BUILDING ON KNOWLEDGE
August 2003
NUMBER 438
USING A SCALA PENETROMETER
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This Bulletin is a guide for using a Scala Penetrometer to determine whether a site has ground conditions that meet the minimum NZS 3604 Timber Framed Buildings criteria. criteria.
BRANZ Bulletin 438
1
This Bulletin replaces Bulletin 212 Soil Testing (with the Scala Penetrometer)
1.0 INTRODUCTION 1.0.1 Section 3 of NZS 3604:1999 Timber Framed Buildings sets out the criteria for ground conditions for buildings constructed to the standard. 1.0.2 The main requirement is that the ultimate bearing pressure of the soil supporting the foundations is not less than 300 kPa.
2.2.3 Each test position must be recorded, and this can conveniently be done on a copy of the foundation floor plan.
1.0.3 Where the ground-bearing capacity does not meet the 300 kPa requirement, the foundations but not necessarily the remainder of the building must be specifically designed.
2.2.4 Clause 3.3.6(c) of NZS 3604 stipulates that the results must be similar across all test positions. If the results indicate significant change in soil profiles across the site then a more detailed investigation is required.
1.0.4 This Bulletin is a guide to using a Scala Penetrometer to determine the ground conditions of a particular site (where there are concerns regarding bearing capacity) to ensure that they meet the minimum NZS 3604 criteria.
2.0 USING A SCALA PENETROMETER A scala penetrometer can be used as a method of determining the bearing capacity of the ground when the “good ground” conditions of NZS 3604 Sections 3.1 and 3.2 are not met.
2.1
2.2
2.3
How to use a Scala Penetrometer
2.3.1 The test method to be used is described in Test 6.5.2 of NZS 4402 Determination of the penetration resistance of a soil . 2.3.2 Begin the test at either the ground level or from the level of the bottom of a proposed nearby footing by excavating to the level of the underside of the footing. 2.3.3 Remove any material, such as crushed rock or gravel, which will be too
The equipment
2.1.1 The Scala Penetrometer equipment, (also called a dynamic cone penetrometer) must conform to Test 6.5.2 of NZS 4402. It consists of two parts (Figure 1): • A stainless steel rod 16 mm in diameter with a conical tip of casehardened steel. The rod is graduated at 50 mm intervals. The rod may be in one piece, or it may be sectioned with threaded joints for easy handling and storage. A multiple-piece rod is easier to commence driving with. • A handle section consisting of a drop hammer and anvil which is fitted to the top of the rod. The anvil is positioned to allow the drop hammer a clear fall of 508 mm. The steel drop hammer weighs 9 kg.
handle
9 kg drop hammer
stainless steel rod
508 mm
anvil
stainless steel probe rod
Test positions
2.2.1 Select test positions to give adequate information about the soil over the entire plan area of the proposed building. 2.2.2 Clause 3.3.8 of NZS 3604 requires at least four test positions for a 2
building not exceeding 200 m plan area with not less than one additional test position for each 100 m2 additional plan area. It is recommended that the chosen locations are near the four corner points of the building foundation.
conical tip
Figure 1. The Scala Penetrometer equipment. BRANZ Bulletin 438
ground surface
no requirements for soil strength over this height
footing
over depth = "W" the "average penetration" per blow shall not exceed 25mm (i.e. a minimum of 12 blows for an interval of 300 mm)
widest footing width (W) this depth is the maximum of 1.2 m and twice footing width "W"
the "average penetration" per blow shall not exceed 38 mm (i.e. a minimum of 8 blows for an interval of 300 mm)
Figure 2. Scala Penetrometer Test Criteria
hard to penetrate or which may damage the equipment. 2.3.4 Hold the Scala Penetrometer vertical and tap the hammer on the anvil until the top of the cone is flush with the soil surface. 2.3.5 Use the drop hammer to drive the tip of the penetrometer to a depth below the underside of the proposed footing of not less than 1.2 m or twice the width of the widest footing, whichever is deeper. 2.3.6 Drop the hammer the full 508 mm on to the anvil for each blow. 2.3.7 Record the number of hammer blows for each interval of penetration. The penetration interval must not exceed 300 mm for any one blow. Measure the penetration from a sight board placed firmly across the ground surface, or pegged alongside the planned Scala Penetrometer shaft hole. 2.3.8 In hard ground it may be difficult to withdraw the steel probe after the test has been completed. To make this easier the probe may be withdrawn BRANZ Bulletin 438
several times during the test. While it is out of the ground, an enlarged hole may be augured on the same centre, taking care not to penetrate closer than 300 mm to the level previously reached by the conical tip. The probe is then reinserted and the test continued.
