It was March 2002, one of my clients sent me a soil reports to examine that it had been caused a conflict between my client and their consulting engineers company who had presented the allowable bearing capacity equal to 20-30 ton for single precast concrete piles in their soil reports as follows:
Dimension of precast piles
L=25-30 m Length of precast piles
Qa=20-30 ton Allowable bearing capacity of precast piles
When I studied Geotechnical reports, I understood that they was taking two great mistakes because the results of Triaxial(U.U) and Shear box(C.D) tests were too much more less than the results of S.P.T tests from depth 18m to final boring (35m) :
1) It sound that they had done Triaxial and Shear box tests on undisturbed samples collected from High Over Consolidation clay (H.O.C clay layers) and it had been caused that undisturbed samples were changed to disturbed samples in Laboratory(the allowable bearing capacity had been calculated in accordance with laboratory tests).
2) It sound that they had taken into the soil characteristics uniformly until depth of 25m.
The results of my views are as follows:
(Regarding to incompatibility between the results of field and laboratory, I have only used of the results of field (S.P.T) except soil classification)
-From depth of 0.00 to 18m:
(G.W.T: Average 1.5m from 0.00 level of bore holes)
1) Soil classification: CL, ML
The soil layers were U.C to N.C (under to normal consolidation) and according to Table 3-5 of J.E Bowles 1996, we have;
-From Depth of 18m to 23m:
1) Soil classification: CL, ML
8) Consistency: Very Stiff
The soil layers were O.C (Over Consolidation).
-From Depth of 23 to 30 m:
1) Soil Classification: CL, ML (Cemented)
The soil layers were H.O.C (High Over Consolidation).
According to above mentioned, allowable bearing capacity of pre-cast concrete piles (Driving) were calculated in accordance with ALPHA-method of M.J.Tomlinson by me as follows:
(In here, the executed dimension has been only calculated that it is 40* 40 cm)
Total settlement (Elastic) had been estimated in accordance with Braja.M.Das by me below cited:
S1 = 0.54 cm Elastic settlement of pile body (concrete)
S2 = 0.32 cm Elastic settlement of pile point soil
S3 = 0.24 cm Elastic settlement of pile sides soil
S = S1+ S2 + S3, S = 1.1 cm
The consolidation settlement had been calculated for piles group and it was negligible just like to elastic settlement.
Therefore, the settlement was not controller and allowable bearing capacity was announced 89 Ton.
This example is included points : A,C,D,F,H,I.
My design was caused a saving money about 3.5 million Euro and so saving time for my client that it was more important than saving money.
Regarding to example(1), I prepared a operating instruction manual accompanied by executive details for construction(pre-fabrication) and transport of pre-cast piles and so method of driving pile ramming that summary of it is as follows:
-Anticipation of amount of piles penetration during ramming at the site in accordance with static bearing capacity obtained in example (1) and modified ENR formula (Dynamic pile formula) for controlling of the results of static bearing capacity (Returning Analyze), and so a Table had been made for this case by me.
-To determine allowable bearing capacity by using of dynamic pile formula (modified ENR formula) as follows:
-Type of diesel piling hammer: K35
-Set final (Refusal): 6 blows/in
-Pile length (L): 23m
-Safety factor: 6
-Efficiency of hammer (E): 80%
-C =0.1 , n = 0.45
Therefore, it was compatible with the result of example (1) and so a Table had been made for this section.
-To determine produced stress in piles during ramming:
Set final (Refusal): 6 blows/in
-To determine minimum length for jointed pre-cast concrete piles:
According to Swedish Code:
Above ratio is for diesel piling hammer.
Therefore, due to diesel piling hammer (K35), minimum length of piles must be 10m.
-The methods for lifting, picking up and transferring of piles:
According to Fig.7.1, Fig.7.2 and Table 7.1 from book of Pile Design and Construction Practice by M.J.Tomlinson (1981) and regarding to length and main reinforcement of pre-cast piles, we can design the methods of lifting piles.
-Maximum cracking on surface of concrete of piles:
Because of ramming, it is possible to be created some crack on surface of concrete.
According to England standard (CP110), maximum opening is 0.3mm for embedded concrete and for expose concrete; it must not be increased from 0.004 times thickness of cover on main reinforcement.
