Transcript Pertemuan 10 Penurunan Pondasi Dangkal Matakuliah : S2094 / Rekayasa Pondasi

Matakuliah
Tahun
Versi
: S2094 / Rekayasa Pondasi
: 2005
: 1.1
Pertemuan 10
Penurunan Pondasi Dangkal
Settlement Analysis Based on
Laboratory Tests
Approach used when good quality
“undisturbed” samples can be obtained
from soil
 Perform consolidation test
 Obtain Cc, Cr , e0, and ´m


Perform settlement analysis
Two Methods of Analysis

Classical Method
 Based
on Terzaghi’s Theory
 One dimensional compression

Skempton and Bjerrum Method
 Considers
distortion settlement
 Uses an adjustment factor for 3D
compression
Classical Method; Foundation
Rigidity Factor
Classical
Method
Thickness of Soil Sub-layers
Settlement Predictions
N.C. Clays
  zf 
Cc

 c  r
H log 
1  e0
  z 0 
Settlement Predictions
O.C. Clays…… Case I
  zf 
Cr

 c  r
H log 
1  e0
  z 0 
Settlement Predictions
O.C. Clays…… Case II
  c  Cc
  zf 
Cr
 

 c  r
H log 
H log 
1  e0
  z 0  1  e0
  c 
Skempton & Bjerrum Method

Settlement, 
  d   c

Distortion Settlement (d)
(q   zD ) B
d 
I1 I 2
Eu

Consolidation Settlement (c)
Distortion Settlement
3D Adjustment Factor, 
Influence Factors, I1 and I2
I1
I2
Example 7.3
Example 7.4
Settlement Analysis based on InSitu Tests
 Techniques for estimating settlements in
sands are nearly always based on in-situ
test results
 In sands, settlement analysis is not
performed based on consolidation analysis
 Instead, we use Equivalent Modulus of
Elasticity, Es
Es from SPT Data (Table 7.4)
Es  0 OCR  1 N60
Es from CPT Data (Table 7.3)
Schmertmann’s Method
I H
  C1 C2 C3 (q   zD )
Es
C1 = depth factor = 1  0.5  zD 
 q   
zD 

C2 =
 t 
Secondary creep factor =1  0.2 log  
 0 .1 
C3 = Shape factor = 1.03  0.03L / B  0.73
Strain Influence Factor
I p
q   zD
 0.5  0.1
 zp
Simplified Schmertmann
Method
 When Es is uniform with depth over the
depth of influence
 For square/circular footings:

C1 C2 C3 (q   zD )( I p  0.025) B

C1 C2 C3 (q   zD )( 2 I p  0.1) B
Es
 For continuous footings:
Es
Settlements in Stratified Soils

When the soil profile primarily consists
of clays and silts – use procedures
described in Section 7.4 to estimate
settlement
 For
clays/silts use laboratory consolidation
tests to determine Cc/(1+e0) and Cr/(1+e0)
 For sands use Cc/(1+e0) or Cr/(1+e0) from
Table 3.7
Settlements in Stratified Soils

When the soil profile primarily consists
of sands and silts – estimate settlement
based on Schmertmann Method
 Use
in-situ test data to determine Es for
sand
 Use following equation to determine Es for
clays
2.30 z
Es 
Cc /(1  e0 )
or
2.30 z
Es 
Cr /(1  e0 )
Settlements in Stratified Soils

Alternative Approach
 Conduct
separate analysis for clays/silts and
sands
 Add the computed settlements
Differential Settlement
 Total Settlement, 
 Differential Settlement, D
 Rotation, 
Differential Settlement
 Variations in Soil Profile
 Variations in Structural Loads
 Design Controlled by Bearing
Capacity
 Construction Tolerances
Computing Differential Settlement

Consider best case/worst case
scenarios



Use D/ ratios
Clayey Soils…. Figure 7.20
Sands …. Figure 7.21
Total and Differential Settlement for
Clays
Total and Differential Settlement for
Sands
Design values of D/ Ratios
Rate of Settlement

In saturated soils rate of settlement
depends largely on how fast the water
drains away

In Clays rates of settlement are slower than
in sands

Consolidation theory allows you to calculate
rate of settlement (Chapters 19 and 20)

Rate of settlement in sands vary depending
upon the pattern of loading
How
Accurate are
our
Settlement
Predictions?
Computer Software SCHMERT (schmertmann.xls)