Dynamic Load Testing of Helical Piles ANNUAL KANSAS CITY SPECIALTY SEMINAR 2014 JANUARY 10, 2014

J o rg e B e i m – J W B C o n s u l t i n g L LC

P i l e D y n a m i c s , I n c .

Main Topics Brief description of the Dynamic Load Test (DLT) Method

Comments on the results that can be obtained from DLT, and how they compare to those of Static Load Tests (SLT) and torque measurements

Brief discussion of Case Studies described in the literature J.G. Cannon (2000) J.W. Beim & S.C. Luna (2012) A. Klesney & F. Rausche (2012) B. White et al (2013)

1. The Dynamic Load Testing Method ANNUAL KANSAS CITY SPECIALTY SEMINAR 2014

JANUARY 10, 2014

The Dynamic Load Testing Method Research program started in 1964 @ Case Institute of Technology (now Case Western Reserve University), with the objective of developing an economical, “practical for field use”, portable pile bearing capacity measurement system

Method is based on electronic measurements of pile top force and velocity during impact of a large hammer

Two kinds of sensors are installed on a location usually close to the pile top: Strain transducers for measuring the force (by multiplying the strain by the elastic modulus and cross-

section area of the pile material) For piles up to about 6” diameter => use strain gages glued to rod For larger piles => use standard reusable gages

Accelerometers for measuring the velocity (by integrating the acceleration data)

The Dynamic Load Testing Method Signals from sensors are sent via cable or radio to the Pile Driving Analyzer® (PDA)

After each blow the PDA processes the data, providing: Pile capacity using the simplified CASE method (valid for uniform piles) Maximum stresses along the pile Transferred energy

The data collected for one blow can be further analyzed using the CAPWAP® program, to determine: Total mobilized resistance Resistance distribution Simulated load-displacement curve

The CAPWAP Analysis CAPWAP is a Signal Matching Program (System Identification Analysis or Reverse Analysis)

System Movement Load

The CAPWAP Analysis The pile is divided in elements roughly 3.3 ft. long => non-uniform piles can be easily modeled The soil is divided in elements usually about 6.6 ft long, plus an additional element for the toe A model based on the work of E.A.L. Smith (1960), with numerous extensions and improvements, is

used for the soil One variable is used as input (for example velocity) and the soil model parameters are interactively

changed until the best possible match between measured and the calculated complementary curves (for example force) is achieved The static resistance from this soil model is the mobilized resistance (can correspond to the ultimate

resistance if sufficient energy was applied to cause substantial permanent displacement) The soil and pile model are used to generate a simulated load-displacement curve

The CAPWAP Analysis A typical simulated load-displacement curve

0 500 1000 1500 2000

0.000

0.200

0.400

0.600

0.800

Load (kips)

Displa

cement

(in)

Pile TopBottom

Ru = 1688.4 kipsRs = 1129.8 kipsRb = 558.6 kipsDy = 0.47 inDx = 0.55 in

2. Results from Dynamic Load Testing ANNUAL KANSAS CITY SPECIALTY SEMINAR 2014

JANUARY 10, 2014

Usual DLT procedure for helical piles Apply blows with increasing drop height, using a suitable impact device Ram with a weight of at least 2% of test load True free fall Good guiding and leveling systems

Interpret the CAPWAP load-displacement curves according to maximum allowable displacement, for example: Davisson: 0.15” + D/120 + elastic deformation (PL/AE) => a set of 0.1” or more on the analyzed blow is

OK (usual for driven piles) FHWA: 5% of diameter Livneh and El Naggar (2008): 8% of largest helical diameter + elastic deformation ½ inch – see B. White (2013) Larger max allowable displacements require larger sets and heavier weights for DLT

Interpretation of CAPWAP load-displacement curve For small allowable displacements (e.g. Davisson), analysis of blow with highest mobilized capacity usually sufficient

For larger displacements, superposition of load-displacement curves for all applied blows is recommended

Other results Stresses along pile: important to determine maximum drop height

Bending (check alignment)

Energy transfer

3. Case studies ANNUAL KANSAS CITY SPECIALTY SEMINAR 2014

JANUARY 10, 2014

J.G. Cannon (2000) Impact System:

Special cable drop hammer made up by one of the Contractors Drop weights consist of 2 and 4 tonne (4.4 and 8.8 kips) solid circular billets of steel of about 350 mm

(13.8 inches) diameter and 2.6 and 5 m (8.5 and 16.4 ft) long respectively Guide frame allows for a stroke of about 2 m (6.6 ft) The frame was supported laterally by 4 guy wires tied to adjacent screw piles, either production piles or

temporary anchors specifically for the test

J.G. Cannon (2000) The following tests were reported:

Sydney, Australia International Airport 89x5.5 mm (3.5x7/32 inches) tubes with single 350 mm (13.8 inches) helix near the toe – working load =

100 kN (22 kips) 168x7 mm (6.6x9/32 inches) tubes with either two 700 mm (27.5 inches) helices (working load 500 kN

[110 kips]) or with 5 helices (working load 600 kN [132 kips]) All piles about 8 m (26 ft) penetration Moderately dense to dense medium sands

