Bearing capacity of floating geosynthetic encased columns (GEC) determined on the basis of CPTU penetration tests

Iwona Chmielewska

doi:10.1515/eng-2020-0078

Article Open Access

Bearing capacity of floating geosynthetic encased columns (GEC) determined on the basis of CPTU penetration tests

Iwona Chmielewska

Published/Copyright: July 14, 2020

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From the journal Open Engineering Volume 10 Issue 1

Abstract

Floating geosynthetic encased columns (GEC) are an increasingly popular method of strengthening weak subsoil. Design of floating columns is a difficult and not fully recognized issue. This paper treats the floating GEC column as a special kind of “pile” and its bearing capacity is calculated using five selected methods for calculating the bearing capacity of piles based on CPTU penetration tests. The calculations were done on the basis of insitu tests carried out on one of the sections of the Bargłów Kościelny bypass. The paper contains a comparison of the bearing capacities of floating GEC columns calculated with different methods based on CPTU penetration tests.

Keywords: GEC columns; bearing capacity of piles; CPTU penetration tests; floating columns

1 Introduction

A floating column is a column with a base in weak subsoil. Floating columns are an increasingly popular method of strengthening weak subsoil, mainly for economic and technological reasons [1]. Guidelines for the design of reinforcement for geosynthetic encased columns (GEC) [2] apply only to end-bearing columns, and there are no guidelines for designing reinforcement for floating columns.

The floating GEC column can be treated as a special kind of “pile” and its bearing capacity can be calculated with methods based on CPTU penetration tests [3].

2 Methods of calculating the bearing capacity of piles based on CPTU tests

In this paper, the bearing capacity of the column was determined by five selected methods: LCPC [4], Beringen and De Ruiter [5], DIN 4014 [6], Schmertmann [7] and Philipponnat [8].

Pile bearing capacity is the sum of the base capacity Qbk and shaft capacity Qsk [9]:

(1)Qk=Qbk+Qsk

(2)Qbk=qbAk

(3)Qsk=fpAs

where Ak and As are the area of the pile base and pile shaft respectively, qb is the unit end-bearing pressure and fp is the average shaft resistance.

2.1 LCPC method

In the Laboratoire Central des Ponts et Chausees (LCPC) method, the unit end-bearing pressure and average shaft resistance can be calculated directly from the tip cone resistance q_c [4].

(4)qb=kcqc,avg

(5)fp=qc,zα′

where k_c, α^′ are coefficients that depend on the pile and soil type, q_c,_avg is the equivalent average cone resistance between 1.5D_k below and 1.5D_k above the pile tip, D_k is the diameter of the pile and q_c,_z is the cone resistance at depth z_k.

The value q_c,_avg should be calculated in three steps:

Calculate q_ac as a mean value of q_c at a depth between 1.5D_k below and 1.5D_k above the pile tip.
Eliminate q_c values higher than 1.3q_ac and lower than 0.7 q_ac.
Calculate q_c_,avg within the range defined in the previous step.

2.2 Beringen and De Ruiter method

In the Beringen and De Ruiter method, also known as the European method, the values of q_b and f_p in Equations 2 and 3 should be determined from the following equations [5]:

(6)qb=9cu

(7)fp=βcu

where β = 1 for normally consolidated soils, β = 0.5 for over consolidated soils and c_u is the undrained shear strength determined from Equation 8 [10].

(8)cu=qt−σv0Nkt

where q_t is the corrected cone resistance, σ_v0 is the total initial stress at the considered depth, N_kt is a coefficient depending on the plasticity index I_p and the degree of soil consolidation. N_kt values are usually in the range of 10 to 20 [11].

2.3 DIN 4014 method

In the DIN 4014 method, the shaft resistance f_p and the unit end-bearing pressure q_b in cohesive soils are determined from the undrained shear strength c_u.

The value of c_u can be determined from Equation 8, using an average q_c value over a zone of three times the pile diameter under the tip.

2.4 Schmertmann method

In the Schmertmann method, the value of q_b is determined from Equation 9 [7].

(9)qb=qc1+qc22

where q_c1 is the minimum average value of the cone resistance at a depth of 0.7D_k or 4D_k below the pile tip and q_c2 is the minimum average value of the cone resistance in the zone equal to 8D_k above the pile tip.

For piles with a base in cohesive soils, the f_p value can be determined from Equation 10 [7].

(10)fp=αcfs

where f_s is the sleeve friction and α_c is a coefficient selected in the range 0.2 to 1.25, depending on the sleeve friction and the type of pile. For values of f_s equal to 30 kPa and less, the value of the coefficient α_c is the same for all types of piles.

2.5 Philipponnat method

In the Philipponnat method, the values of q_b and f_p can be determined from Equations 11 and 12 [8].

(11)qb=kbqca=kbqca(A)+qcb(B)2

(12)fp=αsFsqcs

where k_b is a coefficient that depends on the type of soil in which the pile tip is located (for clays, k_b = 0.5), q_ca_(A) is the average cone resistance in the zone equal to 3B above the pile tip, q_cb_(B) is the average cone resistance at a depth of 3B below the pile tip, F_s is a coefficient depending on the type of soil in which the pile tip is located (for clays, F_s = 50), α_s is a coefficient equal to 1.25 and q_cs is the average cone resistance along the pile shaft. For circular foundations B = B^′ and a B^′ value can be calculated from Equation 13.

