INTRODUCTION
General Introduction
The foundation of a high-rise building is crucial for its stability and sustainability, as it directly transfers the building's entire load to the ground This functional component ensures the structure's firmness and safety, playing a vital role in supporting the entire edifice.
Advancements in science and technology have led to the development of various foundation solutions, including drilled shafts, also known as drilled caissons or bored piles This method involves excavating a pillar, installing a reinforcing cage, and pouring concrete, making it highly effective for supporting large axial and lateral loads while minimizing noise and ground vibration Additionally, drilled shafts are adaptable to various ground conditions, making them a preferred choice for many construction projects due to their technical efficiency and economic benefits.
Drilled shafts are commonly used for highway bridges in seismically active regions due to their superior flexural strength and large diameter, which enhances the structural integrity of reinforced concrete columns They also prevent the reduction of bearing capacity in shallow foundations caused by surface soil erosion, particularly in areas where pile driving is impractical Applications of drilled shafts include structural support, slope stabilization, and earth retention for retaining walls, sound barriers, signage, and high mast lighting, providing essential support against overturning loads.
The rising trend of urbanization is driving up housing demand, necessitating the construction of high-rise buildings due to limited available land As buildings increase in height, the load transferred to the foundation also escalates, which in turn demands a stronger pile capacity to support the structure.
To enhance the bearing capacity of drilled shafts, such as bored piles and barrettes, it is essential to explore cost-effective solutions beyond simply increasing the diameter or length of the piles While enlarging pile dimensions can improve capacity, it often leads to higher expenses and challenges in quality control, particularly with longer piles Therefore, innovative methods to boost the bearing capacity of these drilled shafts should be prioritized to provide effective and economical options for property owners.
Necessity of Research
Research has shown that base grouting and shaft grouting can significantly enhance the bearing capacity of drilled shafts, such as bored piles and barrettes Notably, Gouvenot and Gabaix (1975) found that injecting cement under pressure could increase the capacity of a 660 mm diameter bored pile by 2.5 times compared to gravity sealing Bruce (1986) reported similar enhancements in friction, with post-grouting ultimate loads increasing up to three times for large diameter piles in sands and clays This article focuses on the bearing capacity of shaft grouted bored piles and barrettes for high-rise structures A review by Littlechild et al (2000) of load tests for the Kowloon-Canton Railway Corporation in Hong Kong indicated that shaft grouted barrettes and piles in weathered granite and volcanic materials achieved a two to three-fold increase in shaft friction capacities compared to tests without shaft grouting.
Shaft grouting technology significantly enhances the shaft friction of grouted piles compared to non-grouted piles, with the improvement ratio largely influenced by geological conditions This technology is increasingly utilized in various construction projects across Vietnam by major contractors such as Bachy Soletanche, Bauer Company, and Fecon Company, owing to its ability to boost bearing capacity However, the absence of technical specifications for design highlights the need for comprehensive studies on instrumented shaft-grouted piles and barrettes to better understand their behavior and quantitatively assess the increase in shaft resistance.
Figure 1.1 The section and details of tested piles from Gouvenot‟s research
Objective and Scope of Research
This article assesses the effectiveness of the shaft grouting method in enhancing the shaft resistance of very long bored piles and barrettes in Ho Chi Minh City It aims to propose suitable correlations for the unit shaft resistance of shaft-grouted piles to inform practical design applications.
1 To determine correlations between ultimate unit shaft resistance (r u ) and some parameters such as the corrected SPT N value (N 60 ) and the effective vertical stress (ζ v ) for shaft-grouted and not grouted piles in geological conditions at HCMC
2 To determine soil types that are appropriate for applying the shaft grouting method with consideration of the cost-effectiveness
3 To perform a comparative analysis on-resistance of shorter shaft-grouted piles with longer not grouted ones under the same diameters and geological conditions
To obtain the objectives above, this study focuses on the following:
1 Collect existing data in the literature and data from experiments of the supervisor‟s research program in Ho Chi Minh city
2 Perform analytical analyses to obtain the expected outcomes
LITERATURE REVIEW
Principle of the Shaft Grouting Technology
Shaft Grouting technology is designed to improve shaft resistance in conjunction with piles, utilizing a method similar to drilled shaft installation This innovative approach involves injecting high-pressure grout into the borehole within 24 hours after concrete pouring The modern grouting system features 50 mm diameter mild steel "Tube à manchettes" or grouting tubes, which are equipped with prefabricated holes spaced at 1-meter intervals along the pile These tubes are securely attached to the steel cage using standard tie wire within the concrete cover zone and are typically arranged around the perimeter of bored piles and barrettes, extending the full depth of the shaft grout zone Water and grout are injected through a grouting point using a boring or pushing machine.
The mixing for grout must ensure flexibility, good viscosity, homogenous It consists of cement, water, and bentonite which was mixed with proportions as follows:
The grout mixing process requires the use of a high-speed mixer machine operating at a minimum of 1000 RPM or a drum mixer with at least 150 RPM To ensure a smooth consistency, the mixture must be passed through a nominal 1.2 mm sieve to eliminate any lumps.
