Skempton (1986), mengembangkan koreksi
nilai SPT sebagai berikut.
Tabel
4.3 SPT Hammer Efficiencies
Country
|
Hammer Type
|
Hammer Release
Mechanism
|
Hammer Efficiency
|
Argentina
|
Donut
|
Cathead
|
0.45
|
Brazil
|
Pin Weight
|
Hand Dropped
|
7.2
|
China
|
Automatic
|
Trip
|
0.6
|
Donut
|
Hand Dropped
|
0.55
|
|
Donut
|
Cathead
|
0.5
|
|
Colombia
|
Donut
|
Cathead
|
0.5
|
Japan
|
Donut
|
Tombi Trigger
|
0.78 - 0.85
|
Donut
|
Cathead 2 turns +
Special release
|
0.65 - 0.67
|
|
UK
|
Automatic
|
Trip
|
0.73
|
USA
|
Safety
|
2 turns on cathead
|
0.55 - 0.60
|
Donut
|
2 turns on cathead
|
0.45
|
|
Venezuela
|
Donut
|
Cathead
|
0.43
|
Sumber:
Clayton, 1990 dalam Donald
P. Coduto, 1994
Tabel 4.4 Borehole,
Sampler, and Rood Correction Factors
Factor
|
Equipment Variables
|
Value
|
Borehole diameter
factor, Cb
|
2,5-4,5 in (65 - 115
mm)
|
1
|
6 in (150 mm)
|
1.05
|
|
8 in (200 mm)
|
1.15
|
|
Sampling method
factor, Cs
|
Standard Sample
|
1
|
Sampler without liner
(not recommended)
|
1.2
|
|
Rod length factor, CR
|
10 - 13 ft (3 - 4 m)
|
0.75
|
13 - 20 ft (4 - 6 m)
|
0.85
|
|
20 - 30 ft (6 - 10 m)
|
0.95
|
|
> 30 ft (>10 m)
|
1
|
Sumber: Skempton, 1986 dalam Donald P. Coduto, 1994
Contoh perhitungan:
Titik
BH – 01
Titik
BH – 01 berada pada pondasi untuk dermaga, dengan tanah keras berada pada
kedalaman 20 m ( N SPT > 50).
Tabel
4.5 Deskripsi
Lapisan Tanah Hasil SPT Pada Titik BH-01
Kedalaman
|
Deskripsi
|
N-SPT
|
Klasifikasi*
|
0.00 - 3.50
|
Lanau berlempung,
coklat keabu-abuan
|
2
|
sangat lunak
|
3.50 - 4.00
|
Pasir halus
berkerikil
|
3
|
lunak
|
4.00 - 10.50
|
Pasir berbutir kasar
|
3 - 28
|
lunak hingga sangat
kaku
|
10.50-15.50
|
Batu lempung, kaku
|
42 - 46
|
keras
|
15.50 - 16.50
|
Tanah organik
|
46
|
keras
|
16.50 - 20.00
|
Batu lempung, coklat
keabu-abuan
|
45 - 50
|
keras
|
20.00 - 25.00
|
Batu lempung,
abu-abu
|
> 50
|
keras
|
*
Berdasarkan Terzaghi dan Peck, 1948 dalam Hardiyatmo 2007
Perhitungan selanjutnya ditabelkan pada Tabel
4.6
Tabel 4.6 Nilai N60 dan Qs
pada titik BH – 01
Bagian
|
Kedalaman (m)
|
h/bagian (m)
|
N
|
N60
|
N Rata-rata
|
Qs
|
1
|
0-2,0
|
2
|
2
|
1,667
|
1,667
|
0,852
|
2
|
2,0-4,0
|
2
|
3
|
2,500
|
2,500
|
1,277
|
3
|
4,0-6,0
|
2
|
22
|
18,333
|
18,333
|
9,367
|
4
|
6,0-8,0
|
2
|
24
|
20,000
|
20,000
|
10,218
|
5
|
8,0-10,0
|
2
|
28
|
23,333
|
23,333
|
11,921
|
6
|
10,0-12,0
|
2
|
42
|
35,000
|
35,000
|
17,882
|
7
|
12,0-14,0
|
2
|
43
|
35,833
|
35,833
|
18,307
|
8
|
14,0-16,0
|
2
|
46
|
38,333
|
38,333
|
19,585
|
9
|
16,0-18,0
|
2
|
45
|
37,500
|
37,500
|
19,159
|
10
|
18,0-20,0
|
2
|
50
|
41,667
|
41,667
|
21,288
|
11
|
20,0-30,0
|
10
|
50
|
41,667
|
41,667
|
106,438
|
