Embed Size (px)
Citation preview
17-11-2014
1
Geotechnical Investigation Reports
and
Foundation Recommendations
-Present status in India
-Examples
Prof. V.S.Raju
(Formerly: Director, IIT Delhi & Professor and Dean, IIT Madras)
Email: [email protected] Prof. V.S. Raju
1
CHOICE OF APPROPRIATE FOUNDATION AND EXECUTION
Optimum foundation design should ensure: technical
adequacy, cost effectiveness and ease of execution.
This is not easy, because of many variables including insufficient and inaccurate information at the time of design.
Variation in strata and changes in project requirement during execution
Prof.V.S.Raju 2
17-11-2014
2
TOPICS COVERED
Inadequacy of the Investigations and Inappropriate Foundation Recommendations with Illustrative Examples 1. Fertilizer Plant on the East Coast
2. Fertilizer Plant in Gangetic Belt
3. Office cum Residential Complex at Jodhpur
4. University Campus at Adilabad District
5. L&T Serene County (Residential Campus)
Prof.V.S.Raju
3
FERTILIZER PLANT ON THE EAST COAST
The average soil strata consists of : top layer of dense fine sand 4 to 8 m thick.
followed by soft marine clay up to a depth of 14 to 18 m below ground level.
very stiff clay up to 30 to 40 m below ground level.
Prof.V.S.Raju 4
17-11-2014
3
Prof.V.S.Raju 5
PROBLEMS DUE TO THE PRESENCE OF SOFT CLAY LAYER For site grading, 1- 2 m fill is required - results in a settlement of 12 cm of soft clay. - Along with clay, sand layer also moves down.
In case of pile foundation, large negative drag on piles from soft clay and sand layer.
Considerable reduction in pile capacity (upto 50%), and hence increase in the number of piles.
For structures like bulk storage with large area loads, the stability of soft clay layer and lateral flow to be checked.
Large settlements of floors and lateral forces on neighbouring pile foundations.
Prof.V.S.Raju 6
17-11-2014
4
FOUNDATIONS ADOPTED Precast driven piles were chosen, with bitumen coat in the top layers to reduce the negative drag.
All light structures, not sensitive to settlements, are supported on shallow foundations.
For structures having distributed loads over large areas (silos, water storages), surcharge provided on the periphery to achieve reduced shear stresses on soft clay.
Prof.V.S.Raju 7
The original recommendation was to go for bored piles, 45 cm in diameter. Two alternative pile types have been tested.
a) Precast driven piles 40 cm x 40 cm, 22 m long (to be able to drive). b) Bored cast in situ piles, 45cm dia, 22 m long
Full scale tests for a final decision on the choice of the pile type.
6 test piles, 3 each of the 2 types, installed in 3 different locations in close proximity. Bored cast in-situ piles were installed by bailer boring method.
Prof.V.S.Raju
CHOICE OF THE PILE
8
17-11-2014
5
RESULTS OF PILE LOAD TESTS
Prof.V.S.Raju 9
10
60 60 120 65 B6
82 82 164 85 P5
27 27 54 32 B4
97 97 194 110 P3
54 54 108 55 B2
73 (100)* 80 160 73 P1
Working Load (smaller of 2 and 4) (5)
Half of Failure Load (4)
Failure Load (3)
2 / 3 of Load at 12 mm settlement (2)
Pile No. (1)
The pile length is 19.5 m, and failure load extrapolated for 22 m length is 100 tons.
Table 1: WORKING LOAD IN TONS AS PER IS 2911
Prof.V.S.Raju
17-11-2014
6
CONCLUSIONS
Precast driven piles with design load of 70 to 100 tonnes adopted for the following reasons :
For comparable dimensions, the precast pile has 50 to
80% higher capacity than a bored pile.
Precast pile offers better protection to reinforcement. This is particularly important under the present saline ground water conditions.
The jetting of precast pile up to 12 m will reduce the friction over this depth, and consequently the effects of negative drag.
In case of precast piles, the negative drag can be reduced by applying a slip layer of bitumen.
Prof.V.S.Raju 11
Lower Capacity
Boring operations through stiff fissured clay will result
in softening of this layer, thereby limiting the load
carrying capacity of piles.
There is no possibility of applying any slip layer and to
reduce the negative drag.