The strength at 14 and 18 days age of grout cube (1000 cm 3 ) reach at least 18 Mpa and
The steel cage featured equidistantly arranged grouting tubes (Tube A Manchette) around the pile perimeter These grouting tubes, made of mild steel with external diameters ranging from 49 to 60 mm, are encased in a rubber sleeve equipped with a one-way valve The sleeves are pre-drilled, with spacing intervals of 0.5 to 1.0 m and positioned 1.0 m from the bottom of the pile.
Figure 2.1 Detail of Tube a manchette (Source: Nicholson, P.G, 2014)
Figure 2.2 The land view in the grouting process
Figure 2.3 Simulate the grout affected area and the grout section cross
Figure 2.4 The grouting diagram CINTAC 15 System – JEANLUTZ
(Source: www.jeanlutzsa.fr/en/6869-2/)
The CINTAC 15 System – JEANLUTZ is an advanced automatic recorder designed to ensure precise measurement of grout volume and pressure This fully electronic system effectively monitors grouting pumps, utilizing an analog-to-numeric pressure transducer to accurately measure grouting pressure across various flow rates, determined by the diameter of the grouting pipe.
The construction process of shaft grouting piles closely resembles that of drill shaft piles, with the addition of two key steps: water cracking and shaft grouting Large diameter bored piles and barrettes are created by excavating the ground using rotary drilling equipment and rope grabs, employing bentonite and polymer slurry for support and stabilization of the hole walls While the polymer slurry has minimal impact on the shaft resistance of piles in certain formations, it demonstrates a significant effect in others when compared to bentonite.
2015) After drilling the designed depth, the reinforcing cages were fabricated
Figure 2.6 The process to construct the shaft grouted bored pile, barrettes pile
Grout tubes, covered by a rubber sleeve every meter, will be installed on the steel cage prior to the machette installation After cleaning the drilling hole, rebar cages are set in place, followed by concrete pouring The rubber sleeves of the grout tubes will be opened under water pressure between 5 to 24 hours after the mass pilings are concreted, with records maintained to document which sleeves have been successfully opened and the corresponding pressure levels Shaft grouting for specific piles will occur at least 5 days post-concreting Each test pile, such as PTP1-Saigon center's barrettes pile, features strain gauges attached to the reinforcing cages, with pairs positioned to account for the rectangular shape of the barrettes, ensuring coverage of the cross-section Similarly, TP2-Songviet's project bored pile will have its own gauge arrangements.
The article discusses the placement of three gages, labeled A, B, and C, depicted in the symmetric Figure 2.8 The arrangement of these gage pairs reveals a quasi-symmetrical distribution relative to the center of the barrette and bored pile This analysis is relevant to two specific projects: the Saigon Center Project and the Song Viet Project.
Figure 2.7 Details of grouting and instrumentation of Saigon and Song Viet project a) Saigon center project b) Song Viet project
Figure 2.8 Cross-section and layout of instrumentation
Figure 2.9 The process of water cracking (a) and shaft grouting (b)
Figure 2.10 Illustration diagram of the grout pipe cross-section, flows paths, the section of the grouted sample
In the water cracking step, the manchette is opened with water under pressure (min
To create flow paths in concrete cover, a double packer is inserted into the grouting tube and inflated to inject water at high pressure, forming cracks in the concrete This process must be completed within 24 hours of pouring the concrete to prevent it from hardening Subsequently, cement grout is injected into the tube, starting from the bottom and moving to the highest manchette position, effectively creating a grout zone in the soil Each tube is grouted using a grout pump until the specified conditions are met, which include measuring the shaft grout zone from 500 mm above the first manchette to 500 mm below.
The target grout volume for the bottom manchette is set at 35 litres per square meter If this target is not met, additional grout will be injected into the side pile to ensure that the overall minimum average grout volume for mass piling exceeds the required amount.
25 liters/m2 The limiting pressure shall be set at 40 bar (4.0 Mpa) or resurgence (grout flowing back up to the surface) is identified.
Existing Studies on Shaft-Grouted Piles
Replacement piles, such as bored piles, barrettes piles, and pre-bore H-piles, are often installed into excavated holes, with their bearing capacity primarily derived from tip bearing friction and rock socket friction when resting on bedrock In cases where piles are embedded in soil, their capacity relies on shaft friction, which can be significantly enhanced through shaft grouting Research indicates that grouted piles can exhibit friction capacities 2 to 3 times greater than their non-grouted counterparts Notably, Gouvenot and Gabaix (1975) demonstrated that grouted piles tested in clay and sand showed a 2.5-fold increase in shaft friction Between 1975 and 1985, Bruce analyzed 300 shaft grouted piles along the Jeddah-Mecca Expressway, confirming that post-grouting enhances both shaft and toe friction, allowing for increased working loads Similarly, Stocker (1983) found that grouting improved shaft friction by 1.5 to 3 times in granular and cohesive soils However, limitations in these studies arise from calculations that did not reach or exceeded ultimate bearing capacity, along with reliance on raw SPT-N index data, which can lead to inaccuracies.