-To control of negative skin friction by using of bitumen coating on piles:
After pile ramming, if backfilling with the thickness equal to 1 m is executed on natural soil and the sides of piles, in accordance with Bowles (1996) we have:
Therefore, we have:
L1=16.2 m Distance to the neutral point
Qn = 39 Ton Total force of negative skin friction
In accordance with above mentioned, executive instructions for coating are as follows:
a) Mix bitumen RC-30 as pre-coating (amount of 0.1-0.5 lit/m2).
b) Bitumen 60/70 or 30/50 as final coating with thickness of 10mm plus or minus of 2mm.
c) Maximum length of coating: 16.2 m.
-To prepare ID-card for each pile
-Quality control procedure of concrete for reaching to the design compressive strength of concrete equal to 550 kg/cm2 that it must be used of Micro-silica with super plasticizer additive.
-Quality control of water, cement and aggregates
-Design of pile point:
If dimension of pile point is 10*10 cm, the angle between side plane of pile and vertical plane must be about 30 degree.
If dimension of pile point is 15*15 or more than that, above angle is calculated from below formula:
d = diameter of pile (cm)
This example is included points: B, C, D, F, H, M, Q.
Regarding to example 1 and 2, I had been informed by site manager that some pre-cast piles had been driven no more than depth of 18m. It meant that they had reached to a very hard layer. I checked it out and I found out that the distance between this piles were less than 3m.
Unfortunately, before starting of geotechnical activities, it had not been done any appropriate Geology, Hydrogeology and Geophysics investigations at the site.
Therefore, I ordered to do Seismic Refraction Survey (one of Geophysics methods) and provide Geology records.
We know, in accordance with velocity of “P” waves, we can calculate thickness, hardness and compaction (density) of soil layers as follows:
According to Geology records and results of Seismic Refraction Survey, it was discovered that there was a Dendroidal shape of High Over Consolidation clay (clay stone) after depth of 16-17m. In fact, location of project was on a Delta (connection of river to sea).
Note: Sometimes alluviums or loose fill (made-up gravel) have been laid unconformity on geology formations included: out crops, folded rock stratums that their geomorphology have been shown in shape of up and down (roughness).
Therefore the logs of two bore holes, which have very low distance between them, are not compatible together. It is possible, one of them encounter to rock layer in depth of 1m but another bore hole come in contact with this rock layer in depth of 10m.
One of the best ways to specify this problem is to use of German Light S.P.T equipment (DIN 4094). The specifications of this equipment are as follows:
D =22 mm Rod diameter
D*=35.6 mm Point diameter
a =60 degree Point angle
W = 10 kg Hammer mass
H = 50 cm Free falling height
A = 10 cm2 Point area
We can even change number of blows to ASTM-D1586 in accordance with energy equilibrium’s principle.
This example is included points: B, C, D, E, F, G, H, L, M, N, R, S.
Referring to examples (1), (2) and (3), load test of pre-cast piles (compression tests) was done by my client for controlling of my design.
Regarding to my client’s records, three pre-cast piles had been tested in accordance with ASTM-D 1143-81.The results of compressive loading tests on pre-cast piles adopted from load-settlement curves were as follows:
-Pile number: P059
L = 21 m (because of the problem mentioned in example (3), B = 40*40cm
Q = 280 Ton Maximum load
dz = 21 mm Maximum vertical displacement
For vertical displacement = 11mm, we have: Compressive load = 175 Ton
Pile number: P126-
L = 24 m, B = 40*40 cm
Q = 340 Ton Maximum load
dz = 19 mm Maximum vertical displacement
For vertical displacement = 11mm, we have: Compressive load = 195 Ton
-Pile number: TP11
This pile was a tentative pile not to operate and it had proved that the length of piles must be more than 21m.
L = 18 m, B = 40*40 cm
Q = 140 Ton Maximum load
dz > 55 mm Maximum vertical displacement
In accordance with Load- Vertical displacement curve, we have:
Therefore, the design of pre-cast piles was approved by my client.
On the other hand, there was an important problem in Destacking house (a ware house for storing of galvanized sheets). The problem was “Effect of adjacent surcharge loading on lateral displacement of pre-cast concrete piles” because galvanized sheets, which were stored on finish floor of ware house ( Destacking house), were to maximum distance of 0.25 m from pre-cast piles executed under columns of Destacking house(pile caps) and if lateral displacement of pre-cast piles was increased more than allowable limit, overhead traveling crane was stopped.
