Islamic Univerisy Journal
Vol. 7 No. 1, 1999
Lateral Load Tests on Mini-pIles
DR. Mohammed Awad Department of Engineering Islamic University -of Gaza
ﻤﻠﺨﺹ ﺍﻟﺒﺤﺙ 15
DR. M. Awad
Lateral Load Tests on Miniplles
ﺘﻡ ﺇﻋﺩﺍﺩ ﺒﺭﻨﺎﻤﺞ ﺨﺎﺹ ﻟﺩﺭﺍﺴﺔ ﻗﺩﺭﺓ ﺍﻟﺭﻜﺎﺌﺯ ﺫﺍﺕ ﺍﻷﻗﻁﺎﺭ ﺍﻟﺼﻐﻴﺭﺓ ﻭﺍﻟﺘﻲ ﺘﻡ ﺇﻨﺸﺎﺅﻫﺎ ﻓﻲ ﺍﻟﻤﻭﻗﻊ ﻋﻠﻰ ﺘﺤﻤل ﺍﻟﻘﻭﻯ ﺍﻷﻓﻘﻴﺔ ﻤﻥ ﺨﻼل ﻤﺸﺭﻭﻉ ﺴﻜﻨﻲ ﻓﻲ ﻤﺩﻴﻨﺔ ﺴﺭﺕ ﻤﻴﻼﺩﻱ ﺤﻴﺙ ﺍﺸﺘﻤل ﺍﻟﺒﺭﻨﺎﻤﺞ ﻋﻠﻰ ﻓﺤﺹ ﺭﻜﻴﺯﺘﻴﻥ1993 ﻭ1992 ﺒﻠﻴﺒﻴﺎ ﻤﺎ ﺒﻴﻥ ﻋﺎﻤﻲ .ﺴﻡ20 ﻡ ﻭﺒﻘﻁﺭ10 ،8 ﺒﻁﻭل ﻟﻘﺩ ﺒﺩﺃ ﺍﻨﺘﺸﺎﺭ ﺍﻟﺭﻜﺎﺌﺯ ﺫﺍﺕ ﺍﻷﻗﻁﺎﺭ ﺍﻟﺼﻐﻴﺭﺓ ﻓﻲ ﺍﻟﻌﻘﺩ ﺍﻟﺨﺎﻤﺱ ﻤﻥ ﻫﺫﺍ ﺍﻟﻘﺭﻥ ﻴﺤﺘﻭﻱ ﺍﻟﺒﺤﺙ ﻋﻠﻰ ﻁﺭﻴﻘﺔ ﺇﺠﺭﺍﺀ ﺘﺠﺭﺒﺔ ﺍﻟﺘﺤﻤﻴل ﺍﻷﻓﻘﻲ ﺒﺎﻹﻀﺎﻓﺔ.ﻓﻲ ﺍﻟﺒﻠﺩﺍﻥ ﺍﻷﻭﺭﻭﺒﻴﺔ ﻭﻟﻘﺩ ﺃﻜﺩﺕ ﺍﻟﺩﺭﺍﺴﺔ ﻋﻠﻰ ﺠﻭﺩﺓ.ﺇﻟﻰ ﻁﺭﻴﻘﺔ ﺇﻨﺸﺎﺀ ﺍﻟﺭﻜﺎﺌﺯ ﻭﺼﻔﺎﺕ ﺍﻟﺘﺭﺒﺔ ﻓﻲ ﺍﻟﻤﻭﻗﻊ ﻨﺘﺎﺌﺞ ﺍﻟﺘﺠﺭﺒﺔ ﻭﺒﻤﻘﺎﺭﻨﺘﻬﺎ ﻤﻊ ﺍﻟﻨﺘﺎﺌﺞ ﺍﻟﻤﺤﺴﻭﺒﺔ ﻤﻥ ﺍﻟﻤﻌﺎﺩﻻﺕ ﺍﻟﻤﺨﺘﻠﻔﺔ ﺘﻡ ﺘﺤﺩﻴﺩ ﺍﻟﻁﻭل .ﺍﻷﻤﺜل ﻟﻠﺭﻜﺎﺌﺯ ﻟﺘﺤﻤل ﺍﻷﺤﻤﺎل ﺍﻷﻓﻘﻴﺔ ﺍﻟﻤﻁﻠﻭﺒﺔ ABSTRACT An investigation on the lateral bearing capacity of Mini piles in Sirte city (Libya) was made, in which a special load test program was conducted on Mini piles of 8 m to 10m in length and 20cm in diameter to evaluate the resistance of Mini piles to lateral forces. This type of piling was used in the 50’s in European countries. In this study , a simplified test procedure for the measurement of horizontal load and displacement was developed, and described in simple terms for easy performance. This study describes also the soil conditions, the pile installation, and evaluates the results. The results indicate that this test procedure is reliable and useful. The relation between the lateral load value and the required pile length is studied.
