Civil Engineering Reference
In-Depth Information
Table 9.2
Suggested load increments and holding times
Load
∗
Minimum time of holding load
25% DVL
30 min
50% DVL
30 min
75% DVL
30 min
100% DVL
1 h
75% DVL
10 min
50% DVL
10 min
25% DVL
10 min
0
1 h
100% DVL
6 h
100% DVL
+
25% SWL
1 h
100% DVL
+
50% SWL
6 h
100% DVL
+
25% SWL
10 min
100% DVL
10 min
75% DVL
10 min
50% DVL
10 min
25% DVL
10 min
0
1 h
∗
SWL denotes specified working load; DVL denotes design verification load.
To determine these slow rates of movement a sensitive settlement measuring system
is required. In granular soils or soft rocks the cessation of movement is easy to establish.
However, in clay soils consolidation settlement will occur over an extended period.
Plotting the results as the test proceeds is a useful way of resolving any ambiguities in
the results (see Figure 9.15).
The maintained load test may be used to test contract piles to check acceptance
criteria. Such piles are usually loaded to 100% of the design verification load (DVL)
plus 50% of the specified working load to avoid overstressing the soil whilst still
proving an adequate reserve of strength in the pile-soil system. The use of the DVL was
introduced to take into account items such as downdrag friction load, which will be
in the reverse direction during a test to that which may exist under long-term loading,
and the fact that piles on a site may not be tested from the same level as may pertain
for the piles in normal use. The ICE Specification also contains an extended proof load
procedure which may be useful, particularly for piles bearing on cohesionless soil or on
rock, and where further information may be desirable to verify design. On special test
piles that will not formpart of the permanent works, loading can be increased to failure,
enabling a check to be made on the design parameters (usually by the CRP method).
For particular projects special loading paths may be called for, for example, to
simulate repeated loading. Such loading paths may easily be arranged but careful
supervision is a necessity. Short-period, cyclic or sinusoidal loading requires the use
of sophisticated servo-controlled equipment.
9.2.3.2 Constant rate of penetration test
The constant rate of penetration (CRP) form of testing is confined to special test piles,
and is usually carried out at the pre-contract stage (Whittaker and Cooke, 1961). As
the name implies, the loading is strain controlled and a set rate of penetration of the