J.G. Cannon (2000) Redcliff Hospital, Brisbane, Australia 219x8 mm (8.6x5/16 inches) tubes with 2 helices 0.85 m (33.5 inches) in diameter; 3.0 m (9.8 ft)

penetration. Working load 850 to 1000 kN (191 – 225 kips) “Completely weathered sandstone, which was essentially very stiff-hard clayey sand or sandy clay with

reported undrained shear strength of up to 600-700 kPA (12.5 – 14.6 ksf)”

• Static load test carried to 1.5 times serviceability load. • Dynamic test limited only by stresses in the pile shaft,

so was able to mobilize considerably more resistance. • Initial change in slope of static load test curve was

modeled with acceptable accuracy by CAPWAP, according to author

J.G. Cannon (2000) Redlands Mater Hospital, Brisbane, Australia 89x5.5 mm (3.5x7/32 inches) with a single 350 mm (13.8 inches) helix 114x6 mm (4.5x15/64 inches) with a single 450 mm (17.7 inches) helix Penetrations from 2.2 to 5 m (7.2 to 16.4 ft) Stiff clays over stiff to hard gravely clays 8 piles were tested, comprising 15% of total piles One of the test piles showed high torque resistance during installation, but during testing the set was

very high (37 mm/bl – 8 bl/ft) and the mobilized resistance was also low. According to the author, “this is an example of why installation torque does not give a good indication of pile capacity”.

J.W. Beim & S.C. Luna (2012) Test program performed on helical piles specially installed at the National Geotechnical Experimentation Site of the University of Massachusetts - Amherst campus (UMass-Amherst)

Soil consists of approximately 5 ft of stiff silty-clay fill overlaying a thick 100-ft deposit of late Pleistocene lacustrine varved clay

Seven piles were submitted to Dynamic Load Tests and also to Static Load Tests

Piles were 2 7/8x0.217 inch, with 3 helices (8, 10 and 12 inches)

Three piles were installed to a depth of 12 ft, and consisted of one bottom section and one extension. Five piles were installed to a depth of 18 ft and consisted of one bottom section and two extensions. The shorter piles showed more resistance than the longer ones, as expected due to a drop in soil resistance at 13 ft

J.W. Beim & S.C. Luna (2012) Dynamic Load Tests

3.5 ft-long piece of instrumented rod

Strain gages

PR Accelerometers

Radio transmitters

300 lbs ram

Data sent via radio to a PDA

3 ft max drop height

J.W. Beim & S.C. Luna (2012) Example comparison of DLT and SLT results

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25 30 35

Displacement

inches

Load (kips)

HP5 (12 ft)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12

Displacement

inches

Load (kips)

HP7 (18 ft)

Static

Dynamic

Davidson

J.W. Beim & S.C. Luna (2012) Summary of results

Pile HP5 HP10 HP15 HP7 HP9 HP12 HP14 Depth (ft) 12 12 12 18 18 18 18 Torque (ft-lbs) 1428 1544 1699 869 868 890 1101 SLT - Davisson (kips) 23.1 22.0 22.9 8.6 10.9 11.7 12.7 DLT - Davisson (kips) 22.6 19.3 18.2 8.6 11.2 11.1 12.7 SLT/DLT 1.02 1.14 1.26 1.00 0.97 1.05 1.00 Kt static (ft-1) 16.2 14.3 13.5 9.9 12.6 13.1 11.5

Kt dynamic (ft-1) 15.8 12.5 10.7 9.9 12.9 12.5 11.5

Average Kt static (ft-1) 14.6 11.8

Compressive Bearing Capacity can be determined from torque measured during installation using Qult = Kt x T

The value for Kt recommended for this kind of pile is 9 ft-1 (AC-358). However, in this case: 12 ft piles (HP5, HP10, HP15) => Kt ≈ 15 18 ft piles (HP7, HP9, HP12, HP14) => Kt ≈ 12

The usual Kt value proved to be conservative in this case

A. Klesney & F. Rausche (2012) Ten 2-7/8 inch piles were tested at the Amtrak Passenger Station of Alpine, Texas

Piles installed into primarily granular soil to a minimum depth of 10 ft

The impact system consisted of a “simple, 1,500 lb drop hammer with a reusable, portable pile extension fitted for transducer attachment and drop hammer impact”. Three or four impacts with fall heights up to 3 ft were applied

B. White et al (2013) Four 9 5/8x0.395 inch helical piles tested in Midland, Michigan. The piles had two helices of the same diameter: 20 (2), 22 and 24 inches. The design pile capacities were 87, 96 and 111 kips, respectively.

Piles penetrated 45-46 ft in the soil described as “compact loamy sand to 9 ft depth, stiff clay extending to 45 ft depth, over an extremely hard layer of clay below 45 ft depth”.

Blows were applied by the APPLE VI dynamic load testing system, consisting of a 4.5 ton drop weight supported by a metallic frame. The drop heights ranged from 2 to 18 inches

Load-displacement curves were constructed by superimposing the CAPWAP load-displacement curves of the individual blows. Capacities were evaluated for a ¼ inch and for a ½ inch top displacement (the latter was the specified performance criterion)