2.4
Soil suitability
2.4.1 To have an ultimate bearing pressure of 300 kPa, the results of the penetrometer tests for each interval of penetration and at each test site must satisfy the following (see Figure 2): 1. For a depth equal to the width of the widest footing below the underside of the proposed footing the “average penetration” per blow shall not exceed 25 mm. This is the same as requiring that the number of blows for an interval of 300 mm shall not be less than 12. 2. At greater depths the “average penetration” per blow shall not exceed 38 mm (i.e. the number of blows for an interval of 300 mm shall not be less than 8). 2.4.2 As an example, consider a test starting from ground level near a proposed 350 mm-deep footing using 3
Table 1. Calculations for Scala Penetrometer Example Depth penetrated (mm)
Actual interval (mm)
Number of blows
Criterion satisfied?
300
300
8
Yes (There is no criteria as footing founding 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)
the criteria in 2.4.1. Table 1 shows the test results and calculations to determine whether the measurements satisfy NZS 3604 criteria for deeming the ground to be suitable. The first column of Table 1 records the total depth penetrated by the cone in steps of 300 mm. The actual interval is calculated in Column 2 by subtraction from the previous row. The number of blows used in the interval is tabulated in the third column. Finally the last column checks to see that the criteria are satisfied. In the last row of the table, the interval has been reduced from 300 to 150 mm. Thus the criteria is changed from 8 blows to 8x150/300 = 4 blows.
3.0 SOIL EXAMINATION 3.0.1 Adjacent to each test position augur a bore hole of not less than 50 mm diameter to the same depth as the penetrometer has been driven. The auger must be lifted every 300 mm and the soil examined and classified by inspection. The nature of the ground penetrated must be recorded as per (a) to (f) of Clause 3.3.5 of NZS 3604. Clause 3.3.6 states the requirements for the material withdrawn from the bore hole. 3.0.2 A table is given at the back of this bulletin which may be used for recording Scala Penetrometer test results and the soil inspection information where the same interval is used for both (usually 300 mm) and where both commence from the ground surface. The test position number, as written on a referenced foundation plan, is also recorded on this form. 4
4.0 PROBLEMS 4.0.1 Where a rock is struck or six consecutive blows give less than 20 mm penetration, BRANZ recommends that the test be stopped and replaced with a second and, if necessary, a third test, approximately 0.5 m away. If the second and third tests also give six consecutive blows with less than 20 mm penetration then provided the average penetration per blow over all intervals at these three test positions satisfies the criteria of Section 2.0 then the local ground at this group of tests is deemed to be satisfactory. 4.0.2 The NZS 3604 requirements do not deal with the situation of variable stiffnesses of soil layers within an interval. Even though the number of blows may average less than 25 mm per blow over an interval not exceeding 300 mm, BRANZ recommends independent expert advice where several blows from the total test positions give penetrations of more than 70 mm.
5.0 NEED FOR SPECIFIC DESIGN 5.0.1 If all the requirements of Section 2 are not fulfilled, it will be necessary for an appropriately qualified person to carry out a full site investigation and design suitable foundations. In addition, a specific design is required if the soil consists of expansive clays. These are defined as clays that have properties which exceed the values listed in the definition of good ground in Clause 1.3 of NZS 3604. Section 17 of NZS 3604 provides additional information on expansive soils. BRANZ Bulletin 438
Table …… Results of Scala Penetrometer test and adjacent bore hole data test position Number …… (Refer to drawing number …………) Depth penetrated (mm)
Actual interval (mm)
Number of blows
Record: Soil types Presence of stones, gravel or other hard material Presence of topsoil, peat, fill, or other foreign material Ground water level Soil strength of any peat or clay encountered as tested on natural chunks (not remoulded material or loose shavings) as follows: • Stiff (cannot be moulded in the fingers) BRANZ Bulletin 438
• • •
Criterion satisfied?
Soil description (See aspects needed to be covered below)
Firm (can be moulded in the fingers only by strong pressure) Soft (can be moulded in the fingers easily) Very soft (exudes between the fingers when squeezed in the fist)
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