Figure 2.11 Variation of gain in load with injection
(a) Detail of shaft and toe grouting
(b) Skin friction of cast-in-situ bored pile (at a settlement of 10mm) and anchors in non-cohesive soil Figure 2.12 The Stocker‟s research result
A testing program conducted by Littlechild (2000) for the Kowloon-Canton Railway Corporation in Hong Kong evaluated the shaft friction capacity of six large-scale deep foundation piles, including shaft grouted bore piles and barrettes.
The study reveals that grouted piles exhibit a two to three-fold increase in friction capacity compared to plain piles in weathered granite and volcanic materials The author demonstrates a correlation between mobilized friction and average SPT-N values, indicating that shaft friction is effective for SPT-N values below 60 in Hong Kong's weathered materials, with minimal increases for higher values Sze and Chan (2012) conducted a back analysis using historical data to illustrate the enhanced friction capacity achieved through grouting technology Their experiments confirmed a strong correlation between ultimate skin friction and uncorrected mean blow count of SPT, showing that the frictional capacity of piles utilizing shaft grouting can be significantly improved, up to three times that of plain piles However, much of the analysis relies on average SPT-N values or uncorrected mean blow counts, which may lead to inaccuracies in the results.
Figure 2.13 The comparison mobilized friction value between the grouted pile and plain pile tests in Hong Kong (Source: Littlechild, 2000)
Table 2.1 Summary of typical design parameters following author‟s experience
Shaft grouting is experiencing significant growth in Vietnam, particularly in Ho Chi Minh City This advancement is rooted in foundational research conducted by Phan and Pham in 2013, which explores the relationship between shaft resistance (r s) and the average conditions affecting it.
The study compares the SPT-N values of shaft grouted piles and plain piles, revealing that the raw SPT-N data indicates an increasing value of r_s, suggesting that the piles have not fully mobilized and did not fail under the applied load According to experimental research by Nguyen et al (2019), the ultimate unit shaft resistance (r_u) correlates with the SPT-N 60 value, showing that the r_u value for grouted piles is twice that of plain piles in both sandy and clayey soils Furthermore, the author highlights that the r_u values derived from the β-method are recommended for practical applications.
Figure 2.14 Summary the mobilized shaft resistances of clayey soil (a) and sandy soil (b) versus N value (Source: Phan and Pham, 2013)
Theory for Calculate Bearing Capacity of Drill shaft
2.3.1 Method of FHWA (FHWA-NHI-10-016)
The ultimate bearing capacity of pile have two component side resistance R s and base (tip) resistance R b
R S,i = nominal side resistance for layer i ψ S,i = resistance factor for side resistance in layer i n = number of layers providing side resistance,
B = resistance factor for base resistance
In layered soil profiles, the total (ultimate) shaft resistance of a pile is calculated as:
C s = circumference of pile at depth z For circular pile C s = πD, for square piles C s 4D; δ = effective stress angle of friction for the soil shaft interface;
D = shaft diameter/ length of square;
z i = thickness of layers i; r s : nominal unit side resistance
K = coefficent of horizontal soil stress (K=ζʹ h /ζʹ v ) ζʹ v = average vertical effective stress over the depth interval z; ζʹ h = horizontal effective stress
Figure 2.15 Idealized layering for computation of compression resistances
(Source: FHWA-NHI-10-016 Drilled shafts)
Figure 2.16 Frictional model of unit side resistance for drilled shaft
(Source: FHWA-NHI-10-016 Drilled shafts) For convenience: β = Ktanδ; r s = ζʹv β (2.3) where β: side resistance coefficient (beta method)
Nominal base resistance is calculated as:
A base = the cross-sectional area of bearing at the shaft base q BN = the unit base resistance
2.3.2 Method of Vietnam Standard (TCVN10304-2014)
The allowable capacity for drill shaft follow Architecture institute of Japanese (AIJ):
C s = circumference of pile at depth z For circular pile C s = πD; for square piles C s 4D
D = shaft diameter/ length of square
z i = thickness of layers i r s = nominal unit side resistance ζʹ v = average vertical effective stress over the depth interval z
The unit average shaft resistance in sandy soil layer i
(2.5) The unit average shaft resistance in clayey soil layer i
(2.6) where: c u,i = undrained shear strength for clayey soil layer i: f L = the coefficient depends on the ratio h/d (length/diameter) of pile and with drill shaft f L = 1
p = the coefficient base on the ratio between undrained shear strength and average effective stress, which determined below plot:
Figure 2.17 The diagram to evaluate p coefficient
N s,i = the average SPT index in sandy soil layer i l s,i = length of pile in sandy soil layer i l c,i = length of pile in clayey soil layer i
Theory for Converting Strain to Load
According to the Red Book (Fellenius, 2020), strain gauges on piles measure strain rather than load, and these strains are converted to load values using the modulus of the pile material and its cross-sectional area For steel piles, the modulus is a consistent value of 20.5 x 10^4 MPa In contrast, the modulus of concrete piles varies significantly and depends on the applied stress and strain, decreasing as the imposed strain increases Consequently, when loads are applied to piles, the load-movement relationship is represented by a curve rather than a straight line.