Prof.V.S.Raju
Bored piles not preferred for the following reasons:
Conclusions (Contd.)
12
17-11-2014
7
ANTICIPATED NEGATIVE DRAG
40 tonnes based on theoretical consideration.
Field load tests on
i) Short piles resting on soft clay,
ii) Instrumented piles, where the load distribution with depth has been measured.
Model tests for the proper choice of bitumen coating to reduce negative drag.
i) Precast concrete 10 cm dia and 50 cm long piles were used.
ii) Bitumen coatings using different grades of bitumen were applied, and results showed 80 to 90% reduction in friction.
iii) Finally SAE 80 grade bitumen was adopted. Prof.V.S.Raju
13
17-11-2014
8
17-11-2014
9
17-11-2014
10
17-11-2014
11
17-11-2014
12
17-11-2014
13
17-11-2014
14
17-11-2014
15
The Soil Strata:
Silty sand with low N values (<10) upto 10 m depth,
N > 20 beyond 20 m depth.
N-values ranging between 10 to 20 for the layer between 10 to 20 m depth.
High water table with possibility of liquefaction during earthquake.
Prof.V.S.Raju
FERTILIZER PLANT IN GANGETIC BELT
30
17-11-2014
16
Prof.V.S.Raju
PHASE I : INITIAL FOUNDATION DESIGN
RCC Piles, Driven Cast-in situ, 400 mm dia
Sand Compaction Piles, 2-3 Rows Around RCC Piles,
Pile capacities
Vertical downward : 50 t, Tension : 5t, Lateral : 2.5t,
Result – Total Requirement
16,000 RCC Piles
32,000 Compaction Piles
Problem of execution on time
31
Prof.V.S.Raju
REVIEW AND FURTHER INVESTIGATION (Pile Load Tests)
Revised Pile Capacities
Vertical : 65t
Tension : 25t
Lateral : 3.5t
Reduction in RCC Piles : 40%
Increase in spacing of compaction piles from 3d to 4 d ;
reduction in Compaction Piles : 50%
Saving in construction time : 6 months
Substantial cost savings as well. 32
17-11-2014
17
Prof.V.S.Raju
PHASE II OF THE PLANT
No RCC Piles.
900 mm dia Vibro-Stone Columns with varied spacing (2d,2.25d and 2.5d) to suit the foundation requirement.
Full scale field trials.
Several Footing tests for confirmation.
Substantial savings in time and cost.
33
34 Prof.V.S.Raju
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45
LOAD kg / m2 x 103
SE
TT
LE
ME
NT
(m
m)
7.5
11.77
LOAD SETTLEMENT CURVES FOR SINGLE COLUMN LOAD TEST
17-11-2014
18
Prof.V.S.Raju
LOAD SETTLEMENT CURVES FOR SINGLE AND THREE COLUMN TEST
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 LOAD kg / m2 x 103
SE
TT
LE
ME
NT
(m
m)
10.5 10
8.6
Aonla (Benefield) Aonla (Prill Tower)
Aonla (Compressor House)
Single column Test
Three Column Test
Prof.V.S.Raju 35
17-11-2014
19
2 -3 Storeyed Buildings Investigations Done
8 Bore holes drilled to 6 m depth each.
As per bore logs Rocky strata. Strata starts at Ground level (GL) in all bore holes except in Bore hole 5, where it starts at 1.5 m. BH 5 is at the extreme corner of the plot where nothing is planned to be built. Bore logs do not give the core recovery, which is a must to be given.
Office cum Residential Complex at Jodhpur
Prof.V.S.Raju 37
Recommendations by Investigation Agency
1. Open foundations (footings)
2. Unconfined compressive strength of rock
range given 650 to 850 t/m2
3. Calculated safe bearing capacity (SBC) 80 to
100 t/m2
Prof.V.S.Raju 38
17-11-2014
20
FOUNDATION RECOMMENDATIONS (Contd…)
4. Recommended SBC is 40 t/m2 at 1m
below GL, except in BH 5; where SBC
is 10 t/m2 at 1.2 m depth and 40 t/m2
from 2 m depth onwards.
5. The SBC adopted in Design is not
known. It should have been written on
the drawings.