INTRODUCTION A new project for 7000 inhabitants was built in Sirte city which is situated at the center of Libya in the Mediterranean sea. The 16
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Vol. 7 No. 1, 1999
investigation area was the central area surrounded by the old coastal highway and the inner ring road. This project included housing and buildings for common use. Pile foundations were needed in some parts, where sandy lean clay and medium dense silty sand existed at different depths. Small diameter friction piles were used in this part of the project. Mini piles are used in the same manner as the conventional piles in combination with caps. They can also be used to reduce excessive settlement (Weltman 1981). In the load test program vertical and lateral loading tests were performed to study the efficiency of this type of piles. In this study the lateral capacity of the Mini pile is evaluated. GROUND CONDITIONS The main feature of the site is the distribution of silty sand. The soil consists mainly of fine to coarse sand with silt and traces of clay. The soil investigations which were conducted by Temel Co. (1992) included about 50 bore-holes, Standard Penetration Tests (SPT), test pits, dynamic penetrometer tests (DIN 4094); which consist of a cone driven vertically into the subsoil by 10kg hammer with 50 cm constant free drop height and the number of blows required for 10cm penetration is recorded, plate loading tests, and laboratory analyses of obtained samples. Some variation of sub soil conditions has been found. Typical simplified soil profile is presented in Figure (1). The ground water level during dry season ranges from about 0.25 to 0.50m below the ground surface. The chemical and physical tests are performed for different soil layers for which, the soil properties are listed in the following table.
Table (1): Geotechnical properties: 17
DR. M. Awad
Lateral Load Tests on Miniplles
pH Chloride content Total sulphate content S03 Soluble sulphate content S03 Internal angle of friction Specific gravity Average field dry density Buoyant unit weight Ground water Properties:Chloride contents pH
8.2 - 9.1 0.025 - 0.1% 0.21 - 19% 0.014 - 1% 25o 2.65 16 KN/m3 9.96 KN/m3 1 - 6.2 g/l 8 - 8.3
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Islamic Univerisy Journal
Vol. 7 No. 1, 1999 SPT, (Blows)
60
50
40
30
20
10
0 0.00
0
0.50m Ground water table
Made ground 1.50 Sandy lean clay
5
Depth, (m)
6.5
Silty sand medium dense to dense
10
15 18.00 Calcarenite 20
Figure (1) : Simplified soil profile.
High total sulfate content comes from the presence of gypsum in top soil deposits. Sample descriptions show a stratigraphic sequence containing three major strata; from top to bottom: about
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Lateral Load Tests on Miniplles
1.5m thick made ground loose to very dense silty sand, and calcarenite layer which is about 18m deep.