Here is loading specifications of galvanized sheets:
B*L = 4.5* 12.2 m, Dimension of galvanized sheets
T = 10 mm, Thickness of galvanized sheets
H = 3.5 m, Height of storing (on each other)
P = 7800 kg/cm3, Specific gravity of galvanized sheets
Therefore, loading of galvanized sheets was just like to loading of a rectangular footing that it was equal to:
The summary of my research method in accordance with lay out of galvanized sheets storing is as follows:
-Analysis of loading adjacent pre-cast piles in accordance with Boussinesq equation (JE.Bowles 1996)
-The calculation of bending moment and lateral displacement of pre-cast piles in accordance with equations and curves presented by Davisson and Gill (1963) as follows:
According to above mentioned, it obtained maximum bending moment and lateral displacement below cited:
So, I ordered to do a modeling test of loading adjacent pre-cast pile at the site as follows:
-A pre-cast pile was rammed at the site (L = 15 m, B = 40*40 cm).
-A plate load test (just like to ASTM-D 3966) was done by several rigid plates (maximum dimension of 100*100 cm) and maximum load of 50 Ton with distance equal to 20 cm from pre-cast pile( the distance between center to center of pre-cast pile and rigid plate was 90 cm).
-Three of gauges were installed for measurement of lateral displacement of pre-cast pile during the plate load test.
The results of modeling test showed a maximum lateral displacement equal to 4.5 mm.
The conclusion of research:
-All of analysis and calculations were approximately confirmed by modeling test.
-I offered to control the lateral displacement of pre-cast piles the points as follows:
1) To increase of inertia moment by executing of additional piles.
2) To decrease of at-rest lateral stress ratio(Ko) by using of lean concrete between galvanized sheets and pre-cast piles until critical depth.
3) To use of reinforced concrete tie beam between two against columns.
This example is included the points: A, B, C, D, F, G, H, I, K.
I have mentioned this example because it is included the point: “J” that is one of the most important points for solving of the problem.
Regarding to examples: 1, 2, 3 and 4, there was a great problem in production house because there must be installed a press machinery in depth of (-6 m) while Ground Water Table was about depth of (-1.5 m).
I offered to use sheet piling for the excavation. Before designing of sheet piles, I must control below points:
- “Heaving” because of instability the clay layers of trench floor. In accordance with Bjerrum and Eide(1965), we can calculate ‘Heaving’ as follows:
- “Piping” because of difference between hydraulic gradient outside and inside of excavation (arising of pumping dry) as follows:
h: The difference of water head between outside and inside of excavation
- “Ground loss” because of sides ground settlement arising of movement of sheet piles.
- Over turning of sheet piles in accordance with diagram of design stress envelope (Peck1969).We usually use of Struts and Wales for controlling of over turning.
In here, the important point was not designing of sheet piles but it was the omission of Struts and Wales by me because I ordered to use of the beams for connection of pre-cast piles executed (previously) around of sheet piles (out side of excavation location) to sheet piles.
This example as well as shows the point: “J”.
According to this example, we can see that a soft ware is not enough for designing alone.
This example is also compatible with point “J”.
I received a request of another client that it was the excavation of urban zone till depth of 15 m. After visiting of the site, I offered to execute of the temporary supporting structure by using of Drilled-in-Place piles.
Note: “Always there are too much problems for excavations in loose soils, especially if the depth of them is very high (deep). In addition to soils problems, two important factors are also the controller of stabilization methods that they are Time and Cost (Energy).
One of the simplest methods for excavation is to use of Drilled-in-Place piles as the retaining wall but the most important thing to analyze and design them is to stable against over-turning because Drilled-in-place piles could not be designed and executed for excavations with high height of the walls without Struts and Wales.
In this example, the methods have been proposed for deep excavations by using of Drilled-in-Place piles and acquiring of a wedge failure is shown based on both theoretical considerations and observations of model footing (Jumikis(1962), Ko and Davidson (1973)) and finite elements model for securing passive pressure of soils as resisting force against over turning so that Drilled-in-Place piles can be executed the stepped shape or two row piles that one is supporting the piles near to the wall.
Before an excavation is started to be executed, it must be studied if a vertical wall into soil will be the stable without a supporting structure and what is amount of safety factor or critical height.
In first step, mechanical parameters of soil should be obtained in accordance with Geotechnical investigation and so we can use of returning analysis.
In second step, loading analysis should be considered regarding to surcharges loads and soil specifications.
In third step, we should design a retaining wall as supporting structure, if a vertical wall without a retaining wall is not the stable.”
