle of loading was carried out by adding another 50ton concrete block weight, a new cycle of loading was repeated in the manner earlier described (that is in steps of 10tons) but with a load peak of 75tons.The 75 ton load was maintained up to five hours at which the dial gauge readings (both individual and the average remains practically the same. An unloading cycle in steps of 10 tons was also carried out.

RESULT AND ANALYSIS

Tables 3 and 4 present representative settlements readings of the load tests for the filter and one of the clarifiers respectively;

 

Table 3. Representative readings for pile load test for a pile in zone ‘c’ of filter
Dial gauge readings (mm)
Time Load (Tons) (1) (2) (3) (4) Mean of dial gauge
readings (mm)
8:55 15. 0.84 0.76 0.43 0.39 0.605
9:15 25 1.54 1.40 0.80 0.83 1.142
9:35 35 2.11 1.94 1.17 1.23 1.612
9:55 45 2.70 2.53 1.53 1.62 2.095
10:15 55 3.23 3.04 1.87 1.98 2.530
10:35 65 3.23 3.04 1.87 1.98 2.530
11:55 75 4.41 4.20 2.77 2.95 3.582
13:55 75 4.47 4.28 2.82 3.01 3.645
14:55 75 4.50 4.30 2.85 3.06 3.677
16:05 65 4.40 4.15 2.76 3.01 3.580
16:15 55 4.18 3.87 2.57 2.83 3.262
16:25 45 3.85 3.61 2.40 2.60 3.115
16:35 35 3.45 3.23 2.18 2.35 2.802
16:55 15 1.94 1.80 1.18 1.25 1.542

 

Table 4. Representative readings for a test pile at the clarifier location
Time
Load Dial gauge readings (mm) Mean of dial gauge
readings (mm)
(1) (2) (3) (4)
10:20 15 0.73 0.41 0.28 0.62 0.51
10:40 25 1.04 0.65 0.61 0.93 0.808
11:00 35 1.30 0.86 0.86 1.17 1.050
11:20 45 1.60 1.19 1.16 1.45 1.333
11:40 55 1.90 1.38 1.42 1.71 1.603
12:00 65 2.16 1.72 1.76 2.07 1.928
12:20 75 2.89 2.34 2.39 2.59 2.553
14:00 75 2.24 2.67 2.70 2.92 2.895
16:00 75 3.30 2.74 2.74 3.01 2.950
17:10 65 3.21 2.66 2.63 2.96 2.865
17:20 55 2.99 2.48 2.42 2.76 2.663
17:30 45 2.73 2.25 2.19 2.51 2.420
17:40 35 2.42 1.99 1.86 2.30 2.143
17:50 25 2.05 1.62 1.48 1.88 1.756

 

Table 5. Monitored settlement values for different parts of the filter and clarifier Structures
Filter PositionSettlement (mm) Clarifier A Clarifier B Clarifier C
Clarifier A PositionSettlement(mm) Clarifier B PositionSettlement(mm) Clarifier C PositionSettlement(mm)
T1 6 A1 18 B1 13 C1 21
T2 3 A2 16 B2 13 C2 20
T3 4 A3 16 B3 18 C3 19
FN1 1 A4 7 B4 16 C4 13
FN2 5 Top 22 Top 9
FN3 0
FN4 2
FN5 2
FN6 5
FN7 6
FN8 1

 


Figure 1. Load-Settlement curve for micropile in zone 'C' of Filter Structure

 


Figure 2. Load-Settlement curve for micropile in Clarifier 'B' Structure

 

Figures 1 and 2 show the load -settlement curve for the load tests. The 75 tons load represents one and half times the expected working load of each piles. For the pile group, Tomlinson (1980) stated that for most engineering structures the load to be applied to the group is usually determined by settlement consideration rather than from ultimate carrying capacity of the group divided by an arbitrary factor of safety. Fellenius (2001), in a similar vein argued that settlement should form the basis of determining pile capacity. The pile load test gives such information on settlement. Estimating settlement reliably from pile load tests is somewhat difficult due to time effects and group action. However, empirical relationships for example Skempton et al (1953), gives an expression from which settlement of pile group (driven or bored) in cohesionless soil can be estimated. It predicts a settlement value of 2-16 times the settlement of a single pile.

The Skempton relationship as reported by Broms (2001), can be written as

(1)

Settlement Monitoring

Settlements of the filter were monitored over a period of eight months (November, 2000 to June, 2001). Table 5 presents the observed settlement values for the different parts of the structures. The maximum settlement obtained for the filter over this period was 6 mm at location ‘T’ (Figure 3) while the clarifier records the maximum of all the settlement values over this period at 21 mm at point ‘C1’ on clarifier C.

 


Figure 3. Positions of settlement monitoring in Clarifiers and Filter Structures

 

Table 5. Monitored settlement values for different parts of the filter and clarifier Structures

Filter PositionSettlement (mm) Clarifier A Clarifier B Clarifier C
Clarifier A PositionSettlement(mm) Clarifier B PositionSettlement(mm) Clarifier C PositionSettlement(mm)
T1 6 A1 18 B1 13 C1 21
T2 3 A2 16 B2 13 C2 20
T3 4 A3 16 B3 18 C3 19
FN1 1 A4 7 B4 16 C4 13
FN2 5 Top 22 Top 9
FN3 0
FN4 2
FN5 2
FN6 5
FN7 6
FN8 1

 

DISCCUSSION AND CONCLUSION

No doubt many pile load tests have been carried out and published, but post construction monitoring on them are not very common; however values of settlement reported by Da Costa Nunes and Vargas(1953), showed pile group settlement of between ten to fifteen times the settlements of a single pile from load tests. Fuller and Couper (1955), reports a settlement of between four to seventeen times that of a single pile for the Dartford creek Bridge in Britain, although Skempton et al’s equation predicts a ten times value of settlement of a single pile. For the pile being reported upon the settlement observed so far is about seven times the settlement of a single pile. The ultimate settlements of the pile groups for the structures will only be evident with passage of time; though design called for a maximum allowable settlement of 60mm for the structures, it is doubtful if such a value will be reached during the useful life of the structures. However if pile load test is to be evaluated based on settlement the question arises to what load should a pile test be carried to that will adequately indicate settlement value that will be considered safe for expected structure? It is the authors’ opinion that a load two times the expected working load of the pile should be at least attained during pile test with corresponding settlement used to predict pile behaviour.

 

REFERENCES

 

ACKNOWLEGEMENT

The lead author wishes to express his appreciation to the Cross River state water board and SGI consulting (Nigeria) for the opportunity given to him to participate in the project as one the site engineers.

 

© 2002 ejge