PILE INSTALLATION Mini piles have been installed to depths ranging between 7 and 11 meters and have performed excellently. Lateral load tests were performed on two of them which are 8m and 10m in length and 0.2m in diameter. The Mini piles were designed in accordance with the specifications for construction of Mini piles published in Ground Engineering Journal, (1987). In this project water was used as a drilling fluid together with a flight auger. The water carries away cuttings and fine soil to the ground surface level. A fter forming the hole, the steel reinforcing bars together with spacers and the injection tube are placed in the hole. A cement suspension grout is pumped into the hole through the injection tube which reaches to the bottom of the hole. The injection tube has staggered slots spread every 0.5 m in the vertical direction of the tube. The injection is usually carried out starting from the hole base which reduces the possibility of necking in the pile shaft. In granular or fissured ground, grouting increases the frictional resistance of the pile. The used mixes of pure cement suspension have a water cement ratio ranging from 0.4 to 0.5 with a high cement content. The pile construction technique is shown in the Figure(2). The grout has a strength of 30 MN/m2 and prepared by using a sulphate resisting cement. LATERAL LOADING TEST In order to analyze the lateral force effect on the Mini pile, two load tests were performed on the cast-in-place Mini pile. The usual method of carrying out lateral load tests is to use a pair of piles and 20
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Vol. 7 No. 1, 1999
jack their heads apart. A new arrangement was developed in this study. A schematic diagram of horizontal loading test setup is shown in Figure (3). The lateral deflection of the pile head is measured with a dial gauge at the same level of the applied load. The reaction test weights are placed in an excavated pit up to 0.5 m depth which makes a suitable support to the hydraulic jack which was 98 cm away from the pile surface. The hydraulic jack was placed on two concrete supports. Loads were applied in increments of
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DR. M. Awad
Lateral Load Tests on Miniplles
washing fluid
injection tube
a
a
cutting shoe
reinforcment bars injection tube
Section (a - a)
a. Drilling with external flushing
b. Installation of reinforcment & injection
Figure (2) : Mini pile construction technique
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c. Completed pile
Vol. 7 No. 1, 1999
125cm
Islamic Univerisy Journal
125cm
98cm
400cm Counter Weights
5cm Sand
Jack 50cm
3.5cm
98cm
Hydraulic jack support Mini pile
Figure (3) : Horizontal loading test setup
151 15% of the proposed horizontal design load with a constant time interval between increments of 20 minutes in which a small rate of movement (0.02 mm/5min.) is reached . Load increments were added 23
DR. M. Awad
Lateral Load Tests on Miniplles
until a continuous movement was recorded. In other words, the continuous movement at a constant load or the movement which exceeds 10% of the pile diameter , whichever is reached first, suggests a failure condition that should be considered seriously. Time, load and movement readings were recorded immediately before and after the application of each load increment. The maximum load was recorded when a continuous increase in movement occurred under constant load. The records of two horizontal loading tests are given in Table (2). The horizontal movement of the counter weights was recorded to check the system stability. As a result, horizontal Mini pile tests can be considered as a useful tool in evaluating the safe horizontal force. SAMPLE CALCULATION The method presented here is based on Broms (1964) assumptions: one: neglects the active earth pressure acting on the back of the pile, two: no influence of the pile shape on the ultimate soil distribution pressure, and three: the full lateral resistance is mobilized at the considered movement (Poulos et al 1980). The above Table (2) : Lateral load test results Mini pile of 10m in length
Mini pile of 8m in length Load (ton)
Pile movement (mm)
0 0 2.286 0.11 4.572 0.48 9.144 1.34 11.43 5.6 13.715 14.77 14.85 27.13 0 * 23.91 * Unloading Condition.
Counter weight movement (mm)
Load (ton)
Pile movement (mm)
Counter weight movement (mm)
0 0.02 0.04 0.16 0.56 1.24 1.63 1.24
0 2.286 4.572 6.858 9.144 11.43
0 0.04 0.31 0.53 2.87 5.17 10.44 19.2
0 0.00 0.23 0.38 0.48 0.50 0.50 0.50
13.715 16.001
assumed conditions of the deflection of the pile, the lateral pressure on the pile surface, and the bending moment distribution for long and short piles are shown in Figure (4).
