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  4. Verschiedene Methoden zur Schätzung der Stapelkapazität

Verschiedene Methoden zur Schätzung der Stapelkapazität

Estimating Pile Capacity

Estimating the Pile load-carrying capacity is necessary to determine the ultimate axial load that the pile can carry. The ultimate load capacity of the pile (Qu) is equivalent to the sum of end-bearing capacity (Qp) and frictional resistance (Qs), represented by Fig. 1 and Eq. 1. Numerous published studies and practices determine the pile’s end-bearing capacity and frictional resistance. This article focuses on various methods to estimate the ultimate pile capacity.

figure-load-mechanism, Estimating Pile Capacity,

Zahl 1: Load transfer Mechanism for piles

\( {Q.}_{u} = {Q.}_{p} + {Q.}_{s} \) (1)

Q.u : Ultimate load-carrying capacity

Q.p : End-bearing load capacity

Q.s : Skin-frictional resistance

Universal equations for Qp and Qs


End-bearing capacity (Gl. 2) is the ultimate resistance per unit area developed at the tip of the pile. The unit point resistance at pile tip (qp) can be expressed similarly to the general bearing capacity equation for shallow foundations proposed by Terzaghi (Gl. 3).

\( {Q.}_{p} = {EIN}_{p} \mal {q}_{p} \) (2)

EINp : Pile tip area
qp : p-Delta-Effekte

\( {q}_{p} = (c \times {N.}_{c}) + (q’ \mal {N.}_{q}) + (\gamma \times D \times {N.}_{\p-Delta-Effekte}) \) (3)

c : Soil cohesion at the tip of the pile
q’ : Effective vertical stress at the tip of the pile
D. : Pile width
und : Soil unit weight
N.c , N.q, : Bearing capacity factors
Since the width of the pile is relatively small compared to shallow foundations the third term of Eq. 3 can be neglected, thus Eq. 2 can be re-written as:

\( {Q.}_{p} = {EIN}_{p} \mal[ (c \times {N.}_{c}) + (q’ \mal {N.}_{q}) ] \) (4)

 

The total frictional resistance of the pile, which is developed along its length, can be calculated using this equation:

\( {Q.}_{s} = ∑ (p × ΔL × f) \) (5)

p: Perimeter of the pile

ΔL: Incremental pile length over which p and f are taken

f: Unit frictional resistance at any depth

Methods for Estimating Qp


Meyerhof’s Method

Sandy Soil

According to Meyerhof, the unit point resistance (qp) of piles in sand generally increases with the embedment length until it reaches its maximum value when the embedment ratio (L/D) reaches a critical value. Critical embedment ratio (L/D)Einstellungen für Biege-Torsionsknicken usually varies from 16 zu 18. Bei dieser Methode, piles in the sand are assumed to have zero cohesion (c ≈ 0), and the unit point resistance should not exceed limiting point resistance (ql), which is given by Eq. 7. The bearing capacity factor (Bodenkohäsion an der Pfahlspitze q) values are directly proportional to the soil friction angle of the bearing stratum (Tabelle 1). Based on Meyerhof’s theory, the universal equation for Qp (Eq.4) can be simplified to:

\( {Q.}_{p} = {EIN}_{p} \mal (q’ \mal {N.}_{q}) \leq ({EIN}_{p} \mal {q}_{l}) \) (6)

\( {q}_{l} = 0.5 \mal {p}_{ein} \mal {N.}_{q} \times tan (\Phi’) \) (7)

ql : Limiting point resistance

pein: Atmospheric pressure (≈100 kN/m2)

\( \phi’): Effective soil friction angle at the tip of the pile

Ø
N.q
20
12.4
21
13.8
22
15.5
23
17.9
24
21.4
25
26
26
29.5
27
34
28
39.7
29
46.5
30
56.7
31
68.2
32
81
33
96
34
115
35
143
36
168
37
194
38
231
39
276
40
346
41
420
42
525
43
650
44
780
45
930

Tabelle 1: Interpolated values of Nq (Meyerhof’s theory)

Clay Soil

Gleichung 4 can also calculate the end-bearing capacity of piles in clay or cohesive soils (φ ≈ 0). Since soil friction angle is neglected and the bearing capacity factor (N.c) has a constant value of 9 for cohesive soils, Eq.4 can be written as:

\( {Q.}_{p} = {EIN}_{p} \times c \times {N.}_{c} = 9 \times c \times {EIN}_{p} \) (8)

Vesic’s Method

Vesic’s method of calculating end-bearing capacity on sandy or clayey soils is based on his theory of the expansion of cavities.