1989) the stress-strain curve can with sufficient accuracy be assumed to follow a second-degree line: y = ax 2 +bx+c, where y(stress), x(strain)
Converting Strain to Load Using the Direct Secant Method
The relationship between load and measured strain (Q/ε) is characterized by "direct secant," enhancing the resolution of stiffness in relation to strain for the pile This is represented by the secant stiffness formula, E s A/L, where E s denotes the secant modulus and L represents the unit length of one meter The analysis of strain is crucial for understanding stiffness in this context.
E s A = aε+b (“a” the slope of the line, “b” the ordinate intercept)
Figure 2.18 Near pile-head gage level secant stiffness vs measured strain for the spun pile (Source: Fellenius, 2020)
Converting Strain to Load Using the Tangent Stiffness Method
The construction of tangent stiffness, defined as the change in load over the change in strain versus strain, parallels that of secant stiffness, which is the change in load over strain versus strain The tangent modulus of a pile is represented as a straight line, facilitating the formulation of the secant elastic modulus line This relationship enables the conversion of measured strain values into stress and load using the corresponding strain-dependent secant modulus The equation for the tangent modulus, E_t, is essential in this context.
Which can be integrated to provide a relation for stress as a function of the strain:
A function of secant modulus and strain:
The tangent modulus (E t) and secant modulus (E s) of pile materials are essential for understanding their mechanical behavior under load Stress (ζ) is calculated by dividing the load by the cross-sectional area, while the change in stress (dζ) is determined by the difference between consecutive load increments (ζ n+1 – ζ n) The slope of the tangent modulus line is denoted as 'a', and strain (ε), measured in microstrain (με), reflects the material's deformation under stress The change in strain (dε) is the difference between successive strain measurements (ε n+1 – ε n) Additionally, the y-intercept of the tangent modulus line, represented by 'b', indicates the initial tangent modulus of the material.
In the secant stiffness method, the true stiffness of a pile is represented by the slope of the load versus strain curve (Q/ε) once the shaft resistance is fully mobilized at a gauge level If the shaft resistance exhibits strain-hardening, the slope will be steeper than the true stiffness, while strain-softening will result in a slope that is smaller than the true stiffness In contrast, the tangent stiffness (ΔQ/Δε) is influenced by reduced load increments due to shaft resistance along the pile, preventing a linear relationship from forming until full mobilization occurs above the gauge location Initially, calculated tangent stiffness values are high, but as shaft resistance mobilizes downward, strain increments increase, leading to smaller stiffness values Once all shaft resistance above the gauge is mobilized, the tangent stiffness values calculated at that location accurately reflect the pile's cross-section stiffness.
To estimated unit shaft resistance r s value at fully mobilized the gage location:
(2.11) ΔQ SG : The load increment in a unit pile length at strain gage
SG = strain at each strain gauge level
Q z1 = The load which interpolated from QSG with 0.5m distance above
The load Q z2 is derived from the QSG measurement taken at a distance of 0.5 meters below the pile's circumference, ΔA c The ultimate unit shaft resistance, r u, is identified as the peak or stabilized value of r s in relation to the head load Q HL.
According to Das (2016) the frictional resistance per unit area at any depth z may be determined:
C s = perimeter of the cross section of the pile
Figure 2.19 Load transfer mechanism for piles, the variation of f(z) with depth
When the load Q on the ground surface increases, the maximum frictional resistance along the pile shaft is fully mobilized at a relative displacement of approximately 5-10mm between the soil and the pile, regardless of the pile's size and length L This full mobilization is established based on the t-z curve analysis.
The r u value is estimated by comparing the mobilized shaft resistance (r s) to the pile head load (Q HL) In dense sands and over-consolidated clays, the unit shaft resistance increases and peaks, while in loose sands and normally consolidated clays, it stabilizes at higher loads, as illustrated in Figure 2.20.
Figure 2.20 The mobilized shaft resistance at some strain gauge levels of pile TP4 – Song Viet
Linear Regression Analysis
Regression analysis is a key statistical method used to explore the relationships between variables By examining data points such as (x1, y1), (x2, y2), and so on, a function can be established to describe trends, with common forms including linear equations like y = ax + b In linear regression, the dependent variable y is determined by a single independent variable x, and the intercept, which is the value where the regression line intersects the y-axis, plays a crucial role The best-fit function can generally be expressed as y = f(x) = ax + b.
