Ensoft SHAFT - Drilled Shaft Design

[DirtSmuggler]

Hello
Thanks in advance for all who provide feedback.

I've just started using this software, Ensoft SHAFT, in order to do a drilled shaft design for a project. It's for overhead sign structures.

The problem I have is that tip resistance goes to zero half way into the weathered rock layer, and stays at zero during rock layer. I would think tip resistance (Qb) would do the opposite and jump up higher in weathered rock and rock layer, similar to how the skin friction behaves. What am I doing wrong? What am I missing?

See the output below. Qb goes to zero once entering the weathered rock layer and rock layer.
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New Pile


PROPOSED DEPTH = 10.0 FT
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NUMBER OF LAYERS = 3
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WATER TABLE DEPTH = 0.0 FT.
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FACTOR OF SAFETY APPLIED TO THE ULTIMATE SIDE FRICTION CAPACITY = 2.50
-------------------------------------------------------
FACTOR OF SAFETY APPLIED TO THE ULTIMATE BASE CAPACITY = 3.00
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SOIL INFORMATION
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LAYER NO 1----SAND

AT THE TOP

SIDE FRICTION PROCEDURE: Ko METHOD
LATERAL EARTH-PRESSURE COEFFICIENT - Ko = 5.000E-01
INTERNAL FRICTION ANGLE, DEG. = 4.500E+01
BLOWS PER FOOT FROM STANDARD PENETRATION TEST = 8.800E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.250E+02
MAXIMUM LOAD TRANSFER FOR SOIL, LB/SQ FT = 1.000E+10
DEPTH, FT = 0.000E+00

AT THE BOTTOM

SIDE FRICTION PROCEDURE: Ko METHOD
LATERAL EARTH-PRESSURE COEFFICIENT - Ko = 5.000E-01
INTERNAL FRICTION ANGLE, DEG. = 3.500E+01
BLOWS PER FOOT FROM STANDARD PENETRATION TEST = 1.900E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.250E+02
MAXIMUM LOAD TRANSFER FOR SOIL, LB/SQ FT = 1.000E+10
DEPTH, FT = 6.500E+00

LAYER NO 2----DECOMPOSED ROCK

AT THE TOP

ELASTIC MODULUS OF GRAVEL, LB/SQ IN = 2.089E+06
POISSION RATIO OF GRAVEL = 2.500E-01
BLOWS PER FOOT FROM STANDARD PENETRATION TEST = 5.000E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.450E+02
MAXIMUM LOAD TRANSFER FOR SOIL, LB/SQ FT = 1.000E+10
DEPTH, FT = 6.500E+00


AT THE BOTTOM

ELASTIC MODULUS OF GRAVEL, LB/SQ IN = 2.089E+06
POISSION RATIO OF GRAVEL = 2.500E-01
BLOWS PER FOOT FROM STANDARD PENETRATION TEST = 5.000E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.450E+02
MAXIMUM LOAD TRANSFER FOR SOIL, LB/SQ FT = 1.000E+10
DEPTH, FT = 8.500E+00


LAYER NO 3----STRONG ROCK

AT THE TOP

DIAMETER OF SOCKET, FT = 2.000E+00
SPACING OF DISCONTINUITIES,FT = 1.000E+00
THICKNESS OF INDIVIDUAL DISCONTINUITIES,FT = 1.000E-03
UNIAXIAL COMPRESSION STRENGTH OF ROCK,LB/SQ FT = 1.440E+05
UNIAXIAL COMPRESSION STRENGTH OF CONCRETE,LB/SQ FT= 7.200E+05
ELASTIC MODULUS FOR THE INTACT ROCK, LB/SQ IN. = 1.204E+07
ROCK QUALITY DESIGNATION (RQD) % = 8.800E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.600E+02
DEPTH, FT = 8.500E+00


AT THE BOTTOM

DIAMETER OF SOCKET, FT = 2.000E+00
SPACING OF DISCONTINUITIES,FT = 1.000E+00
THICKNESS OF INDIVIDUAL DISCONTINUITIES,FT = 1.000E-03
UNIAXIAL COMPRESSION STRENGTH OF ROCK,LB/SQ FT = 1.440E+05
UNIAXIAL COMPRESSION STRENGTH OF CONCRETE,LB/SQ FT= 7.200E+05
ELASTIC MODULUS FOR THE INTACT ROCK, LB/SQ IN. = 1.204E+07
ROCK QUALITY DESIGNATION (RQD) % = 8.800E+01
SOIL UNIT WEIGHT, LB/CU FT = 1.600E+02
DEPTH, FT = 2.000E+01