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Vol. 7 No. 1, 1999
The first step is to check the assumption that the pile may be considered long. To do this, the ultimate horizontal load “Hu” is computed as: Hu
= 0.5 γ′ d L3 Kp/(e +L)
--------- (1)
Where = coefficient of passive earth pressure Kp = (1+sin φ′)/(1-sin φ′) φ′ = angle of internal friction. γ′ = effective unit weight of the soil L = pile length. e = eccentricity of load Hu d = pile diameter. For the free-head pile conditions, call the soil properties and pile dimensions: Kp = 2.5, γ′ = 0.996 t/m3, e = 0.0, d = 0.2m, L = 8m = 0.5(0.996)(0.2)(L)3(2.5)/(0+L) Hu = 0.249L2 = 15.94 tons
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DR. M. Awad
Lateral Load Tests on Miniplles
Hu e f L
g
Mmax
3γ dLKp Deflection
Soil Reaction
Bending Moment
a. Short pile
Hu e f
Myield Deflection
Soil Reaction
Bending Moment
b. Long pile
Figure (4) : Free-head piles in a cohesionless soil: (after Broms, 1964) 26
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Vol. 7 No. 1, 1999
The maximum moment occurs at a distance “f” below the surface, where = 0.82 √(Hu/(d Kp γ′)) = 0.82 √(0.249L2)/((0.2)(2.5)(0.996)) = 0.58L = 4.64m Mmax. = Hu (e+ (2/3)f) = (0.249L2) ((0.0+(2/3)(0.58L)) = 0.096L3 = 49.15 t.m
f
--------- (2)
Table (3) : The calculated values of ultimate horizontal loads for different pile lengths. Pile length (m) Hu (ton)
1
1.5
2
2.5
3
3.5
4
5
6
8
10
12
0.249
0.56
0.996
1.56
2.24
3.05
3.98
6.23
8.96
15.94
35.86
f (m)
0.58
0.87
1.16
1.45
1.74
2.03
2.32
2.9
3.48
4.64
M max. (ton.m)
0.096
0.32
0.77
1.5
2.59
4.12
6.14
12
20.74
49.15
24 .9 5. 8 96
6.96 165.9
The calculated value of Hu results in Mmax. > My (yield moment of the pile section ). Thus, the pile will act as a “long” pile and “Hu” may then be calculated from the two following equations: Hu = (3/2) γ′ d Kp f2 Mmax = My = Hu (e +(2/3)f) . Hu = 3/2 (0.996)(0.2)(2.5)f2 = 0.747 f2
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--------- (3)
DR. M. Awad
Lateral Load Tests on Miniplles
Then My = Hu (0.0+(2/3)f) and f = 3 My/2Hu Substitute the value of “f” in equation (3) Then: Hu = 0.747 (3My/2Hu)2 Then: Hu3 = 1.68 My2 Hu = (1.68 My2)1/3 f = (Hu/0.747)0.5 The ultimate horizontal load “Hu” has been obtained for different values of yield moments of the pile section “My” and are listed in the following table. Table (4): Calculated ultimate horizontal loads for different values of yield moments of the Mini pile section: My (ton.m) Hu (ton)
1.5 1.56
2 1.89
2.5 2.19
3 2.47
3.5 2.74
4 3
5 3.48
10 5.52
15 7.23
20 8.76
31.67 11.89
40 13.9
The foregoing example shows that after a limited pile length, the pile material strength becomes the main factor in determining the ultimate horizontal force that can be carried by the free-head pile. RESULTS ANALYSIS An analysis of pile test results and the calculated values was made. The data in Figure (5) and the given data in Tables (3 and 4) can be rearranged and compared to illustrate the relations between the calculated and the measured results. In this study (McNulty, 1956), criteria of the ultimate load definition is used in which the ultimate horizontal load is taken as the load required to produce a specified deflection (6.35mm). Using a factor of safety of 3; the allowable horizontal force will be 4 tones which is more than the code specified value (i.e. 10% of the applied axial load). In this project the working 28
Islamic Univerisy Journal
Vol. 7 No. 1, 1999