For simple linear regression, the slope (a), intercept (b), correlation coefficient (R) and coefficient of determination (R 2 ) and can analytically be calculated as follows:
When the intercept = 0 the function y = ax with the slope of linear:
METHODOLOGY
Introduction
This article presents an analytical back analysis method aimed at assessing the effectiveness of shaft grouting in enhancing the shaft resistance of very long bored piles and barrettes in Ho Chi Minh City It proposes suitable correlations for the unit shaft resistance of shaft-grouted piles to aid practical design The study is based on data collected from geological conditions, static load tests, strain gauge tests, and extensometer tests, encompassing 12 projects with a sample of 34 piles.
The Procedure for Determining Correlation between r u and SPT N 60 ; Correlation
The correlation of variables is commonly utilized in various studies, with their values determined through practical experiments Specifically, the relationship between the undrained shear strength (r u) and effective vertical stress (ζ‟ v) is assessed using the beta method, where the beta value is influenced by geological conditions and construction techniques This relationship is expressed mathematically as r u = β.σ’ v (3.1).
The relationship between the undrained shear strength (r_u) and the Standard Penetration Test (SPT) N-index (N_60) can be expressed as r_u = k.N_60 The SPT N-index is determined using a 0.62 kN hammer dropped from a height of 76.0 cm, recording the number of blows needed to penetrate the soil by 30 cm at various depths This index is essential for assessing soil strength parameters at each project site After adjusting for 60% energy efficiency and other factors as outlined by Skempton (1986), the raw SPT N-index is converted to N_60 Additionally, the normalized blow count is corrected for borehole diameter (C_B), rod length (C_R), and sample type (C_S) to ensure accurate soil strength evaluations.
The methodology for determining the correlation of variables \( r_u \), \( N_{60} \), and \( \zeta'_{v} \) is illustrated in Figure 3.1 First, data collected from geological condition tests is utilized to calculate \( N_{60} \) and \( \zeta'_{v} \) Next, data obtained from static load tests and extensometer tests is analyzed to complete the correlation assessment.
To calculate the incremental load at the strain gauge (ΔQ SG), refer to equation 2.12 in Chapter 2, following the methodology of Fellenius (1989) Utilize the r s value to determine the mobilized shaft resistance (r u) at the strain gauge level from the t-z curve Additionally, establish the correlation between r u and N 60, as well as the correlation between r u and ζ’ v using least-squares regression analysis.
Regression and Statistic method following theory and equation 2.19, 2.20
Collect data and analyze geological condition (soil profile)
Collect data from static load test & strain gauge test and extensometer
& corrected N-index for 60% efficient energy N 60
Q SG (strain gage load); ΔQ SG (increment load at strain gage)
Find correlation by R 2 square r u = kN 60 r u = βζʹ v
Determine the fully mobilized shaft resistance ( r u ) at strain gauge level
Figure 3.1 Flow chart to evaluate correlation between r u with N 60 , ζʹv
Evaluation of Appropriate Soil Type for Applying Shaft Grouting Technology
The research subject for each pair of piles which is located same site, same length, same geological condition
Figure 3.2 Illustration plain pile and grouted pile to evaluate shaft resistance increment The procedure was followed as below: i) From data was collected, R s value was calculated following equation 2.2 in chapter
2, where r s value was taken from Objective 1 results for clayey soils and sandy soils ii) The cost to inflect grout was calculated based on the market cost with equation
C g = shaft grouting cost; c g = shaft grouting cost over the unit area (1m 2 )
L gp = length of grouted iii) The shaft resistance increment R s was calculated for clayey soil and sandy soil:
R s,gp = shaft resistance of grouted pile
R s,ngp = shaft resistance of not grouted pile iv) Calculate the ratio between the shaft grouting cost and the shaft resistance increment
To assess the suitability of the shaft grouting method, it is essential to analyze the correlation between the ratio of cost to shaft resistance increment (C g /ΔR s) and the average Standard Penetration Test (SPT-N) value This evaluation will help determine the appropriate soil conditions for effective application of the shaft grouting technique.
Following the AASHTO,1988, the property of the soil was classified by SPT-N value
Table 3.1 SPT-N value soil property correlations for (a) granular (sandy) soil and (b) cohesive (clayey) soil
(Source: FHWA-IF-02-034 Evaluation of soil and rock properties)
Calculate ultimate shaft resistance R s for every soil layer
Calculate shaft grouting cost following depth
Calculate Shaft resistance increment (R s ) for every Soil layer with not grouted & grouted Pile Evaluate average SPT-N
Calculate the ratio between the shaft grouting cost and shaft resistance increment (C g /R s ) for every Soil layer at one bore hole
Collect data and analyze geological condition (soil profile)
Determine the correlation between the ratio
(C g /R s ) and average SPT-N Evaluate the appropriate soils to apply shaft grouting Figure 3.3 Flow chart to evaluate suitable soil type
The Procedure to Comparison of Shaft Grouted Pile and Plain Pile
This section focuses on comparing three pairs of piles from the SongViet project: TP1 & TP2, TP4 & TP5, and TP1-1 & TP3-1 The comparison procedure involves several steps: first, utilizing data from static load tests, strain gauge tests, and extensometer tests to determine the secant and tangent stiffness moduli Next, the Q SG is calculated at the gauge level using the appropriate equations to fit the target load to SG-1, leading to the establishment of the load distribution curve (Q SG – R s) Finally, the accumulated resistance (in kN) is calculated from the pile head to the gauge levels.