INPUT DRILLED SHAFT INFORMATION
------------------------------

MINIMUM SHAFT DIAMETER = 2.500 FT.
MAXIMUM SHAFT DIAMETER = 2.500 FT.
RATIO BASE/SHAFT DIAMETER = 0.000 FT.
ANGLE OF BELL = 0.000 DEG.
IGNORED TOP PORTION = 5.000 FT.
IGNORED BOTTOM PORTION = 0.000 FT.
ELASTIC MODULUS, Ec = 3.500E+06 LB/SQ IN






COMPUTATION RESULTS
-------------------



- CASE ANALYZED : 1
VARIATION LENGTH : 1
VARIATION DIAMETER : 1



DRILLED SHAFT INFORMATION

-------------------------

DIAMETER OF STEM = 2.500 FT.
DIAMETER OF BASE = 2.500 FT.
END OF STEM TO BASE = 0.000 FT.
ANGLE OF BELL = 0.000 DEG.
IGNORED TOP PORTION = 5.000 FT.
IGNORED BOTTOM PORTION = 0.000 FT.
AREA OF ONE PERCENT STEEL = 7.069 SQ.IN.
ELASTIC MODULUS, Ec = 3.500E+06 LB/SQ IN
VOLUME OF UNDERREAM = 0.000 CU.YDS.
SHAFT LENGTH = 10.000 FT.



PREDICTED RESULTS
-----------------

QS = ULTIMATE SIDE RESISTANCE;
QB = ULTIMATE BASE RESISTANCE;
WT = WEIGHT OF DRILLED SHAFT (FOR UPLIFT CAPACITY ONLY);
QU = TOTAL ULTIMATE RESISTANCE;
QBD = TOTAL ALLOWABLE LOAD USING A FACTOR OF SAFETY
APPLIED TO THE ULTIMATE BASE RESISTANCE;
QDN = TOTAL ALLOWABLE LOAD USING FACTORS OF SAFETY
APPLIED TO THE ULTIMATE SIDE RESISTANCE AND
THE ULTIMATE BASE RESISTANCE.

LENGTH VOLUME QS QB QU QBD QDN QU/VOLUME
(FT) (CU.YDS) (TONS) (TONS) (TONS) (TONS) (TONS) (TONS/CU.YDS)
6.0 1.09 0.54 54.66 55.20 18.76 18.44 50.60
7.0 1.27 1.16 55.02 56.18 19.50 18.80 44.14
8.0 1.45 7.93 55.02 62.95 26.27 21.51 43.28
9.0 1.64 14.88 0.00 14.88 14.88 5.95 9.09
10.0 1.82 50.65 0.00 50.65 50.65 20.26 27.86




AXIAL LOAD VS SETTLEMENT CURVES
-------------------------------


LOAD SETTLEMENT RELATIONSHIP
------------------------------------------

TOP LOAD TOP MOVEMENT
TONS IN.
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02
5.0138E+01 1.0637E-02

 

[EireChch]

I used SHAFT regularly and it worked fine. You have definitely made a mistake in your input and I dont have the time to review that text. You need to contact a colleague and ask them

 

[BridgeSmith]

Not familiar with the software, but our shaft designs often do have the tip resistance very low when combined with side friction, due to the differences in movement required for mobilization. In order to count tip resistance and side friction, even at ultimate strength, the bottom of the hole must be cleaned and inspected.

Typically, shafts for overhead sign structures are have relatively light axial loads. If the side friction resistance is morethan the load, it may show zero tip resistance because the required capacity is zero. IOW, it doesn't include it because it doesn't need it.

 

[EireChch]

SHAFT doesnot account for strain incompatibility between shaft and tip. What software are you using BS, I didnt know any software that did this. We typically get round this by applying a larger fos on tip resistance than skin friction

 

[BridgeSmith]

The software we use doesn't do that, AFAIK. We have Allpile and Lpile, but I don't use them for axial; I do that with hand comps or tabulate it in Excel. We've gone through the calcs to use both the side friction and tip resistance, and found it was not worth the trouble for approximately 5% of the tip resistance added to side friction. Now, we just take the simple approach, and use one or the other.

We only use the software to get the moment and deflection from the lateral load, which is what's always controlled for the sign structure foundations I've done. Axial resistance was always plenty adequate by the time I got it deep enough to limit the deflections.

Anyway, it appears the program is dropping the tip resistance to zero in the rock layer. Maybe it's due to the spacing of the discontinuities, or it's not including the end resistance because it's counting the side friction within 1 diameter of the bearing elevation. I'm fairly sure the AASHTO spec directs to do it the opposite way - ignore the skin friction within 1 shaft diameter of the tip.

 

[DirtSmuggler]

Thank you BridgeSmith for you insight.