Specifications of the project:
6-1) In the southern part of the project ground:
Under footing of a building (two floor) had been excavated to depth of 12 m without any supporting structure and it showed a section of cemented sandy gravel layers (Hezar Dareh conglomerate of geology formations).
Regarding to the stabilization time (about 1year) of excavation, I estimated minimum the parameters of soil layers by using of Retaining Analysis as follows:
-Taylor Method (1937):
Therefore, minimum selected characteristics of Conglomerate layer are:
6-2) In the western part of the project ground:
Concerning to Geotechnical investigations included: Test pits, Standard Penetration Tests and Remolded Shear Box Tests, soil characteristics were selected as follows:
- From depth of 0.00 to depth of 5 m (very loose sandy gravel with clay):
- After depth of 5 m:
There was Conglomerate layer just like to (6-1) as follows:
Therefore, the problem was point (6-2). In the western part of the project ground, I offered to use of Drilled-in-Place concrete piles as the temporary supporting structure.
Step1) Loading Analysis:
-There was a traffic load as the surcharge load with distance at least 70 cm from wall edge, that I considered it as a model of continuous foundation as follows:
B = 30 cm, Width of continuous foundation
Q = 5.6 kg/cm2, Loading by continuous foundation
According to Bowles (1996) and Boussinesq equation, the load diagram was obtained just like to a parabolic curve that total sum of load and load resultant are as follows:
- Lateral pressure of soil:
Step 2) Geotechnical design of Drilled-in-Place piles:
It was included the points as follows:
- Length of piles (L)
- Diameter of piles (d)
- Numbers of total piles (N)
- Distance of center to center the piles from each other (r)
- Length of clamping (fixity) of piles into cemented sandy gravel layer (h)
For Geotechnical design (above points), below parameters must be controlled:
- Stabilization controlling against over-turning.
- Stabilization controlling against sliding forward.
- Stabilization controlling for allowable bearing capacity of foundation.
Usually, the important problem is over-turning and the other cases are ok.) )
H = 500 cm, Excavation from depth of 0.00 to 500cm
h = 250 cm (0.5H)
L = H + h = 750 cm
d = 80 cm
N = 7 In accordance with length of excavation wall
r = 125 cm In accordance with length of excavation wall
- Calculations for controlling of over-turning:
Therefore, we have:
It is possible only using of passive pressure (Pp), if it was executed a stair with width equal to 2.5 m (from Conglomerate layer) in front of piles for securing of passive pressure. Width of stair could be calculated in accordance with model footing of Jumikis(1962), Ko and Davidson(1973) accompanied by using of finite element model. Of course, it can be a research work at the university.
So, we can obtain characteristics of soil by Returning Analysis where we can use of a Grab Bucket Crane at the site so that the excavation is done in center of the project ground (limited section) until appropriate depth by a Grab Bucket Crane for reaching to a critical safety factor.
Therefore, this example as well as shows the points: “J” and “Q”.
This example is compatible with the point: “O”.
Concerning to example (6), I was persuaded to do structural design of Drilled-in-Place concrete piles by my client because of lack of time (I present the Geotechnical design for the projects because I have only studied BSc degree in the field of Geology at the university). For structural design of concrete piles, I had to read chapters: 1, 2, 4, 5, 6 and 10 from Mechanic of Materials book (by Popov.EP) and ACI-318.
Here is my design method:
Drilled-in-Place concrete piles had been considered as Cantilever beam.
According to free body diagram:
Regarding to bending moment and shear diagrams, maximum main longitudinal reinforcement must be executed from bottom of piles (on ground) to the height of
4.5 m and according to ACI-318, maximum bars are 6% of the piles cross section
For acquisition the equivalent section of concrete and bars and calculating the moment of inertia, I considered total cross section of reinforcements just equal to thin-wall steel pipe as follows:
t: Thickness of thin-wall steel pipe (cm)
Therefore, all of assumptions were ok.
Since bending moment was the controller (not shearing force), this design was approved to execute by me.
This example as well as present the points: “O” and “M”.
This example is a general idea or a hypothesis for highlighting of the point: “N”.
Maybe, one of the ways to improve saturated clay soils (N.C) will be to prevent their drainage in the future.
I have brought this example because it is compatible with the point: “P”
In one of the earth dam projects, we had to inject Red-colored water into the joints and cracks of Rocks under dam foundation for finding out direction (orientation) and distribution of them.
Red-colored water had been appeared on the ground surface until radius of 30 km far away from the location of Dam. It as well as showed connection among the joints and cracks.