H-2 Stiffness Factors

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 599

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H-2.1 The lateral soil resistance for granular soils and normally consolidated clays which have varying soil modulus is modeled according to the equation:

\frac {p}{y} = \eta_ {\mathrm{h}} z

where

p = lateral soil reaction per unit length of pile at depth z below ground level;

$y =$ lateral pile deflection; and

$\eta_{\mathrm{h}} =$ modulus of subgrade reaction for which the recommended values are given in Table 10.

H-2.2 The lateral soil resistance for over-consolidated clays with constant soil modulus is modeled according to the equation:

\left| \frac {p}{y} = K \right|

where

K = \frac {k _ {1}}{1 . 5} \times \frac {0 . 3}{B}

where $k_{1}$ is Terzaghi's modulus of subgrade reaction as determined from load deflection measurements on a 30 cm square plate and B is the width of the pile (diameter in case of circular piles). The recommended values of $k_{1}$ are given in Table 11.

Table 10Modulus of Subgrade Reaction for Granular Soils and Normally Consolidated Clays, \eta_{\mathrm{h}}, kN/m³ (Clause H-2.1)

Sl No.	Soil Type	N (Blows/ 30 cm)	Range of $\eta_{h}$ kN/m $^{3}$ x 10 $^{3}$
Sl No.	Soil Type	N (Blows/ 30 cm)	Dry (4)	Submerged (5)
(1)	(2)	(3)	Dry (4)	Submerged (5)
i)	Soft organic silt	-	-	0.15
ii)	Soft normally consolidated clay	0 - 4	-	0.35 - 0.7
iii)	Very loose sand	0 - 4	<0.4	< 0.2
iv)	Loose sand	4 - 10	0.4 - 2.5	0.2 - 1.4
v)	Medium sand	10 - 35	2.5 - 7.5	1.4 - 5.0
vi)	Dense sand	>35	7.5 - 20.0	5.0 - 12.0

NOTE — The $\eta_{h}$ values may be interpolated for intermediate standard penetration values, N.

Table 11Modulus of Subgrade Reaction for Cohesive Soil, k_{1}, kN/m^{3} (Clause H-2.2)

Sl No.	Soil Consistency	Unconfined Compression Strength, $q_{u}$ kN/m $^{3}$ × 10 $^{3}$	Range of $k_{1}$ kN/m $^{2}$
(1)	(2)	(3)	(4)
i)	Soft	25-50	4.5-9.0
ii)	Medium stiff	50-100	9.0-18.0
iii)	Stiff	100-200	18.0-36.0
iv)	Very stiff	200-400	36.0-72.0
v)	Hard	> 400	>72.0

NOTE — For $q_{\mathrm{u}}$ less than 25, $k_{1}$ may be taken as zero, which implies that there is no lateral resistance.

H-2.3 Stiffness Factors

H-2.3.1 For Piles in Sand and Normally Consolidated Clays

Stiffness factor $T$ , in $\mathrm{m} = \sqrt[5]{\frac{EI}{\eta_{\mathrm{h}}}}$ where

$E =$ Young's modulus of pile material, in $\mathrm{MN / m^2}$ ;

$I =$ moment of inertia of the pile cross-section, in $\mathrm{m}^4$ ; and

$\eta_{h} = \text{modulus of subgrade reaction variation, in } MN/m^{3} (see Table 10).$

H-2.3.2 For Piles in Over-Consolidated Clays

\text {Stiffness factor} R, \text {in} \mathrm{m} = \sqrt [ 4 ]{\frac {E I}{K B}}

$E =$ Young's modulus of pile material, in $\mathrm{MN / m^2}$ ;

$I =$ moment of inertia of the pile cross-section, in $\mathrm{m}^4$ ;

$K = \frac{k_{1}}{1.5} \times \frac{0.3}{B}$ (see Table 11 for values of $k_{1}$ , in MN/m³ (see also Note); and

B = width of pile shaft (diameter in case of circular piles), in m.

NOTE - Table 11 shall be referred if the average unconfined compression strength, $q_{\mathrm{u}}$ is greater than or equal to $100\mathrm{kN / m^2}$ .

H-3 Criteria for Short Rigid Piles and Long Elastic Piles

Having calculated the stiffness factor $T$ or $R$ , the criteria for behaviour as a short rigid pile or as a long elastic pile are related to the embedded length $L_{\mathrm{e}}$ as given in Table 12.