Collect data from static load test & strain gauge test and extensometer test report
Calculate construction cost for plain piles and shaft grouted piles
Comparison between Not Grouted (Plain/Normal) Pile and Grouted Pile
Perform load-settlement curve for plain piles and shaft grouted piles
Figure 3.4 Flow chart to compare grouted pile and plain pile
DATABASE AND ANALYSIS RESULTS
Database of Test Piles
The data Geological Condition, the physico-mechanical properties of the soil layer, Standard Penetration value (N 60 ), static load test, strain test… on research is collected from the following sources:
- Test size in 12 constructions projects with 34 piles in Ho Chi Minh City such as:
Landmark 81, Vinhome Bason, Friendship Tower, Empire City, Exim bank, German House, Song Viet, Everrich, Sai Gon Center, Riviera, Palm Garden, Lancaster Lincoln
Figure 4.1 The location of projects in Ho chi minh city
Table 4.1 The information of projects in Ho Chi Minh city
No Project name Test pile name Pile type
(MN) IST/SG Grouted depths (m) Borehole Installation date Test date
2 Vinhome Bason TN 6 Barrette 67.19 0.82.8 64/30 Y/Y 58.0-68.0 HK-8 5/2/2016 27/2/2016 22 Polymer
TSBP1-MU4 Bored 62.20 1.2 26/13 Y/Y 32.0-59.0 BH4-1 7/6/2017 29/6/2017 22 Bentonite
TSBP4-MU4 Bored 62.20 1.2 26/13 Y/Y 32.0-59.0 BH4-3 10/6/2017 6/7/2017 26 Bentonite
TSBP1-MU7 Bored 62.00 1.2 25/12.5 Y/Y 32.0-59.0 BH7-4 21/6/2017 14/7/2017 23 Bentonite
TSBP4-MU7 UD16 62.00 1.2 25/12.5 Y/Y 32.0-59.0 BH7-2 24/6/2017 21/7/2017 27 Bentonite
TSBP7-MU7 Bored 62.20 1.2 37/12.5 Y/Y 32.0-59.0 BH7-2 20/10/2017 27/12/2017 68 Bentonite
TP1 (Ocell) Bored 80.25 2 44/22 Y(10)/N - HK4 31/1/2010 13/3/2010 41 Bentonite
TP2 (Ocell) Bored 80.60 2 48.4/22 Y(10)/N - HK-5 26/1/2010 5/3/2010 38 Bentonite
TP3 (Ocell) Bored 84.70 2 46/22 Y(10)/Y 66.0-84.7 HK-6 30/1/2010 16/3/2010 45 Bentonite
TPH (Ocell) Bored 80.26 1.5 33/22 Y(10)/Y 60-80.26 HK-7 30/1/2010 22/3/2010 51 Bentonite
10 Riveria TP7 Bored 68.75 1.2 33.9/11.3 Y(10)/Y 25.25-68.35 BH-15 5/2/2019 5/3/2019 28 Bentonite
Shaft grouting has been extensively utilized in Ho Chi Minh City, located in the Mekong Delta region The survey site reveals that the ground is primarily composed of middle to early Holocene sediments, including marine-origin sediments, with additional layers of river and sea sediments reaching depths of up to 100 meters The Mekong Delta basin is characterized by deposits from the Mekong River, consisting of thick layers of alternating alluvial soil, organic soft clay, compact silty sand with gravel, and medium to dense silty sand, all underlain by dense to very dense silty sand.
1977) Specially, almost all the projects are located near Saigon river, the more complicated the geology
The geological profile consists of five distinct layers: Layer 1 (0-30 m) features very soft silty clay and organic silt with an SPT N-index of 0-4 Layer 2 (30-37 m) comprises loose to medium sand mixed with silty and clayey components, exhibiting an SPT N-index of 15-20 Layer 3 (37-46 m) contains dense to very dense clayey silty sand, with stiff to hard clay and sandy lean clay, reflected by an SPT N-index of 20-30 Layer 4 (46-55 m) consists of clayey sand and silty sand, transitioning to medium to dense sand, with an SPT N-index of 35-40 Finally, Layer 5 (from 55 m) is characterized by dense to very dense clayey silty sand and gravel, alongside semi-stiff to stiff silty clay, with an SPT N-index exceeding 50.