Sandy Soil

Based on his theory, end-bearing capacity of piles in sand can be estimated using the following equations:

\( {Q.}_{p} = {EIN}_{p} \times \bar{\sigma’}_{Das} \mal {N.}_{\sigma} \) (9)

\(\Bar{\sigma’}_{Das} = frac{1 + (2 \mal {K.}_{Das})}{3} \times q’\) (10)

\( {K.}_{Das} = 1 – sin \phi’\) (11)

\( {N.}_{\sigma} = frac{3 \mal {N.}_{q}}{1 + (2 \mal {K.}_{Das})} \) (12)

\(\Bar{\sigma’}_{Das} \) : Mean effective normal ground stress at the level of the pile point

Ko: Earth pressure coefficient at rest

: Bearing capacity factor

Clay Soil

Same with Meyerhof’s method, Gl. 4 is also applicable to calculate the end-bearing capacity of piles in clay. jedoch, the value of the bearing capacity factor (N.c) is a factor of rigidty index (ichr). According to his theory of expansion of cavities, N.c und ichr can be estimated by:

\( {N.}_{c} = (\frac{4}{3}) \mal [ln({ich}_{r}) + 1] + \frac{\Pi}{2} + 1 \) (13)

\( {ich}_{r} = frac{{E.}_{s}}{3 \times c} \) (For φ ≈ 0)(14)

ichr: Rigidity index

E.s: Modulus of elasticity of soil

 

Coyle and Castello’s Method (Sandy Soil)

Based on 24 large-scale field load tests of driven piles in sand, Coyle and Castello suggested that the end-bearing capacity of piles can be calculated using Eq.15. The values of the bearing capacity factor (Bodenkohäsion an der Pfahlspitze q) is a factor of both embedment ratio (L/D) and the soil friction angle (φ’), wie in Abb. 2

\( {Q.}_{p} = {EIN}_{p} \mal (q’ \mal {N.}_{q}) \) (15)

Zahl 2: Variation of Nq with L/D & Phi’ (Redrawn after Coyle & Castello, 1981)
Quelle: Das, Braja. Grundlagen des Grundbaus (7th Ausgabe, p.564)

Methods for Estimating Qs


Frictional Resistance of Piles in Sand

The unit frictional resistance of piles in sand, as shown in Eq. 5, considers multiple factors which are quite difficult to calculate. It includes the earth pressure coefficient (K.) & soil-pile friction angle, which both have varying values depending on which approach to use or to the available soil data.

\( f = K\times {\sigma}_{Das}’ \times tan (\p-Delta-Effekte) \) (15)

K.: Effective earth pressure coefficient

σ’Das: Effective vertical stress at the depth under consideration

D: Soil-pile friction angle

The following are the different ways to estimate the effective earth pressure coefficient and soil-friction angle values. These variables are a factor of soil frictional angle (φ’) or pile type.

Effective earth pressure coefficient

The soil exerts lateral earth pressure to the pile surface. It is necessary to account for this pressure on the design or analysis for stability. The following are the different ways to determine the earth pressure coefficients to calculate the unit frictional resistance of piles in sand.

NAVFAC DM 7.2

Pile Type Kompression Uplift
Driven H-piles
0.5-1.0
0.3-0.5
Round/Square Driven displacement piles
1.0-1.5
0.6-1.0
Tapered Driven displacement piles
1.5-2.0
1.0-1.3
Driven jetted piles
0.4-0.9
0.3-0.6
Bored piles (< 24″Phi)
0.7
0.4

Tabelle 2: Earth pressure coefficient, K. (NAVFAC DM 7.2)

Average K Method

The earth pressure coefficient (K.) can also be evaluated by taking the average of earth pressure coefficient at rest (K.0), active earth pressure (K.ein), and passive earth pressure (K.p), as shown from Equations 16-19.

\( K =\frac{{K.}_{0} + {K.}_{ein} + {K.}_{p}}{3} \) (16)

\( (K.)_{0} =1sin \phi \) (17)

\( (K_{ein} =1 – {tan}^{2}( \frac{45 – \phi}{2}) \) (18)

\( (K_{p} =1 + {tan}^{2}( \frac{45 + \phi}{2}) \) (19)

Mansur and Hunter (1970)

Based on different field load test results, Mansur and Hunter concluded the values of earth pressure coefficient with the corresponding pile types.

