BRANZ Bulletin 438 1

NUMBER 438August 2003

USING A SCALAPENETROMETER

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B U I L D I N G O N K N O W L E D G E

This Bulletin is a guide for using a Scala Penetrometer to determine whether a site hasground conditions that meet the minimum NZS 3604 Timber Framed Buildings criteria.

2 BRANZ Bulletin 438

This Bulletin replaces Bulletin212 Soil Testing (with theScala Penetrometer)

1.0 INTRODUCTION1.0.1 Section 3 of NZS 3604:1999Timber Framed Buildings sets out thecriteria for ground conditions for buildingsconstructed to the standard.

1.0.2 The main requirement is that theultimate bearing pressure of the soilsupporting the foundations is not less than300 kPa.

1.0.3 Where the ground-bearingcapacity does not meet the 300 kParequirement, the foundations but notnecessarily the remainder of the buildingmust be specifically designed.

1.0.4 This Bulletin is a guide to using aScala Penetrometer to determine theground conditions of a particular site(where there are concerns regardingbearing capacity) to ensure that theymeet the minimum NZS 3604 criteria.

2.0 USING A SCALAPENETROMETER

A scala penetrometer can be used as amethod of determining the bearingcapacity of the ground when the “goodground” conditions of NZS 3604Sections 3.1 and 3.2 are not met.

2.1 The equipment2.1.1 The Scala Penetrometerequipment, (also called a dynamic conepenetrometer) must conform to Test6.5.2 of NZS 4402. It consists of twoparts (Figure 1):• A stainless steel rod 16 mm in

diameter with a conical tip of case-hardened steel. The rod isgraduated at 50 mm intervals. Therod may be in one piece, or it may besectioned with threaded joints foreasy handling and storage. Amultiple-piece rod is easier tocommence driving with.

• A handle section consisting of a drophammer and anvil which is fitted tothe top of the rod. The anvil ispositioned to allow the drop hammera clear fall of 508 mm. The steeldrop hammer weighs 9 kg.

2.2 Test positions2.2.1 Select test positions to giveadequate information about the soilover the entire plan area of theproposed building.

2.2.2 Clause 3.3.8 of NZS 3604requires at least four test positions for a Figure 1. The Scala Penetrometer equipment.

building not exceeding 200 m2 plan areawith not less than one additional testposition for each 100 m2 additional planarea. It is recommended that thechosen locations are near the fourcorner points of the building foundation.

2.2.3 Each test position must berecorded, and this can conveniently bedone on a copy of the foundation floor plan.

2.2.4 Clause 3.3.6(c) of NZS 3604stipulates that the results must besimilar across all test positions. If theresults indicate significant change in soilprofiles across the site then a moredetailed investigation is required.

2.3 How to use a ScalaPenetrometer

2.3.1 The test method to be used isdescribed in Test 6.5.2 of NZS 4402Determination of the penetration resistanceof a soil.

2.3.2 Begin the test at either the groundlevel or from the level of the bottom of aproposed nearby footing by excavating tothe level of the underside of the footing.

2.3.3 Remove any material, such ascrushed rock or gravel, which will be too

handle

9 kg drop hammer

stainless steel rod

508 mm

anvil

stainless steelprobe rod

conical tip

BRANZ Bulletin 438 3

hard to penetrate or which maydamage the equipment.2.3.4 Hold the Scala Penetrometervertical and tap the hammer on theanvil until the top of the cone is flushwith the soil surface.

2.3.5 Use the drop hammer to drivethe tip of the penetrometer to a depthbelow the underside of the proposedfooting of not less than 1.2 m or twicethe width of the widest footing,whichever is deeper.

2.3.6 Drop the hammer the full 508mm on to the anvil for each blow.

2.3.7 Record the number of hammerblows for each interval of penetration.The penetration interval must notexceed 300 mm for any one blow.Measure the penetration from a sightboard placed firmly across the groundsurface, or pegged alongside theplanned Scala Penetrometer shafthole.

2.3.8 In hard ground it may be difficultto withdraw the steel probe after thetest has been completed. To make thiseasier the probe may be withdrawn

several times during the test. While itis out of the ground, an enlarged holemay be augured on the same centre,taking care not to penetrate closer than300 mm to the level previously reachedby the conical tip. The probe is then re-inserted and the test continued.

2.4 Soil suitability2.4.1 To have an ultimate bearingpressure of 300 kPa, the results of thepenetrometer tests for each interval ofpenetration and at each test site mustsatisfy the following (see Figure 2):1. For a depth equal to the width of

the widest footing below theunderside of the proposed footingthe “average penetration” per blowshall not exceed 25 mm. This isthe same as requiring that thenumber of blows for an interval of300 mm shall not be less than 12.