applied axial load was 28 tones. The value of 4 tones horizontal force corresponds to less than 0.5 mm deflection as shown in Figure (5)which is an acceptable deflection of working loads. Using the specified horizontal movement of 6.35mm, Figure (5) shows that the measured ultimate horizontal load for the Mini pile of 8m long is 11.89 tons and for the pile of 10m long is 11.62 tons. The comparisons of the analytical results show the influence of the pile length on the calculated value of the ultimate horizontal force as shown in Figure (6). For the used soil and pile material properties, the comparisons lead to the result that ultimate horizontal force can be calculated from the soil resistance distribution up to 6.91 m pile long , after this, it is noticed that the
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Islamic Univerisy Journal
Vol. 7 No. 1, 1999 Horizontal load (ton) 5 10
0
15
Horizontal movment, (mm)
0 5 (11.89,6.35)
10 15 20 25 30
a. Mini pile with 8m in length
Horizontal movement, (mm)
0
5
Horizontal load (ton) 10
0 2 4 6 8 10 12 14 16 18 20
15
(11.62,6.35)
b. Mini pile with 10m in length Figure (5) : Lateral load versus horizontal movement
17
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DR. M. Awad
Lateral Load Tests on Miniplles 180 160
Max. moment, (ton.m) .
140 120 100 80 60 40 31.67 20 0 0
2
4
6 8 6.91 Pile length, (m) a. Maximum bending moment
10
12
10
12
Ultimate horizontal force, (ton)
40 35
applied pile length calculated pile length
30 25 20 15
11.89
10 5 0 0
2
4
6.91 6
8
Pile length, (m) b. Ultimate horizontal load force Figure (6) : Calculated maximum moments & ultimate horizontal forces versus pile 18 length for the studied Mini pile.
Islamic Univerisy Journal
Vol. 7 No. 1, 1999
calculated ultimate horizontal force is more than the measured value of the ultimate horizontal force, which means small or almost no effect of the elongation of the pile after the limited depth on the lateral force capacity of the Mini pile. This result is confirmed by the measured ultimate results which show no much difference in the ultimate horizontal load determined in two tests of piles 8 m and 10 m long with the same material and cross section area. For design, it is important to realize that the limited pile length which can carry the required horizontal force should be confirmed by running a field test. CONCLUSIONS The foregoing analysis involving a laterally loaded pile may lead to the following conclusions. 1. The use of analytical results for determining the ultimate horizontal load may lead to overestimated values for the long piles. 2. Mini piles may, in some instances, transmit lateral forces. 3. It is important to carry out more in depth studies on this subject, together with vertical load analysis to determine the required limited pile length.
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DR. M. Awad
Lateral Load Tests on Miniplles
REFERENCES 1. Broms , B.B : Lateral Resistance of Piles in Cohesionless Soils : J.S.M.F.D, ASCE, Vol. 90, SM3, 1964, PP 123-156. 2. Devecon : Overall Soil and Ground Water Study for Sirte. February 1993. 3. DIN, German Institute for Standardization : Dynamic and Static Penetrometers. DIN 4094, Part 1 and Part 2. 4. Fed. of Piling Specialists : Specification for the construction of Mini piles. Ground Engineering, Vol. 20 , No.3. April 1987, PP 15-17. 5. McNulty, J.F.: Thrust Loading on Piles. J.S.M.F.D ASCE , Vol. 82, No. SM4, 1956, Paper 1081. 6. Poulos, H.G. and Davis, E.H. : Pile Foundation Analysis and Design. Jon Wiley and Sons, 1980, PP. 143-162. 7. Temel Inv. Inc.: Geotechnical Factual Report, Sirte, 1992. 8. Weltman, A.J.: A Review of Micro Pile Types. Ground Engineering. Vol. 14, No. 4 , May 1981, PP 43-44, 47-49.
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