Pile Type K.
H-piles
1.65
Steel pipe piles
1.26
Precast concrete piles
1.5

Tabelle 3: Earth pressure coefficient, K. (Mansur and Hunter, 1970)

Soil-pile Friction Angle

The friction angle between the soil and the surface of the pile is an essential aspect of foundation design. Practically, many engineers approximate this value as equal to 2/3 of the internal friction angle of the soil. jedoch, based on the study of Coyle and Castello in 1981, the soil-pile friction angle is approximately equivalent to 80% of the internal friction angle of the soil. Andererseits, NAVFAC DM7.2 uses these values to estimate the friction angle between the soil and pile:

Pile Type D
Steel pile
20°
Timber pile
3/4 Phi
Concrete pile
3/4 Phi

Tabelle 4: Soil-pile friction angle (D) (NAVFAC DM 7.2)

 

Frictional Resistance of piles in Clay

Calculating the frictional resistance of piles in clayey soils can be as challenging as the one in sandy soils due to the introduction of new variables, which are also not as easy to determine. jedoch, there are several available methods to obtain the values of these variables.

λ Method

Based on the study of Vijayvergiya and Focht in 1972, the total frictional resistance of piles in clay can be estimated by determining the average unit frictional resistance of the pile, as shown by Equations 20 und 21. λ values changes as the depth of the penetration of pile increases. Tabelle 5 shows the variation of λ with the embedment length of the pile.

\( {f}_{av} = \lambda \times [\Bar{\sigma’}_{Das} +( 2 \mal {c}_{u})] \) (20)

\({Q.}_{s} = p \times L \times {f}_{av} \) (21)

\( \Bar{\sigma’}_{Das} \): Mean effective vertical stress for the entire embedment length

cu: Mean undrained shear strength

L. (m) λ
0
0.5
5
0.336
10
0.245
15
0.200
20
0.173
25
0.150
30
0.136
35
0.132
40
0.127
50
0.118
60
0.113
70
0.110
80
0.110
90
0.110

Tabelle 5: Variation of λ with pile embedment length (L.)

α Method

The α method suggests that unit frictional resistance of piles is equivalent to the product of the undrained cohesion of the soil layer and its corresponding empirical adhesion factor (ein). Tabelle 6 shows the corresponding value of the adhesion factor with the ratio of undrained cohesion and atmospheric pressure (cu/pein).

\(f = \alpha \times {c}_{u}\) (22)

Deshalb, the total frictional resistance of pile in clay using this method can be re-written as:

\({Q.}_{s} = sum (f \times p \times \Delta L) = sum (\alpha \times {c}_{u} \times p \times \Delta L)\) (23)

 

cu/pein ein
≤ 0.1
1.0
0.2
0.92
0.3
0.82
0.4
0.74
0.6
0.62
0.8
0.54
1.0
0.48
1.2
0.42
1.4
0.40
1.6
0.38
1.8
0.36
2.0
0.35
2.4
0.34
2.8
0.34

pein = atmospheric pressure ≈ 100 kN / m2

Tabelle 6: Variation of α (p-Delta-Effekte, Peck, and Mesri, 1996)

 

β Method

Pore water pressure around the pile increases when the pile is driven into saturated clays. This method, based on effective stress analysis, is suited for long-term (drained) analyses of the pile load capacity as it considers the gradual dissipation of the excess pore water pressure over time. According to Tomlinson (1971), piles driven in soft clays assume that failures occur in the remolded soil close to the pile surface. Based on Eq. 15, der Begriff (K × tanδ) for unit frictional resistance of piles in sand shall be represented by β. The soil-friction angle (D) shall be replaced by a remolded drained friction angle of the soil (Phi’R.). Thus the unit frictional resistance of piles in clay is estimated to be equal to:

\(f = \beta \times {\sigma’}_{Das}\) (24)

\(\beta = K \times tan {\Phi ‘}_{R.}\) (25)

Conservatively, the earth pressure coefficient (K.) is equivalent to the earth pressure coefficient at rest (K.0) which varies for normally consolidated clays and overconsolidated clays, as shown in the following equations:

\( K = {K.}_{0} = 1 – ohne {\Phi ‘}_{R.}\) (Normally consolidated clays) (26)

\( K = {K.}_{0} = (1 – ohne {\Phi ‘}_{R.}) \mal sqrt(OCR)\) (Overconsolidated clays) (27)

OCR: Overconsolidation ratio

 

 

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Verweise:

  • Das, B.M. (2007). Grundlagen des Grundbaus (7th Ausgabe). Global Engineering
  • Rajapakse, R.. (2016). Pile Design and Construction Rule of Thumb (2nd Ausgabe). Elsevier Inc.
  • Tomlinson, M.J. (2004). Pile Design and Construction Practice (4th Ausgabe). E. & FN Spon.
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