34 Figure 4.2 The soil properties and SPT-N value from data of Ho Chi Minhʹs project
(a) Sai Gon center (PTP-1) (b) Everrich (TP1)
This study conducts a comparative analysis of the resistance between shorter shaft-grouted piles and longer non-grouted piles, utilizing six selected piles for evaluation The analysis involves three specific pairs: Pair 1 consists of TP-01 and TP-02, Pair 2 includes TP-04 and TP-05, and Pair 3 features TP1-1 and TP1-3 The detailed characteristics of each pile are outlined for further examination.
Table 4.2 The information of three test pair piles
Plot Pair Test pile name Pile type
(MN) IST/SG Grouted depths (m) Test date
Figure 4.3 Illustration of the three test pair piles
The Correlation between r u and SPT N 60
This section explores the relationship between the mobilized unit shaft resistance (r u) and the corrected SPT-N value, as illustrated in Figures 4.4 and 4.5, focusing on both clayey and sandy soils.
Figure 4.4 Correlation of r u and N 60 for Clayey soil for not grouted and grouted pile
Figure 4.5 Correlation of r u and N 60 for Sandy soil for not grouted and grouted pile
Based on data from 34 piles and the procedures discussed above, there are 180 out of
A total of 360 strain gauge levels were fully mobilized, with 180 data points analyzed Among these, 124 soils were grouted, while 56 levels remained ungrouted The classification revealed 80 strain gauge levels in clayey soil and 100 in sandy soil Some data points were excluded due to low SPT-N index and effective vertical stress, particularly those close to the ground surface, which could lead to unreliable results For very soft to soft clays, where N60 is less than 8 (Clayton, 1993), the SPT-N values were notably low, prompting this study to disregard the SPT results.
To ensure more stable regression, it is recommended to maintain an N index smaller than 8 Statistical analysis established a correlation between the parameters r u and N 60, verified through least squares regression For ungrouted data points, the k values were approximately 3.21 for clayey soil and 3.61 for sandy soil In contrast, for grouted data points, the k values were found to be 5.64 for clayey soil and 6.24 for sandy soil.
Table 4.3 The resistance ratio of grouted over not grouted
The table illustrates the relationship between the r u value and the N 60 index, while also comparing the k value ratios of grouted and non-grouted piles The findings indicate that the r u value for grouted piles is approximately 1.75 times greater than that of non-grouted piles (plain piles) in both clayey and sandy soils.
The Correlation between r u and Effective Vertical Stress ζʹ v
In the method, the r u value is proportional to the effective vertical stress following form:
Figures 4.6 and 4.7 illustrate the correlation between the parameters r u and ζʹ v using fully mobilized data points The analysis reveals that for the non-grouted data points, the β values are approximately 0.25 for clayey soil and 0.28 for sandy soil In contrast, the grouted data points yield a β value of 0.47 for both soil types.
Figure 4.6 Correlation of r u and ζʹ v for Clayey soil for not grouted and grouted pile
Figure 4.7 Correlation of r u and ζʹ v for Sandy soil
39 for not grouted and grouted pile
Table 4.4 illustrates the relationship between the r u value and the β coefficient, highlighting the ratio of β values between grouted and non-grouted piles In clayey soil, the r u value for grouted piles is approximately 1.83 times greater than that of non-grouted piles, while in sandy soil, it is around 1.68 times larger.
Table 4.4 The resistance ratio of grouted over not grouted
Evaluation the Appropriate Soil Type for Applying Shaft Grouting Technology
Figure 4.8 illustrates the significant variation in cost-benefit analysis across different types of clayey soils The cost to increase shaft resistance by 1 kPa is notably higher for soft clay at an average of 18,252 VND, compared to 10,768 VND for stiff clay, 4,375 VND for very stiff clay, and approximately 3,511 VND for hard clay When compared to soft clay, the construction costs decrease to 0.58 times for stiff clay, 0.24 times for very stiff clay, and 0.19 times for hard clay The grouting method proves to be most effective in hard clay (SPT-N: 30-60), followed by very stiff clay (SPT-N: 15-30), stiff clay (SPT-N: 9-15), and soft clay (SPT-N < 8).
The correlation between the cost-benefit and SPT-N index was shown by the power equation
Grouted Not grouted Ratio σʹ v Clay r u = 0.468 ζʹ v r u = 0.256 ζʹ v 1.828
Figure 4.8 The correlation of the shaft grouting cost to increase one shaft resistance unit versus Standard penetration test blow count for clayey soil
Figure 4.9 illustrates the significant variation in cost-benefit ratios across different types of sandy soils The cost to increase shaft resistance by 1 kPa is notably higher in loose sand, averaging 11,390 VND, compared to 6,500 VND for medium sand, 2,924 VND for dense sand, and approximately 2,116 VND for very dense sand Consequently, when increasing shaft resistance, the construction costs decrease to 0.57 times for medium dense sand, 0.28 times for dense sand, and 0.19 times for very dense sand, relative to loose sand The grouting method proves to be most effective in sandy soils, ranked from most to least effective as follows: dense sand (SPT-N 25-50), medium dense sand (SPT-N 11-24), and loose sand (SPT-N < 10).