2. At greater depths the “averagepenetration” per blow shall notexceed 38 mm (i.e. the number ofblows for an interval of 300 mmshall not be less than 8).

2.4.2 As an example, consider a teststarting from ground level near aproposed 350 mm-deep footing using

Figure 2. Scala Penetrometer Test Criteria

ground surface

footing

no requirements for soilstrength over this height

over depth = "W" the"average penetration" per blowshall not exceed 25mm(i.e. a minimum of 12 blows for aninterval of 300 mm)

the "average penetration" per blowshall not exceed 38 mm(i.e. a minimum of 8 blows for aninterval of 300 mm)

this depth is the maximum of1.2 m and twice footingwidth "W"

widest footingwidth (W)

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the criteria in 2.4.1. Table 1 shows thetest results and calculat ions todetermine whether the measurementssatisfy NZS 3604 criteria for deemingthe ground to be suitable. The firstcolumn of Table 1 records the totaldepth penetrated by the cone in stepsof 300 mm. The actual interval iscalculated in Column 2 by subtractionfrom the previous row. The number ofblows used in the interval is tabulatedin the third column. Finally the lastcolumn checks to see that the criteriaare satisfied. In the last row of thetable, the interval has been reducedfrom 300 to 150 mm. Thus the criteriais changed from 8 blows to 8x150/300= 4 blows.

3.0 SOIL EXAMINATION3.0.1 Adjacent to each test positionaugur a bore hole of not less than 50mm diameter to the same depth as thepenetrometer has been driven. Theauger must be lifted every 300 mmand the soil examined and classifiedby inspection. The nature of theground penetrated must be recordedas per (a) to (f) of Clause 3.3.5 of NZS3604. Clause 3.3.6 states therequirements for the materialwithdrawn from the bore hole.

3.0.2 A table is given at the back ofthis bulletin which may be used forrecording Scala Penetrometer testresults and the soil inspectioninformation where the same interval isused for both (usually 300 mm) andwhere both commence from the groundsurface. The test position number, aswritten on a referenced foundationplan, is also recorded on this form.

4.0 PROBLEMS4.0.1 Where a rock is struck or sixconsecutive blows give less than 20mm penetration, BRANZ recommendsthat the test be stopped and replacedwith a second and, if necessary, a thirdtest, approximately 0.5 m away. If thesecond and third tests also give sixconsecutive blows with less than 20 mmpenetration then provided the averagepenetration per blow over all intervals atthese three test positions satisfies thecriteria of Section 2.0 then the localground at this group of tests is deemedto be satisfactory.

4.0.2 The NZS 3604 requirements donot deal with the situation of variablestiffnesses of soil layers within aninterval. Even though the number ofblows may average less than 25 mm perblow over an interval not exceeding 300mm, BRANZ recommends independentexpert advice where several blows fromthe total test positions give penetrationsof more than 70 mm.

5.0 NEED FORSPECIFIC DESIGN

5.0.1 If all the requirements of Section2 are not fulfilled, it will be necessary foran appropriately qualified person to carryout a full site investigation and designsuitable foundations. In addition, aspecific design is required if the soilconsists of expansive clays. These aredefined as clays that have propertieswhich exceed the values listed in thedefinition of good ground in Clause 1.3 ofNZS 3604. Section 17 of NZS 3604provides additional information onexpansive soils.

Table 1. Calculations for Scala Penetrometer Example

Depth Actual Number Criterionpenetrated interval of satisfied?(mm) (mm) blows

300 300 8 Yes (There is no criteria as footingfounding depth has not been reached.

600 300 12 Yes (Number of blows not less than 12)

900 300 14 Yes (Number of blows not less than 12)

1200 300 16 Yes (Number of blows not less than 12)

1500 300 11 Yes (Number of blows not less than 8)

1650 150 5 Yes (Number of blows not less than 8/2=4)

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Depth Actual Number Criterion Soil description (See aspectspenetrated interval of blows satisfied? needed to be covered below)(mm) (mm)

Record: Soil typesPresence of stones, gravel or other hardmaterialPresence of topsoil, peat, fill, or other foreignmaterialGround water levelSoil strength of any peat or clay encounteredas tested on natural chunks (not remouldedmaterial or loose shavings) as follows:• Stiff (cannot be moulded in the fingers)

• Firm (can be moulded in the fingersonly by strong pressure)

• Soft (can be moulded in the fingerseasily)

• Very soft (exudes between thefingers when squeezed in the fist)

Table …… Results of Scala Penetrometer test and adjacent bore hole data test positionNumber …… (Refer to drawing number …………)