(13)B′=12πDk≈0.886Dk

3 Calculation arrangements

The GEC column is treated as a special kind of “pile” and its bearing capacity is calculated using five selected methods for determining the pile bearing capacity based on CPTU tests.

This paper considers the characteristic value of the bearing capacity Q_k (Figure 1) of a single column for reinforcingweak subsoil for plans with large dimensions (road embankments, spatial structures). Negative friction was omitted in the calculations because the reinforced soil mattress on the subsoil causes settlement of the column and the weak subsoil surrounding the column to be equal. The calculations were carried out for columns with diameter D_k equal to 0.8 m.

Figure 1

Geometry of the problem.

The bearing capacity of floating GEC columns was determined on the basis of CPTU tests carried out on one of the sections of the Bargłów Kościelny bypass, where a weak subsoil was reinforced with floating GEC columns.

On the analyzed section of the Bargłów Kościelny bypass, there are organic soils, mainly peat with a thickness of from about 3 m to about 9 m. Below the organic soils there are glacial deposits in the form of sandy clays. On top of the layer of glacial deposits, the clays are soft. With depth, the moisture of the cohesive soils decreases, and thus the liquidity index decreases, dropping to a value corresponding to stiff clay. The ground water level is located at a depth of 0.2–0.3 m below the ground surface.

The relationships between the liquidity index I_L of sandy clay and the cone resistance q_c, undrained shear strength c_u and sleeve friction f_s were determined on the basis of the CPTU penetration tests (Figure 2).

Figure 2

Relationships between: a) q_c – I_L, b) c_u – I_L, c) f_s – I_L.

Bearing capacity of the columns was calculated for soil conditions determined in four selected locations located on the section of Bargłów Kościelny bypass under consideration. The analyzed locations were spaced about 30mapart. The calculations weremade for different depths of columns in the weak subsoil z_k (Figure 1) equal to 0, 0.75, 1.5 and 3.0 m for location 1 and 4; 0, 0.75, 1.5 and 2.5 for location 2; and 0, 0.75, 1.5 and 2.0 for location 3.

Different values of the maximum depth for locations 2 and 3 are due to the fact that a depth of 3 m would mean that the tip of the column would be located in stiff soil and the column would not be a floating column.

Figure 3 presents the results of the CPTU tests in the four selected locations located on the section of the Bargłów Kościelny bypass under consideration and the thickness of the layers of peat and sandy clay with a variable liquidity index I_L.

CPTU tests were carried out to a depth of about 12 m, which wouldbe insufficient to make calculations based on the cone resistance graphs; therefore a forecast of the liquidity index was made to a depth of 20 m (Figure 3).

Figure 3

Results of CPTU tests: a) location 1, b) location 2, c) location 3, d) location 4.

Table 1 shows the values of the cone resistance, sleeve friction and undrained shear strength from Figure 2 for different liquidity indexes of the sandy clay located at the tip of the floating GEC column.

Table 1

Parameters of the sandy clay determined on the basis of the CPTU penetration tests.

Degree of plasticity I_L	Cone resistance q_c	Sleeve friction f_s	Undrained shear strength c_u
[-]	[kPa]	[kPa]	[kPa]
0.58	676.1	11.05	30.10
0.42	1193.4	23.67	56.98
0.31	1679.9	34.67	81.90

4 Bearing capacity of the floating GEC columns

Figure 4 shows the bearing capacity of the floating columns at different depths of the column tip in the soft sandy clay for the five selected calculation methods.

Figure 4

Bearing capacity of the floating GEC columns: a) location 1, b) location 2, c) location 3, d) location 4.

5 Conclusions

From the calculations, comparable values of the column bearing capacity were obtained for the five calculation methods analyzed in this paper. Differences in the values for the different calculation methods are due to the different assumptions on which the methods were based.

In all the analyzed locations, the lowest bearing capacity of the column was determined using the LCPC method. The highest bearing capacity was calculated using the European and Schmertmann methods, and for location 3, using the DIN 4014 method.

The bearing capacity of the columns increased with increasing depth of the column tip in sandy clay. A more than three times higher bearing capacity that varied with the depth was observed in location 1.

The highest bearing capacity was determined for the columns in location 3, while the smallest was for the columns in location 1. This can be related to the soil conditions in each individual location.

The methods of determining the bearing capacity of floating GEC columns described in this paper do not allow the distribution of the load in the columns and weak subsoil to be included in the calculations; thus, it does not take into account cooperation between the column and the surrounding soil. However, the author believes that these methods can be successfully used to pre-estimate the bearing capacity of floating GEC columns based on the results of CPTU penetration tests.

Main Nomenclature

A_k: column base area
A_s: column shaft area
c_u: undrained shear strength
D_k: column diameter
f_p: shaft resistance
f_s: sleeve friction
I_L: liquidity index
Q_bk: column base capacity
q_b: unit end-bearing pressure
q_c: cone resistance
Q_k: column bearing capacity
Q_sk: column shaft capacity
q_t: corrected cone resistance
z_k: depth of the column base
CPTU: cone penetration test
GEC: geosynthetic encased column

Acknowledgement

This paper, carried out at Bialystok University of Technology, was supported by Polish financial resources on science under project WZ/WBiIŚ/7/2019.

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Received: 2019-07-25

Accepted: 2020-05-13

Published Online: 2020-07-14

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/eng-2020-0078

Keywords for this article

GEC columns; bearing capacity of piles; CPTU penetration tests; floating columns

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