The correlation between the cost-benefit and SPT-N index was showed by the power equation
Figure 4.9 The correlation of the shaft grouting cost to increase one shaft resistance unit versus Standard penetration test blow count for sandy soil
Comparison of Shaft Grouted Pile and Plain Pile
The study analyzed three pairs of piles by comparing their load-deflection characteristics and accumulated shaft resistance The load-deflection responses at the pile head for both grouted and non-grouted (plain) piles are illustrated in Figures 4.10, 4.11, and 4.12.
Figure 4.10 illustrates a comparison between the test results of the TP-02 shaft grouted pile and the TP-01 non-grouted pile At a design load of 18.5 MN (140% of the design load), the TP-01 pile experienced a total settlement of 20.22 mm, which increased to 34.62 mm while maintaining the load level In contrast, the TP-02 pile, under the same design load, recorded a settlement of 18.8 mm.
Figure 4.11 illustrates a comparison between the TP-04 grouted pile and the TP-05 non-grouted pile under a design load of 17.3 MN (186% of the design load), revealing a total settlement of 30.6 mm for TP-05, while TP-04 experienced a settlement of 20.5 mm Additionally, Figure 4.12 indicates that at a design load of 14.1 MN (125% of the design load), the TP1-1 pile head settled by 22 mm, compared to a settlement of 16.82 mm for the TP3-1 pile.
Figure 4.10 The load – settlement relation chart of pair 1 (TP-01 and TP-02)
Figure 4.11 The load – settlement relation chart of pair 2 (TP-04 and TP-05)
Figure 4.12 The load – settlement relation chart of pair 3 (TP1-1 and TP3-1)
Figures 4.13, 4.14, and 4.15 illustrate the shaft resistance accumulated for each pair of piles The grouted depths are as follows: TP-02 ranges from 36 m to 60 m, TP-04 from 36 m to 59 m, and TP3-1 from 34.3 m to 60.3 m Analysis reveals that shaft resistance was evaluated at various load levels at the pile head and correlated with the depth of each strain gauge Notably, the shaft resistance from the pile head to the grouting starting point is equal for both grouted and non-grouted piles However, there is a significant increase in the shaft resistance of the grouted piles within the grouted area compared to the non-grouted piles at greater depths.
36 m with TP-02 and TP-04, from the depth 34.3 m with TP3-1 to the toe pile
Figure 4.13 The shaft resistance accumulated of pair 1 (TP-01 and TP-02)
Figure 4.14 The shaft resistance accumulated of pair 2 (TP-04 and TP-05)
Figure 4.15 The shaft resistance accumulated of pair 2 (TP1-1 and TP3-1)
Table 4.5 reveals that the test grouted pile TP-02 offers a 10.9% reduction in construction costs compared to the test plain pile TP-01 Similarly, the grouted piles TP-04 and TP-03 exhibit savings of 11.3% and 7.8% in construction costs when compared to the plain piles TP-05 and TP-01, respectively.
Table 4.5 The construction cost saving percentage comparison with plain pile and shaft grouted pile
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
This study analyzes the effectiveness of the shaft grouting method in enhancing the shaft resistance of very long bored piles and barrettes through an analytical approach Key conclusions highlight the significant improvements in shaft resistance achieved by implementing this method, demonstrating its viability for optimizing deep foundation performance.
1) In both clayey soil and sandy soil, the ultimate shaft resistance r u value of grouted piles would roughly be 1.8 times larger than r u value of not grouted piles (plain piles)
2) Results from analyses on suitable soil types indicate that, in general, the stiffer/denser the soil is, the better the effectiveness of the method The soil has SPT-N larger than 10 (stiff clay, very stiff clay, hard clay, medium dense sand, dense sand and very dense sand) is suitable to apply shaft grouting method
3) Long drilled shafts installed along the Saigon river can be shorted significantly by shaft grouting the lower portion of the piles The test pair piles from actual projects show that the shorter grouted piles (10 to 15 m shorter) still satisfy the design resistance required for the longer plain piles The reduction of the pile length reduces many risks from the construction of very long piles Especially, the shorter grouted piles help reduce the construction cost up to 10% compared with that for the longer plain ones.
Limitations and Suggestions
The research presented in my thesis has certain limitations, particularly in the finite element method (FEM) analysis, as it does not simulate the grouted zone of the shaft This omission restricts a detailed examination of soil behavior during and after the shaft grouting process at varying pressure levels.
Suggestion: The research result about the correlation of ultimate shaft resistance r u and
The N 60 index, along with parameters r u and σʹ v, was directly utilized during the design and construction phases Research findings on the evaluation of suitable soil types for shaft grouting methods enable engineers to swiftly predict and provide solutions for structural design plans in projects.
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