Prof.V.S.Raju 39
1. Foundation Depth adopted: 1.5m.
2. Foundation sizes 1.5m x 1.5m to 2.3m x 2.3m
3. For excavation Rock Blasting has been done
4. Instead of excavating / blasting individual pits
for each footing, the entire foot print of all the
buildings has been blasted and excavated.
WHAT HAPPENED AT SITE
Prof.V.S.Raju 40
17-11-2014
21
5. This resulted in excavated rock material ranging
from huge boulders to rubble of volume of about
25,000 m3 (Actually needed ≈ 10% of this).
6. Additional issues:
(a) How to dispose of the excavated material.
(b) Huge quantity of soil material for plinth
filling needed.
What Happened at Site (Contd…)
Prof.V.S.Raju 41
Basis : Site visit
Inspection of the strata in the
excavated pits
Study of the soil report
1. The investigation is not as per the
relevant Indian Standards.
Prof. V.S. Raju’s Investigation of the Situation
and Foundation Recommendations
Prof.V.S.Raju 42
17-11-2014
22
2. The recommendations in the report are
wrong and are less by a Factor 3 to 4.
3. The correct SBC values are 150 t/m2 (on a
conservative side) with a minimum size of
footing as 0.8m x 0.8m.
Depth of footing 0.5m to 1m.
Prof. V.S. Raju’s Recommendations (Contd …)
Prof.V.S.Raju 43
4. There is absolutely no justification to stipulate
a foundation depth of 1.5m for the entire site.
5. There was no need at all to make the footing
sizes so big as given (1.5m x 1.5m to 2.3m x
2.3m)
6. There is no need to blast the strata over the
entire foot print of the building.
Prof. V.S. Raju’s Recommendations (Contd …)
Prof.V.S.Raju 44
17-11-2014
23
7. No need to blast for the individual footings also as the required likely sizes are 0.8m x 0.8m to 1.2m x 1.2m, depth 0.5 m to 1.0m only.
Pavement breakers (jack hammers) will do the job.
Prof. V.S. Raju’s Recommendations (Contd …)
Prof.V.S.Raju 45
Photographs of Blasted Rocks Prof.V.S.Raju
46
17-11-2014
24
rap
Photographs of Blasted Rocks Prof.V.S.Raju
47
University Campus at Adilabad District
Total area ~ 300 acres For Phase I development, Only 7 boreholes Bore logs improper and inadequate. First SPT at 10 m below GL. Pile Foundations recommended and executed, which are not at all required. Pile safe capacity for 600 mm dia, 12 m length : 83 tons (Very low) - Settlement of pile up to 10 % of pile diameter could not be attained
by 3 times the design load.
48 Prof. V.S. Raju
17-11-2014
25
5.8
m t
o 2
5m
soft
ro
ck
1.80 mto 5.80 m yellow clay & Murrum
0.00 to 1.80 m Brown clay
First SPT at 10m below G.L ,
Required every 1m to 1.5m
Typical Bore Profile Prof. V.S. Raju
49
Pile Load Tests:
Five Initial Load Tests
At 3 times the design load, settlements are only 2 mm to 27 mm as against permitted 45 mm to 60 mm.
Pile capacities are not revised.
Piles, which are not required in the first place are grossly over designed.
50 Prof. V.S. Raju
17-11-2014
26
• 30 Acres site
• Strata – Rocks and Boulders with local depressions, highly uneven.
• 10 towers between 11 and 14 floors
• Recommended SBC by the soil Investigation Agency 30 t/m2
- One value for the entire site?
L&T Serene County
Prof.V.S.Raju 51
L&T Serene County
52 Prof. V.S. Raju
17-11-2014
27
Extreme Issue was with one of the 10 towers built
Tower Dimension : 75 m x 30 m
Highly variable strata at Founding Level.
Hard Rock SBC 400 t/m2
Hard Murrum 80 t/m2
Soft Clay 4 m thick 0 t/m2
75 m
30 m
Tower Foot Print 53 Prof. V.S. Raju
Important:
• These type of variations do occur in rocky and bouldry strata
• The soft clay is due to a old pond, which normally gets covered up during site grading.
Prof.V.S.Raju 54
17-11-2014
28
All these examples reaffirm the requirement of high quality Geotechnical Investigation, Interpretation by a qualified Foundation Engineer in close collaboration with the Structural Designer.
Prof.V.S.Raju 55
THANK YOU
JAI HIND
